Frequency Of Genetic Recombination Research Articles

Past innovations and future possibilities in plant chromosome engineering.

Plant chromosome engineering has emerged as a pivotal tool in modern plant breeding, facilitating the transfer of desirable traits through the incorporation of alien chromosome fragments into plants. Here, we provide a comprehensive overview of the past achievements, current methodologies and future prospects of plant chromosome engineering. We begin by examining the successful integration of specific examples such as the incorporation of rye chromosome segments (e.g. the 1BL/1RS translocation), Dasypyrum villosum segments (e.g. the 6VS segment for powdery mildew resistance), Thinopyrum intermedium segments (e.g. rust resistance genes) and Thinopyrum elongatum segments (e.g. Fusarium head blight resistance genes). In addition to trait transfer, advancements in plant centromere engineering have opened new possibilities for chromosomal manipulation. This includes the development of plant minichromosomes via centromere-mediated techniques, the generation of haploids through CENH3 gene editing, and the induction of aneuploidy using KaryoCreate. The advent of CRISPR/Cas technology has further revolutionized chromosome engineering, enabling large-scale chromosomal rearrangements, such as inversions and translocations, as well as enabling targeted insertion of large DNA fragments and increasing genetic recombination frequency. These advancements have significantly expanded the toolkit for genetic improvement in plants, opening new horizons for the future of plant breeding.

Concurrent Disruption of Genetic Interference and Increase of Genetic Recombination Frequency in Hybrid Rice Using CRISPR/Cas9.

Manipulation of the distribution and frequency of meiotic recombination events to increase genetic diversity and disrupting genetic interference are long-standing goals in crop breeding. However, attenuation of genetic interference is usually accompanied by a reduction in recombination frequency and subsequent loss of plant fertility. In the present study, we generated null mutants of the ZEP1 gene, which encodes the central component of the meiotic synaptonemal complex (SC), in a hybrid rice using CRISPR/Cas9. The null mutants exhibited absolute male sterility but maintained nearly unaffected female fertility. By pollinating the zep1 null mutants with pollen from indica rice variety 93-11, we successfully conducted genetic analysis and found that genetic recombination frequency was greatly increased and genetic interference was completely eliminated in the absence of ZEP1. The findings provided direct evidence to support the controversial hypothesis that SC is involved in mediating interference. Additionally, the remained female fertility of the null mutants makes it possible to break linkage drag. Our study provides a potential approach to increase genetic diversity and fully eliminate genetic interference in rice breeding.

A local Bacillus spp.

Background Increasing chemical nematicide and fertilizer application has disturbed the natural balance and increased human health risks, which in turn compelled and increased the urgency to discover novel and more safe and effective biological tools to control root-knot nematode (Meloidogyne incognita) distributed throughout Egypt. Objective Isolation and screening of rhizobacteria from soil samples was the first step. The second step was the evaluation of the isolated rhizobacteria capability as nematicides under laboratory and screenhouse conditions. The third was to employ protoplast fusion as genetic technique to improve the nematicidal effect of rhizobacterial bacteria strains and comparison between parent and fusant protein profiles to analyze the genetic recombination among them. Materials and methods Serial dilution method was used to isolate rhizobacteria on Tryptic Soy Agar plates. The evaluation test against M. incognita J2 of bacterial isolates was achieved under laboratory conditions and nematode parameters under screenhouse. The most potent rhizobacteria were exposed to molecular identification using 16s rDNA sequencing. Bacillus thuringiensis subsp tenebrionis (Bt) was evaluated for the first time as a bioagent that acted as a nematicide. Protoplast fusion was employed between the most potent bacterial strain, Bacillus cereus (Bc) and B. thuringiensis subsp tenebrionis (Bt). Selection of bacterial fusants was achieved on complete media containing selectable antibiotics marker. Parental and fusant strains were evaluated against J2 under laboratory and screenhouse conditions and their ability in plant-growth promotion by assessment of plant parameters. SDS-PAGE analysis was used to analyze protein profiles. Results and conclusion Out of 15 bacterial isolates, NRC12 recorded the highest percentage mortality 88.3% after 24 h compared with control. NRC12 was identified as B. cereus NRC12 (Bc) and deposited in Genbank under accession number MW548408. Protoplast fusion between B. cereus (Bc) and B. thuringiensis (Bt) resulted in formation of 10 stable bacterial fusants that gave higher nematicidal effect compared with parental strains. Bacterial fusant, F7, achieved the highest mortality and reduction in J2 in soil by 98.3 and 87.19% under laboratory and screenhouse conditions, respectively. The highest reduction in the number of galls and egg masses on root system was also recorded by F7 with 77.18 and 72.35%, respectively, compared with its parents. Also, F7 registered the highest significant (P≤0.05) increment in length, fresh weight, and dry weights in the shoot system of eggplant in pot experiments. The bacterial fusant, F7, exhibited more ability to fix atmospheric nitrogen compared with bacterial parents. Protein profile of F7 proved that this fusant was the only one that acquired equal protein bands from each parent. The highest frequency of genetic recombination might occur in F7 and subsequently, induced effective nematicidal effect more than its parents.

Genetic and phylogenetic analysis of porcine circovirus type 2 on Jeju Island, South Korea, 2019-2020: evidence of a novel intergenotypic recombinant.

Porcine circovirus type 2 (PCV2) is the most ubiquitous viral pathogen of pigs and has persistently affected the global swine industry. Since first being identified in South Korea in 1999, the virus has undergone considerable genetic change and genotype shifts during the past two decades. These events have contributed to the coexistence of genotypes PCV2a, PCV2b, and PCV2d in Korean pig populations, which may promote viral recombination. The genotypic and phylogenetic characteristics of PCV2 strains circulating in pig herds on Jeju Island from 2019 to 2020 were the focus of this study. Genotype-specific PCR indicated that PCV2d is the dominant viral genotype and that coinfections with PCV2d and PCV2a (75%) or PCV2a and PCV2b (25%) are common in provincial pig herds. The complete genome sequences of 11 PCV2 strains, including three PCV2a, two PCV2b, and six PCV2d strains, were determined. A genomic comparison showed that all of the viruses had the highest nucleotide sequence identity to their corresponding genotypic reference strain. Notably, genetic and phylogenetic analysis revealed that one PCV2d strain, KNU-1931, exhibited nucleotide sequence variation in the ORF1 gene when compared to other PCV2d strains but showed a high degree of similarity to the PCV2b strains. Comprehensive recombination analysis suggested that KNU-1931 originated from natural recombination within ORF1 between PCV2b (the minor parent) and PCV2d (the major parent) strains. Our findings provide information about the frequency of genetic recombination between two different PCV2 genotypes circulating in the field domestically, illustrating the importance of continual intergenotypic recombination for viral fitness when multiple genotypes are present.

Deciphering the Impact of a Bacterial Infection on Meiotic Recombination in Arabidopsis with Fluorescence Tagged Lines

Plants are under strong evolutionary pressure to maintain surveillance against pathogens. One major disease resistance mechanism is based on NB-LRR (NLR) proteins that specifically recognize pathogen effectors. The cluster organization of the NLR gene family could favor sequence exchange between NLR genes via recombination, favoring their evolutionary dynamics. Increasing data, based on progeny analysis, suggest the existence of a link between the perception of biotic stress and the production of genetic diversity in the offspring. This could be driven by an increased rate of meiotic recombination in infected plants, but this has never been strictly demonstrated. In order to test if pathogen infection can increase DNA recombination in pollen meiotic cells, we infected Arabidopsis Fluorescent Tagged Lines (FTL) with the virulent bacteria Pseudomonas syringae. We measured the meiotic recombination rate in two regions of chromosome 5, containing or not an NLR gene cluster. In all tested intervals, no significant difference in genetic recombination frequency between infected and control plants was observed. Although it has been reported that pathogen exposure can sometimes increase the frequency of recombinant progeny in plants, our findings suggest that meiotic recombination rate in Arabidopsis may be resilient to at least some pathogen attack. Alternative mechanisms are discussed.

The completed genome sequence of the pathogenic ascomycete fungus Fusarium graminearum.

BackgroundAccurate genome assembly and gene model annotation are critical for comparative species and gene functional analyses. Here we present the completed genome sequence and annotation of the reference strain PH-1 of Fusarium graminearum, the causal agent of head scab disease of small grain cereals which threatens global food security. Completion was achieved by combining (a) the BROAD Sanger sequenced draft, with (b) the gene predictions from Munich Information Services for Protein Sequences (MIPS) v3.2, with (c) de novo whole-genome shotgun re-sequencing, (d) re-annotation of the gene models using RNA-seq evidence and Fgenesh, Snap, GeneMark and Augustus prediction algorithms, followed by (e) manual curation.ResultsWe have comprehensively completed the genomic 36,563,796 bp sequence by replacing unknown bases, placing supercontigs within their correct loci, correcting assembly errors, and inserting new sequences which include for the first time complete AT rich sequences such as centromere sequences, subtelomeric regions and the telomeres. Each of the four F. graminearium chromosomes was found to be submetacentric with respect to centromere positioning. The position of a potential neocentromere was also defined. A preferentially higher frequency of genetic recombination was observed at the end of the longer arm of each chromosome. Within the genome 1529 gene models have been modified and 412 new gene models predicted, with a total gene call of 14,164. The re-annotation impacts upon 69 entries held within the Pathogen-Host Interactions database (PHI-base) which stores information on genes for which mutant phenotypes in pathogen-host interactions have been experimentally tested, of which 59 are putative transcription factors, 8 kinases, 1 ATP citrate lyase (ACL1), and 1 syntaxin-like SNARE gene (GzSYN1). Although the completed F. graminearum contains very few transposon sequences, a previously unrecognised and potentially active gypsy-type long-terminal-repeat (LTR) retrotransposon was identified. In addition, each of the sub-telomeres and centromeres contained either a LTR or MarCry-1_FO element. The full content of the proposed ancient chromosome fusion sites has also been revealed and investigated. Regions with high recombination previously noted to be rich in secretome encoding genes were also found to be rich in tRNA sequences. This study has identified 741 F. graminearum species specific genes and provides the first complete genome assembly for a Sordariomycetes species.ConclusionsThis fully completed F. graminearum PH-1 genome and manually curated annotation, available at Ensembl Fungi, provides the optimum resource to perform interspecies comparative analyses and gene function studies.Electronic supplementary materialThe online version of this article (doi:10.1186/s12864-015-1756-1) contains supplementary material, which is available to authorized users.

DROSOPHILA MELANOGASTER MG. ЯК ТЕСТ-ОБ’ЄКТ СКРИНІНГУ КСЕНОБІОТИКІВ НА АКАРИЦИДНУ АКТИВНІСТЬ

<p> </p><p>The development of new effective test systems for screening xenobiotics on biological activity, including its possible harmful influence on the organism is the modern challenge. Test systems of eukaryotic origin are needed for testing of xenobiotics on their gonadotoxical, embryotoxic, teratogenic, and carcinogenic effects. Such testing should include the monitoring of genetic effects of bioactive compounds and the possible mutagenic effect. The <em>Drosophila melanogaster</em> Mg. is very convenient test object as a classical genetics object, for which the genome is fully sequenced. It is convenient and easy to find out the frequency of crossingover in different parts of the genome, and study the key indicators of vitality and ontogenetic adaptation (fertility, longevity, survival under stress conditions, etc.) on <em>Drosophila. </em>The crossover frequency, in our opinion, can be an integral factor for the general state of genotype.so we could use the mark of genetic recombination frequency as one of the indicators of the impact of xenobiotics.</p> <p>In this study we examined the possibility of using <em>Drosophila melanogaster</em> Mg. (Diptera, Drosophilidae) as a test object for screening xenobiotics on acaricidal activity. Sensitivity of flies to acaricides piridaben (<em>Sanmayt</em>) and proparhit (<em>Omayt</em>) was studied by their fitness, longevity, fertility, and frequency of meiotic recombination in the area b-vg second chromosome. It was found that the studied compounds - piridaben and proparhit have significant influence on vitality of <em>D. melanogaster.</em> The presence of these acaricides in the nutrition of flies testifies the harmful influence on their crossingover and reduced fecundity and longevity. We suggested to use the <em>D. melanogaster</em> as a test object for screening xenobiotics on acaricidal activity.</p> <p><em>Keywords: Drosophila, the test object, acaricides, crossingover, fertility, longevity.</em></p><p><strong><a href="http://dx.doi.org/10.7905/bbmspu.v5i1.970">http://dx.doi.org/10.7905/bbmspu.v5i1.970</a></strong><strong> </strong></p>

Molecular Characterization of Southern Rice Blacked‐Streaked Dwarf Virus (SRBSDV) from Vietnam

AbstractSouthern rice black‐streaked dwarf virus (SRBSDV) is a novel putative member of the genus Fijivirus, family Reoviridae. We report here the genomic sequences of a Vietnamese isolate (SRBSDV‐V). The total genome of SRBSDV‐V has 29 115 nucleotides (nt), nine nt shorter than SRBSDV‐GD or ‐HN, but similar in organization to these two Chinese isolates. Nucleotide diversities among SRBSDV isolates were much lower than those among the corresponding ORFs of the available RBSDV isolates and there was a lower purifying selection pressure on SRBSDV than RBSDV, providing first molecular evidence for the view that SRBSDV is of recent origin. In studies of all available SRBSDV sequences, there was no obvious correlation between geographic distances and phylogenetic distribution. A high frequency of genetic recombination was found among both Chinese and Vietnam SRBSDV isolates, suggesting that recombination may play an important role in the molecular variation and evolution of SRBSDV.

Genetic Control of Hotspots

Both chromatin and DNA sequence account for individual differences in the location and frequency of genetic recombination.

Effect of temperature on genetic recombination in barley

The genetic recombination frequencies in barley at 12, 18 and 24°C were compared in two three-point tests with markers on chromosome 6. Two loci, o (orange lemma) and Amyl (α-amylase isoenzyme), 27% recombination apart, were used in both tests. Locus nec2 (necrotic leaf spotting), or a breakpoint in translocation T4-6a, both closely linked with locus o, constituted the third marker point. Significant differences between the estimated recombination frequencies at the three temperatures were not found. If temperature has an effect on the recombination frequency, it is apparently of no importance for mapping gene loci or for barley breeding. However, the recombination frequency may be affected more by temperature when a translocation is involved. A reduced pollen sterility at 12°C was found in the translocation heterozygote; it may be caused by an increased frequency of alternate orientation of the translocation quadrivalent.

Distribution of mating types and genetic diversity induced by sexual recombination in Setosphaeria turcica in northern China

Mature ascocarps and ascospores in the heterothallic ascomycete fungus, Setosphaeria turcica, were successfully produced in Sach’s medium with barley culm as the mating stimulator after four weeks’ coincubation of two opposite mating type isolates at 25°C in darkness. A single isolate could not produce ascospores or ascocarps. The ascocarps were produced on the exposed surface and embedded parts of barley culm or in the upper layer of the medium. The asci linked themselves to ascocarp with their short handles and assembled at the bottom of the ascocarp. Many asci had four to six colorless mature ascospores with one to six septa. But asci with eight ascospores were also found. Using isolate 9914 and isolate 9961 as standard testers for mating types (MAT1 and MAT2), respectively, 94 isolates of S. turcica collected from northern China in 1999, 2003, and 2004 were grouped into three mating types: MAT1 (53 isolates), MAT2 (31 isolates) and MAT12 (10 isolates). The MAT12 isolates, which were first found in China, were compatible with not only MAT1 isolates but also MAT2 isolates. No MAT12 isolates were found in 1999, but 2 MAT12 isolates and 8 MAT12 isolates were found in 2001 and 2003, respectively. The geographic distribution of different mating types was unequal among locations. Generally the frequency of MAT1 was significantly higher than that of MAT2 and MAT12. The unequal distribution of mating types suggested a low frequency of genetic recombination. The pathogenicity of different mating type isolates was tested on the susceptible corn inbred B37 and the results revealed that the disease latency period, disease incidence, lesion area and conidia production were not significantly different among the three mating type groups. However, the pathogenicity of the progeny isolates of isolate 99-12 (MAT2, race 1) and isolate 99-15 (MAT1, race 0) was significantly different from the parent isolates, isolate 99-12 and isolate 99-15, suggesting that sexual recombination could cause significantly virulence variation in S. turcica. Random amplification of polymorphic DNA (RAPD) analysis also revealed high genotype diversity among the progeny isolates, indicating that the sexual recombination could also produce significant genetic variation in the fungal pathogen.

Teaching molecular genetics: chapter 4—positional cloning of genetic disorders

Positional cloning is the approach of choice for the identification of genetic mutations underlying the pathological development of diseases with simple Mendelian inheritance. It consists of different consecutive steps, starting with recruitment of patients and DNA collection, that are critical to the overall process. A genetic analysis of the enrolled patients and their families is performed, based on genetic recombination frequencies generated by meiotic cross-overs and on genome-wide molecular studies, to define a critical DNA region of interest. This analysis culminates in a statistical estimate of the probability that disease features may segregate in the families independently or in association with specific molecular markers located in known regions. In this latter case, a marker can be defined as being linked to the disease manifestations. The genetic markers define an interval that is a function of their recombination frequencies with the disease, in which the disease gene is localised. The identification and characterisation of chromosome abnormalities as translocations, deletions and duplications by classical cytogenetic methods or by the newly developed microarray-based comparative genomic hybridisation (array CGH) technique may define extensions and borders of the genomic regions involved. The step following the definition of a critical genomic region is the identification of candidate genes that is based on the analysis of available databases from genome browsers. Positional cloning culminates in the identification of the causative gene mutation, and the definition of its functional role in the pathogenesis of the disorder, by the use of cell-based or animal-based experiments. More often, positional cloning ends with the generation of mice with homologous mutations reproducing the human clinical phenotype. Altogether, positional cloning has represented a fundamental step in the research on genetic renal disorders, leading to the definition of several disease mechanisms and allowing a proper diagnostic approach to many conditions.

Modelling the stability of Stx lysogens

Shiga-toxin-converting bacteriophages (Stx phages) are temperate phages of Escherichia coli, and can cause severe human disease. The spread of shiga toxins by Stx phages is directly linked to lysogen stability because toxins are only synthesized and released once the lytic cycle is initiated. Lysogens of Stx phages are known to be less stable than those of the related lambda phage; this is often described in terms of a ‘hair-trigger’ molecular switch from lysogeny to lysis. We have developed a mathematical model to examine whether known differences in operator regions and binding affinities between Stx phages and lambda phage can account for the lower stability of Stx lysogens. The Stx phage 933W has only two binding sites in its left operator region (compared to three in phage lambda), but this has a minimal effect on 933W lysogen stability. However, the relatively weak binding affinity between repressor molecules and the second binding site in the right operator is found to significantly reduce the stability of its lysogens, and may account for the hair-trigger nature of the switch. Reduced lysogen stability can lead to increased frequency of genetic recombination in bacterial genomes. The development of the mathematical model has considerable utility in understanding the behaviour and evolution of the molecular switch, with implications for phage-related diseases.

A plant-transformation-competent BIBAC/BAC-based map of rice for functional analysis and genetic engineering of its genomic sequence

Sequencing of the rice genome has provided a platform for functional genomics research of rice and other cereal species. However, multiple approaches are needed to determine the functions of its genes and sequences and to use the genome sequencing results for genetic improvement of cereal crops. Here, we report a plant-transformation-competent, binary bacterial artificial chromosome (BIBAC) and bacterial artificial chromosome (BAC) based map of rice to facilitate these studies. The map was constructed from 20 835 BIBAC and BAC clones, and consisted of 579 overlapping BIBAC/BAC contigs. To facilitate functional analysis of chromosome 8 genomic sequence and cloning of the genes and QTLs mapped to the chromosome, we anchored the chromosomal contigs to the existing rice genetic maps. The chromosomal map consists of 11 contigs, 59 genetic markers, and 36 sequence tagged sites, spanning a total of ca. 38 Mb in physical length. Comparative analysis between the genetic and physical maps of chromosome 8 showed that there are 3 "hot" and 2 "cold" spots of genetic recombination along the chromosomal arms in addition to the "cold spot" in the centromeric region, suggesting that the sequence component contents of a chromosome may affect its local genetic recombination frequencies. Because of its plant transformability, the BIBAC/BAC map could provide a platform for functional analysis of the rice genome sequence and effective use of the sequencing results for gene and QTL cloning and molecular breeding.

High Frequency of Genetic Recombination Is a Common Feature of Primate Lentivirus Replication

Recent studies indicate that human immunodeficiency virus type 1 (HIV-1) recombines at exceedingly high rates, approximately 1 order of magnitude more frequently than simple gammaretroviruses such as murine leukemia virus and spleen necrosis virus. We hypothesize that this high frequency of genetic recombination is a common feature of primate lentiviruses. Alternatively, it is possible that HIV-1 is unique among primate lentiviruses in possessing high recombination rates. Among other primate lentiviruses, only the molecular mechanisms of HIV-2 replication have been extensively studied. There are reported differences between the replication mechanisms of HIV-1 and those of HIV-2, such as preferences for RNA packaging in cis and properties of reverse transcriptase and RNase H activities. These biological disparities could lead to differences in recombination rates between the two viruses. Currently, HIV-1 is the only primate lentivirus in which recombination rates have been measured. To test our hypothesis, we established recombination systems to measure the recombination rates of two other primate lentiviruses, HIV-2 and simian immunodeficiency virus from African green monkeys (SIVagm), in one round of viral replication. We determined that, for markers separated by 588, 288, and 90 bp, HIV-2 recombined at rates of 7.4%, 5.5%, and 2.4%, respectively, whereas SIVagm recombined at rates of 7.8%, 5.6%, and 2.7%, respectively. These high recombination rates are within the same range as the previously measured HIV-1 recombination rates. Taken together, our results indicate that HIV-1, HIV-2, and SIVagm all possess high recombination frequencies; hence, the high recombination potential is most likely a common feature of primate lentivirus replication.

Heterozygosity of Knob-Associated Tandem Repeats and Knob Instability in Mitotic Chromosomes of Zea (Zea mays L. and Z. diploperennis Iltis Doebley)

: Knobs are blocks of heterochromatin present on chromosomes of maize (Zea mays L.) and its relatives that have effects on the frequency of genetic recombination, as well as on chromosome behavior. Knob heterozygosity and instability in six maize inbred lines and one Z. diploperennis Iltis Doebley line were investigated using the fluorescence in situ hybridization (FISH) technique with knob-associated tandem repeats (180 bp and 350 bp (TR-1)) as probes. Signals of seven heterozygous knobs containing 180-bp repeats and of one heterozygous knob containing TR-1 were captured in chromosomes of all materials tested according to the results of FISH, which demonstrates that the 180-bp repeat is the main contributor to knob heterozygosity compared with the TR-1 element. In addition, one target cell with two TR-1 signals on one homolog of chromosome 2L, which was different from the normal cells in the maize inbred line GB57, was observed, suggesting knob duplication and an instability phenomenon in the maize genome. (Managing editor: Wei WANG)

Reduced chromosome association between the short arms of 5H homologues in Hordeum vulgare L. at metaphase I

Rod or ring bivalents at metaphase I (MI) reflect whether crossing-over has occurred on one or both pairs of chromosome arms, respectively, during meiotic prophase (pachytene). In cultivated barley (Hordeum vulgare) the highest frequencies of rod bivalents were found for satellite chromosomes (either 5H or 6H). To distinguish between chromosome 5H and 6H associations, a recombinant line derived from an H. vulgare × H. bulbosum hybrid with a hemizygous introgression from H. bulbosum on the short arm of chromosome 6H was probed with 45S rDNA and pSc119.2 using fluorescent in situ hybridization and compared with non-recombinant H. vulgare. The frequencies of rod bivalents, caused by failure of the short arms of 5H chromosomes (5HS) to associate during meiosis in the recombinant line, were similar to the frequencies of rod bivalents involving the satellite chromosomes in H. vulgare. Hence, rod bivalent formation in H. vulgare satellite chromosomes is probably based on reduced association of 5HS, corresponding to a low frequency of genetic recombination. 5HS is, therefore, more likely to show linkage disequilibrium than the other chromosome arms.

Nonrandom dimerization of murine leukemia virus genomic RNAs.

Retroviral genomes consist of two unspliced RNAs linked noncovalently in a dimer. Although these two RNAs are generally identical, two different RNAs can be copackaged when virions are produced by coinfected cells. It has been assumed, but not tested, that copackaging results from random RNA associations in the cytoplasm to yield encapsidated RNA homodimers and heterodimers in Hardy-Weinberg proportions. Here, virion RNA homo- and heterodimerization were examined for Moloney murine leukemia virus (MLV) using nondenaturing Northern blotting and a novel RNA dimer capture assay. The results demonstrated that coexpressed MLV RNAs preferentially self-associated, even when RNAs were identical in known packaging and dimerization sequences or when they differed overall by less than 0.1%. In contrast, HIV-1 RNAs formed homo- and heterodimers in random proportions. We speculate that these species-specific differences in RNA dimer partner selection may at least partially explain the higher frequency of genetic recombination observed for human immunodeficiency virus type 1 than for MLV.

Involvement of a replicative DNA helicase of bacteriophage T4 in DNA recombination.

Bacteriophage T4 gene 41 encodes a replicative DNA helicase that is a subunit of the primosome which is essential for lagging-strand DNA synthesis. A mutation, rrh, was generated and selected in the helicase gene on the basis of limited DNA replication that ceases early. The survival of ultraviolet-irradiated phage and the frequency of genetic recombination are reduced by rrh. In addition, rrh diminishes the production of concatemeric DNA. These results strongly suggest that the gene 41 replicative helicase participates in DNA recombination.

Genetic recombination. Patterns in the genome.

Recent analyses suggest that the GC content of gene sequences is related to the local frequency of genetic recombination; this may reflect the greater efficacy of natural selection when recombination is frequent.