Leu Codon Research Articles

Characterization of the Complete Chloroplast Genome of the Dragonhead Herb, Dracocephalumheterophyllum (Lamiaceae), and Comparative Analyses with Related Species

Dracocephalum heterophyllum (Lamiaceae: tribe Mentheae) is an annual aromatic herb native to East Asia with a long record of human uses, including medicinal, alimentary, and ornamental values. However, no information is available about its molecular biology, and no genomic study has been performed on D. heterophyllum. Here, we report the complete chloroplast (cp) genome of D. heterophyllum and a series of comparative genomic analyses between this and closely related species of Lamiaceae. Results indicated that the cp genome has a typical circular structure of 150,869 bp in length, consisting of a long single-copy (LSC) region with 82,410 bp, a short single-copy (SSC) region with 17,098 bp, and two inverted repeat (IR) regions of 51,350 bp. A total of 133 genes were identified, including 37 tRNA genes, 8 rRNA genes and 88 protein-coding genes, with a GC content of 37.8%. The gene content, organization, and GC values observed here were similar to those of other Dracocephalum species. We detected 99 different simple sequence repeat loci, and the codon usage analysis revealed a preferential use of the Leu codon with an A/U ending. Comparative analysis of cp genome sequences revealed five highly variable regions with remarkably higher Pi values (>0.03). The mean Ka/Ks between D. heterophyllum and three other Dracocephalum species ranged from 0.01079 (psbB) to 1.0497 (ycf2). Two cp genes, ycf2 and rps11, were proven to have high ratios of Ka/Ks, implying that cp genes may had undergone positive selection in the evolutionary history. We performed multiple sequence alignments using the cp genome of 22 species and constructed maximum likelihood (ML) and Bayesian trees, and found that D. heterophyllum were more closely related to D. moldavica and D. palmatum. In addition, the phylogenetic relationships between Dracocephalum and other members of Lamiaceae were consistent with previous results. These results are valuable for further formulating effective strategies of conservation and management for species in Dracocephalum, as well as providing a foundation for future research on the genetic resources of Dracocephalum.

Complete chloroplast genome of Oryza sativa L. (B810s) and its phylogenetic analysis

The complete chloroplast (cp) genome of Oryza sativa L.(B810S) was 134546 bp in length in the study, which contains 149 genes including 99 coding protein genes, 41 transfer RNA genes, 8 ribosomal RNA genes and 1 non-coding region by gene annotation. A total of 20879 amino acids were encoded by this cp genome, TTT (Phe) and TTG (Leu) codon were the most frequent amino acids, whereas the ACC (Thr), GCC (Ala), CTC (Leu), and AAC (Asn) codon were the least frequent ones. The content of the four bases on the cp genome were 30.6% for A, 30.4% for T, 19.4% for C and 19.6% for G, respectively. Obviously, the A+T (61.0%) content is more higher than G+C (39.0%). The gene order and content are the same as those of previously reported cp genome of Rice. Phylogenetic analysis was implemented based on concatenated amino acid sequences of 99 protein-coding genes using Neighbor-Joining method (NJ) method. Therefore, the complete B810S cp genome provides interesting insights and valuable information that can be used to identify related species and reconstruct its phylogeny. Bangladesh J. Bot. 50(3): 895-901, 2021 (September) Special

Improving Phylogenetic Signals of Mitochondrial Genes Using a New Method of Codon Degeneration.

Recovering deep phylogeny is challenging with animal mitochondrial genes because of their rapid evolution. Codon degeneration decreases the phylogenetic noise and bias by aiming to achieve two objectives: (1) alleviate the bias associated with nucleotide composition, which may lead to homoplasy and long-branch attraction, and (2) reduce differences in the phylogenetic results between nucleotide-based and amino acid (AA)-based analyses. The discrepancy between nucleotide-based analysis and AA-based analysis is partially caused by some synonymous codons that differ more from each other at the nucleotide level than from some nonsynonymous codons, e.g., Leu codon TTR in the standard genetic code is more similar to Phe codon TTY than to synonymous CTN codons. Thus, nucleotide similarity conflicts with AA similarity. There are many such examples involving other codon families in various mitochondrial genetic codes. Proper codon degeneration will make synonymous codons more similar to each other at the nucleotide level than they are to nonsynonymous codons. Here, I illustrate a “principled” codon degeneration method that achieves these objectives. The method was applied to resolving the mammalian basal lineage and phylogenetic position of rheas among ratites. The codon degeneration method was implemented in the user-friendly and freely available DAMBE software for all known genetic codes (genetic codes 1 to 33).

Characterization of the Complete Mitochondrial Genome of Fischoederius elongatus Derived from Cows in Shanghai, China.

A study was conducted to reveal the characterization of the complete mitochondrial genome of Fischoederius elongatus derived from cows in Shanghai, China. Results indicated that the complete mt genome of F. elongatus was 14,288 bp and contained 12 protein-coding genes (cox1-3, nad1-6, nad4L, atp6, and cytb), 22 transfer RNA genes, and two ribosomal RNA genes (l-rRNA and s-rRNA). The overall A + T content of the mt genome was 63.83%, and the nucleotide composition was A (19.83%), C (9.75%), G (26.43%), and T (44.00%). A total of 3284 amino acids were encoded by current F. elongatus isolate mt genome, TTT (Phe) (9.84%) and TTG (Leu) (7.73%) codon were the most frequent amino acids, whereas the ACC (Thr) (0.06%), GCC (Ala) (0.09%), CTC (Leu) (0.09%), and AAC (Asn) (0.09%) codon were the least frequent ones. At the third codon position of F. elongatus mt protein genes, T (50.82%) was observed most frequently and C (5.85%) was the least one. The current results can contribute to epidemiology diagnosis, molecular identification, taxonomy, genetic, and drug development researches about this parasite species in cattle.

The complete chloroplast genome sequence of yellow mustard (Sinapis alba L.) and its phylogenetic relationship to other Brassicaceae species

As a member of the large Brassicaceae family, yellow mustard (Sinapis alba L.) has been used as an important gene pool for the genetic improvement of cash crops in Brassicaceae. Understanding the phylogenetic relationship between Sinapis alba (S. alba) and other Brassicaceae crops can provide guidance on the introgression of its favorable alleles into related species. The chloroplast (cp) genome is an ideal model for assessing genome evolution and the phylogenetic relationships of complex angiosperm families. Herein, we de novo assembled the complete cp genome of S. alba by integrating the PacBio and Illumina sequencing platforms. A 153,760 bp quadripartite cycle without any gap was obtained, including a pair of inverted repeats (IRa and IRb) of 26,221 bp, separated by a large single copy (LSC) region of 83,506 bp and a small single copy (SSC) region of 17,821 bp. A total of 78 protein-coding genes, 30 tRNA genes, and four rRNA genes were identified in this cp genome, as were 89 simple sequence repeat (SSR) loci of 18 types. The codon usage analysis revealed a preferential use of the Leu codon with the A/U ending. The phylogenetic analysis using 82 Brassicaceae species demonstrated that S. alba had a close relationship with important Brassica and Raphanus species; moreover, it likely originated from a separate evolutionary pathway compared with the congeneric Sinapis arvensis. The synonymous (Ks) and non-synonymous (Ks) substitution rate analysis showed that genes encoding “Subunits of cytochrome b/f complex” were under the lowest purifying selection pressure, whereas those associated with “Maturase”, “Subunit of acetyl-CoA”, and “Subunits of NADH-dehydrogenase” underwent relatively higher purifying selection pressures. Our results provide valuable information for fully utilizing the S. alba cp genome as a potential genetic resource for the genetic improvement of Brassica and Raphanus species.

Abstract 2573: DNA damaging drugs stimulate tRNA cleavage through SLFN11 to inhibit protein synthesis and induce cell death

Abstract DNA damaging agents (DDAs) based chemotherapies are the first and still the most widely used anticancer treatments. However, quickly developed drug resistance of cancer cells often impedes their efficacies. Two independent transcriptome analyses in vast collections of cancer cell lines discovered that human SLFN11 is the only gene whose expression significantly sensitizes cancer cells to DNA damaging agents (DDAs) including Top1 and Top2 inhibitors, DNA alkylating agents, and DNA synthesis inhibitors. But the mechanism was unclear. We showed previously that SLFN11 inhibits HIV viral proteins syntheses by modulating tRNAs abundances and exploiting the biased codon usage of the virus. We hereby report that upon DDAs treatment, SLFN11 preferentially inhibits translation of genes involved in DNA damage response and repair, including ATR and ATM, based on their distinct codon usages, while the early DNA damage response signaling remains unaffected. Type II tRNAs, which include all serine and leucine tRNAs, are cleaved in a SLFN11-dependent manner in response to DDAs. The syntheses of proteins encoded by genes with high TTA (Leu) codon usage such as ATR display utmost susceptibility to SLFN11 expression due to the extreme low abundance of cognate tRNA-Leu-TAA. Specific inhibition of tRNA-Leu-TAA expression with special antisense oligonucleotides (Gapmers) suppressed ATR protein expression and restored DDA sensitivity of SLFN11-deficient cancer cells. Our study uncovered a novel mechanism of DNA damage response regulation mediated by SLFN11-dependent tRNA cleavage and codon-specific translational inhibition. Further investigation will elucidate the roles of SLFN11 in DNA damage-related tumorigenesis. More importantly, bioinformatics analysis revealed high frequency of codon TTA (Leu) usage in most genes involved in DNA damage response signaling and repair mechanisms including Homology Directed Repair (HDR), Nonhomologous End-Joining (NHEJ), and Mismatch Mediated Repair (MMR). Instead of targeting single gene product, novel therapies based on the modulation of tRNA-Leu-TAA expression would effect by inhibiting the protein synthesis of a wide spectrum of genes involved in DNA damage response and repair, thus preventing the development of drug resistance by cancer cells. Citation Format: Manqing Li, Elaine Kao, Dane Malone, Xia Gao, Jean Y. Wang, Michael David. DNA damaging drugs stimulate tRNA cleavage through SLFN11 to inhibit protein synthesis and induce cell death [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 2573.

Human intrinsic immunity factor SLFN11 promotes cell death upon DNA damages through cleavage of distinct type II tRNAs and inhibition of proteins syntheses related to DNA damage responses and repairs

Abstract Previously we showed that as an intrinsic immunity factor, human SLFN11 inhibits HIV viral proteins syntheses by modulating tRNAs abundances and exploiting the biased codon usage of the virus. Transcriptome analysis in vast collection of cancer cell lines revealed that SLFN11 is the only gene whose expression significantly sensitizes tumor cells to DNA damaging agents (DDAs). We hereby report that SLFN11 preferentially inhibits translation of genes involved in DNA damage response and repair, including ATR and ATM, upon DDAs treatment based on their distinct codon usages without disrupting early DNA damage response signaling. Type II tRNAs, which include all serine and leucine tRNAs, are cleaved in a SLFN11-dependent manner in response to DDAs. Genes with high TTA (Leu) codon usage such as ATR display utmost susceptibility to translational suppression by SLFN11 due to the extreme low abundance of cognate tRNA-Leu-TAA. Specific attenuation of tRNA-Leu-TAA with antisense oligonucleotides (ASO) sufficed to ablate ATR protein expression and restore DDA sensitivity of SLFN11-deficient tumor cells. Our study uncovered a novel mechanism of DNA damage response regulation mediated by SLFN11-dependent tRNA cleavage and codon-specific translational inhibition. The immunomodulation of SLFN11 expression and activation can be a potent novel approach to overcome the wide-spreading resistance of tumor cells to chemo-/radio-therapies.

DNA damage-induced cell death relies on SLFN11-dependent cleavage of distinct type II tRNAs.

Transcriptome analysis revealed a strong positive correlation between human SLFN11 expression and the sensitivity of tumor cells to DNA damaging agents (DDAs). We show here that SLFN11 preferentially inhibits translation of ATR or ATM upon DDAs treatment based on distinct codon usage without disrupting early DNA damage response signaling. Type II tRNAs, which include all serine and leucine tRNAs, are cleaved in a SLFN11-dependent manner in response to DDAs. mRNAs encoded by genes with high TTA (Leu) codon usage such as ATR display utmost susceptibility to translational suppression by SLFN11. Specific attenuation of tRNA-Leu-TAA sufficed to ablate ATR protein expression and restore DDA sensitivity of SLFN11-deficient cells. Our study uncovered a novel mechanism of codon-specific translational inhibition via SLFN11-dependent tRNA cleavage in the DNA damage response, and supports the notion that SLFN11-deficient tumor cells can be resensitized to DDAs by targeting ATR or tRNA-Leu-TAA.

Forced Ambiguity of the Leucine Codons for Multiple-Site-Specific Incorporation of a Noncanonical Amino Acid.

Multiple-site-specific incorporation of a noncanonical amino acid into a recombinant protein would be a very useful technique to generate multiple chemical handles for bioconjugation and multivalent binding sites for the enhanced interaction. Previously combination of a mutant yeast phenylalanyl-tRNA synthetase variant and the yeast phenylalanyl-tRNA containing the AAA anticodon was used to incorporate a noncanonical amino acid into multiple UUU phenylalanine (Phe) codons in a site-specific manner. However, due to the less selective codon recognition of the AAA anticodon, there was significant misincorporation of a noncanonical amino acid into unwanted UUC Phe codons. To enhance codon selectivity, we explored degenerate leucine (Leu) codons instead of Phe degenerate codons. Combined use of the mutant yeast phenylalanyl-tRNA containing the CAA anticodon and the yPheRS_naph variant allowed incorporation of a phenylalanine analog, 2-naphthylalanine, into murine dihydrofolate reductase in response to multiple UUG Leu codons, but not to other Leu codon sites. Despite the moderate UUG codon occupancy by 2-naphthylalaine, these results successfully demonstrated that the concept of forced ambiguity of the genetic code can be achieved for the Leu codons, available for multiple-site-specific incorporation.

The Kinetics of Ribosomal Peptidyl Transfer Revisited

The speed of protein synthesis determines the growth rate of bacteria. Current biochemical estimates of the rate of protein elongation are small and incompatible with the rate of protein elongation in the living cell. With a cell-free system for protein synthesis, optimized for speed and accuracy, we have estimated the rate of peptidyl transfer from a peptidyl-tRNA in P site to a cognate aminoacyl-tRNA in A site at various temperatures. We have found these rates to be much larger than previously measured and fully compatible with the speed of protein elongation for E. coli cells growing in rich medium. We have found large activation enthalpy and small activation entropy for peptidyl transfer, similar to experimental estimates of these parameters for A site analogs of aminoacyl-tRNA. Our work has opened a useful kinetic window for biochemical studies of protein synthesis, bridging the gap between in vitro and in vivo data on ribosome function.

Multiple factors contribute to keeping levels of the symbiosis regulator RscS low

Increased activity alleles (rscS1 and rscS2) of the symbiosis regulator RscS induced both syp transcription and biofilm formation in Vibrio fischeri. Neither allele encodes a protein variant; instead they carry mutations near the putative ribosome-binding site and, in the case of rscS1, an additional silent mutation at codon Leu25. In this study, we found that endogenous levels of RscS are very low under the culture conditions examined and that the increased activity alleles dramatically increased the levels of protein. Of the two mutations present in rscS1, the Leu25 mutation, which replaces a rare with a more common Leu codon, appeared to make the greater contribution to increased activity. Our results suggest that RscS levels are maintained at low levels in the cell by the presence of a weak promoter, possible inefficient ribosome binding and the presence of rare codons in the 5' end of the gene. Restriction of RscS levels may be important to prevent spurious signalling by this sensor kinase in the absence of a squid host.

Evolutionary Potential of (β/α)8-Barrels: Stepwise Evolution of a “New” Reaction in the Enolase Superfamily

The molecular details of the processes involved in divergent evolution of "new" enzymatic functions are ill-defined. Likely starting points are either a progenitor promiscuous for the new reaction or a progenitor capable of catalyzing the new reaction following a single substitution that results from a single base change. However, the molecular (sequence) pathway by which the selective advantage provided by this protein can be improved and ultimately optimized is unclear. In the mechanistically diverse enolase superfamily, we discovered that a monofunctional progenitor could acquire the ability to catalyze a "new" reaction by a single base change: the D297G mutant of the monofunctional l-Ala-d/l-Glu epimerase (AEE) from Escherichia coli catalyzed a low level of the o-succinylbenzoate synthase (OSBS) reaction as well as a reduced level of the AEE reaction [Schmidt, D. M. Z., Mundorff, E. C., Dojka, M., Bermudez, E., Ness, J. E., Govindarajan, S., Babbitt, P. C., Minshull, J., and Gerlt, J. A. (2003) Biochemistry 42, 8387-8393]. We then discovered that the selective advantage and OSBS activity of the D297G mutant are both enhanced by the I19F substitution [Vick, J. E., Schmidt, D. M. Z., and Gerlt, J. A. (2005) Biochemistry 44, 11722-11729]. Both the D297G and I19F substitutions are positioned to alter the substrate specificity so that the substrate for the OSBS reaction is more productively positioned vis a vis the active site catalytic groups. We now report that both the selective advantage and OSBS activity of the D297G/I19F double mutant are enhanced by the R24C (one base change from the wild type Arg codon), R24W (two base changes from the wild type Arg codon and one base change from the R24C codon), and L277W (one base change from the wild type Leu codon) substitutions. The effects of the R24C and L277W mutants are "additive" in the D297G/I19F/R24C/L277W mutant. The greatest selective advantage and OSBS activity are associated with the D297G/I19F/R24W mutant. These "new" substitutions that enhance both the selective advantage and kinetic constants are positioned in the active site where they can alter the specificity, highlighting that the evolution of the "new" OSBS function can be accomplished by changes in substrate specificity.

Complete mitochondrial genome of a Chinese scorpionMesobuthus martensii(Chelicerata, Scorpiones, Buthidae)

The complete mitochondrial genome (15,034 bp) of a Chinese scorpion Mesobuthus martensii (Buthidae) was sequenced and characterized in detail. The genome contains 13 protein-coding genes, 21 transfer RNA genes, two ribosomal RNA genes and a large non-coding region ( = CR). Its gene arrangement pattern is identical to that of Limulus polyphemus (Chelicerata, Xiphosura), with the exceptions of the tRNA(Glu)-tRNA(Ile)-tRNA(Met) (Q-I-M) arrangement and tRNA(Asp)-loss. Additional interesting features are found and discussed: high frequency of Leu(UUG) codon use, low A+T content of the genome (66.75%), and six repeat units (five 60-nt-long and one 58-nt-long repeats) in the 998-nt CR. Bayesian analysis based on amino acid sequences of the 12 proteincoding genes (excluding ATP8) reveals that the family Buthidae (Order Scorpiones) and the class Arachnida form strong monophyletic groups within Chelicerata, respectively. It indicated that the scorpions are the most ancestral arachnids.

A site-directed integration system for the nonuniversal CUGSer codon usage species Pichia farinosa by electroporation

Halotolerant yeast, Pichia farinosa, is a valuable yeast strain in fermentation industry because it produces high yield of glycerol and xylitol, and can tolerate both contamination and high-density growth during fermentation. However, the lack of genetic manipulation tools makes it less popular as a gene engineering strain. Expression systems commonly used in other yeast systems, such as Saccharomyces cerevisiae and Pichia pastoris cannot be used in P. farinosa because it translates universal Leu codon CUG as Ser. Here we reported a modified expression vector and a transformation system with enhanced efficiency in P. farinosa. The results showed that cells of OD(600 )0.8-1.0 with DTT treatment can obtain high transformation efficiency. The optimized electroporation condition was 900 V, 25 microF, and 200 Omega. The DNA concentration did not influence the transformation. Our system provides the potential not only for applying P. farinosa as an industrial strain of gene engineering, but also for studying gene function in its native host.

Over Expression of a tRNA Leu Isoacceptor Changes Charging Pattern of Leucine tRNAs and Reveals New Codon Reading

During mRNA translation, synonymous codons for one amino acid are often read by different isoaccepting tRNAs. The theory of selective tRNA charging predicts greatly varying percentages of aminoacylation among isoacceptors in cells starved for their common amino acid. It also predicts major changes in tRNA charging patterns upon concentration changes of single isoacceptors, which suggests a novel type of translational control of gene expression. We therefore tested the theory by measuring with Northern blots the charging of Leu-tRNAs in Escherichia coli under Leu limitation in response to over expression of tRNA(GAG)(Leu). As predicted, the charged level of tRNA(GAG)(Leu) increased and the charged levels of four other Leu isoacceptors decreased or remained unchanged, but the charged level of tRNA(UAG)(Leu) increased unexpectedly. To remove this apparent inconsistency between theory and experiment we postulated a previously unknown common codon for tRNA(GAG)(Leu) and tRNA(UAG)(Leu). Subsequently, we demonstrated that the tRNA(GAG)(Leu) codon CUU is, in fact, read also by tRNA(UAG)(Leu), due to a uridine-5-oxyacetic acid modification.

Association of a Single Codon Deletion in Bone Morphogenetic Protein 15 Gene with Prolificacy in Small Tail Han Sheep

Small Tail Han Sheep has significant characteristics of high prolificacy and non-seasonal ovulatory activity and is an excellent local sheep breed in P. R. China. Recently a novel member of the transforming growth factor β (TGFβ) superfamily termed bone morphogenetic protein 15 (BMP15) was shown to be specifically expressed in oocytes and to be essential for female fertility. Therefore, BMP15 is a candidate gene for reproductive performance of Small Tail Han Sheep. The whole genomic nucleotide sequence of BMP15 gene in Small Tail Han Sheep was searched for polymorphisms by PCR-SSCP and direct sequencing, and only one polymorphism was found. The polymorphism was a result of a 3 base pair deletion, which eliminated a single Leu codon (CTT). The allelic frequencies for A (without deletion) and B (with a codon deletion) are 0.73 and 0.27 respectively. The effects of BMP15 genotype on litter size were evaluated using the least squares model. This indicated that there was a significant association between litter size of Small Tail Han Sheep and a deletion in BMP15 gene (p=0.02<0.05). Small Tail Han Sheep ewes with AA and AB genotype produce on average 0.5 and 0.3 more lambs per litter than those ewes with BB genotype.

Molecular Mechanism of Type I Congenital Heparin Cofactor (HC) II Deficiency Caused by a Missense Mutation at Reactive P2 Site: HC II Tokushima

We found a 66-year-old Japanese patient with type I congenital heparin cofactor (HC) II deficiency manifesting multiple atherosclerotic lesions. To investigate the molecular pathogenesis of our patient, we performed sequencing analysis and expressed recombinant human wild-type and mutant HC II molecules in COS-1 and CHO-K1 cells. Sequencing analysis following amplification of each of all 5 exons and its flanking region showed a single C to T transition at nucleotide position 12,854 in exon 5, which changed a Pro443 codon (CCG) to Leu codon (CTG). Because this mutation generates a new Bhv I site, the Bbv I digestion pattern of the PCR-amplified exon 5 fragments from each family member was analyzed. In all cases, the patterns were consistent with the activities and antigen levels of plasma HC I1 in those members. Transient transfection, metabolic labeling and pulse-chase experiments followed by immunoprecipitation analysis showed that the recombinant mutant HC II molecules were secreted from COS-1 cells in reduced amounts compared with the wild-type, and that an enhanced intracellular association of the mutant molecules with a chaperone, GRP78/BiP, was observed in CHO-K1 cells. Northern blot analysis indicated that the mutant HC I1 mRNA was transcribed at a similar level as that of wild-type. Immunohistochemical staining of the transfected cells revealed that COS-1 cells expressing the mutant HC II molecules were stained mainly in the perinuclear area. We conclude that the impaired secretion of the mutant HC II molecules, due to intracellular degradation, is the molecular pathogenesis of type I congenital HC II deficiency caused by a Pro443 to Leu mutation at reactive P2 site.

Regulation of the Escherichia coli tna operon: nascent leader peptide control at the tnaC stop codon.

Expression of the tryptophanase (tna) operon of Escherichia coli is regulated by catabolite repression and by tryptophan-induced transcription antitermination at Rho-dependent termination sites in the leader region of the operon. Tryptophan induction is dependent on translation of a short leader peptide coding region, tnaC, that contains a single, crucial tryptophan codon. Recent studies suggest that during induction, the TnaC leader peptide acts in cis on the translating ribosome to inhibit its release at the tnaC stop codon. In the present study we use a tnaC-UGA-'lacZ construct lacking the tnaC-tnaA spacer region to analyze the effect of TnaC synthesis on the behavior of the ribosome that translates tnaC. The tnaC-UGA-'lacZ construct is not expressed significantly in the presence or absence of inducer. However, it is expressed in the presence of UGA suppressors, or when the structural gene for polypeptide release factor 3 is disrupted, or when wild-type tRNATrP is overproduced. In each situation, tnaC-UGA-'lacZ expression is reduced appreciably by the presence of inducing levels of tryptophan. Replacing the tnaC UGA stop codon with a sense codon allows considerable expression, which is also reduced, although to a lesser extent, by the addition of tryptophan. Inhibition by tryptophan is not observed when Trp codon 12 of tnaC is changed to a Leu codon. Overexpression of tnaC in trans from a multicopy plasmid prevents inhibition of expression by tryptophan. These results support the hypothesis that the TnaC leader peptide acts in cis to alter the behavior of the translating ribosome.

RNA editing of the tobacco mitochondrial orf38/220.

An alloplasmic, male-sterile tobacco line obtained by sexual hybridization, resulting in introgression of the Nicotiana bigelovii cytoplasm into the Nicotiana tabacum species, is characterized by male-sterile flowers that have filaments with flat-fringed ends instead of normal stamens, and a split corolla (Burk, 1960; Chaplin, 1964). A mitochondrially encoded chimeric gene, orf38/220, corresponding to a fusion of an orf coding for 38 amino acids and the sequence coding for the carboxy-terminal 220 amino acids of the ATPase a subunit (atpA), was found to be associated with the cytoplas'mic male-sterile phenotype (P. Bergman, W. Kofer, G. Hakansson, and K. Glimelius, unpublished data). Our studies show that this chimeric gene is a target for posttranscriptional editing in which seven cytosines are replaced by uridines in the mitochondrial mRNA (Table I). At the protein level, three Pro codons (codons 180,221, and 246) were changed to Leu codons, as previously described for sugar beet atpA mRNAs (Senda et al., 1993). We have identified four additional sites of editing, codons 96 and 115 in which the Pro codon is edited to Ser, codon 142 in which Ser becomes a Leu codon, and codon 155 in which the Leu codon is transformed into a Phe codon. Eight independent clones were sequenced; three of them were fully edited, whereas in five of the clones codon 115 was unedited. Furthermore, in one of these clones codon

Effect of a rare leucine codon, TTA, on expression of a foreign gene in Streptomyces lividans

Streptomyces are bacteria with a very high chromosomal G + C composition (> 70 mol%) and extremely biased codon usage. In order to investigate the relationship between codon usage and gene expression in Streptomyces, we used ssi ( Streptomyces subtilisin inhibitor) as a reporter gene and monitored its secretory expression in S. lividans. In consequence of alteration of the native codons of Leu, Lys and Ser of ssi to minor ones by site-directed mutagenesis, i.e., Leu 79-Leu 80: CTG-CTC to TTA-TTA, Lys 89: AAG to AAA, Ser 108-Ser 109: TCG-AGC to TCT-TCT, respectively, the production of SSI was reduced remarkably in the case of TTA codons, while it was slightly increased in the case of AAA and almost the same in TCT codons. This conspicuous decrease found for Leu codon replacement was probably due to the low availability of intracellular tRNA Leu (UUA), a product of bldA which has been reported to be expressed only during the late stage of growth.