Comparative Sequence Analysis Research Articles

Flavobacterium frigoritolerans sp. nov. and Flavobacterium shii sp. nov., isolated from glaciers on the Tibetan Plateau.

The genus Flavobacterium belongs to the family Flavobacteriaceae and its members are widely distributed in the environment. Taxonomic descriptions of strains LS1R47T and LS1R49T isolated from the Laigu glacier on the Tibetan Plateau, China, are presented in this study. Both strains were psychrotolerant, Gram-stain-negative, aerobic and rod-shaped. The comparative analysis of 16S rRNA gene sequences showed that strain LS1R47T was closest to Flavobacterium bizetiae CIP 105534T (98.90 %) and strain LS1R49T was closest to Flavobacterium collinsii 983-08T (98.73 %). The 16S rRNA gene sequence similarity, average nucleotide identity (ANI) and digital DNA-DNA hybridization (dDDH) values between the two novel isolates were 99.4, 86.0 and 30.9 %, respectively. The ANI and dDDH values between strains LS1R47T and LS1R49T and their closely relatives were below 87.6 and 33.3 %, respectively. Phylogenomic analysis showed that the two strains cluster together with Flavobacterium hydatis ATCC 29551T. Both strains contained MK-6 as sole quinone, phosphatidylethanolamine as the principal polar lipid, and summed feature 3 (iso-C15 : 0 2-OH and/or C16 : 1 ω7c), iso-C15 : 0 3-OH, C15 : 0 3-OH and iso-C17 : 0 3-OH as the main fatty acids. These results indicated that strains LS1R47T and LS1R49T represented two novel species within the genus Flavobacterium. Therefore, we propose two novel species, Flavobacterium frigoritolerans sp. nov. (LS1R47T=CGMCC 1.11577T=NBRC 113654T) and Flavobacterium shii sp. nov. (LS1R49T=CGMCC 1.11581T=NBRC 113652T).

Comparative analysis of SilA-laccase mediated degradation of ciprofloxacin, norfloxacin and ofloxacin and interpretation of the possible catalytic mechanism

Fluoroquinolones (FQs) are the most commonly used antimicrobial drugs and regardless of their advantages in the healthcare sector, the pollution of these antimicrobial drugs in the environment has big concerns about human and environmental health. The presence of these antibiotic drugs even at the lowest concentrations in the environment has resulted in the emergence and spread of antibiotic resistance. Hence, it is necessary to remediate these pollutants from the environment. Previously alkaline laccase (SilA) from Streptomyces ipomoeae has been demonstrated to show degrading potentials against two of the FQs, Ciprofloxacin (CIP) and Norfloxacin (NOR); however, the molecular mechanism was not elucidated in detail. In this study, we have analyzed the possible molecular catalytic mechanism of FQ degrading SilA-laccase for the degradation of the FQs, CIP, NOR and Ofloxacin (OFL) using three-dimensional protein structure modeling, molecular docking and molecular dynamic (MD) studies. The comparative protein sequence analysis revealed the presence of tetrapeptide conserved catalytic motif, His102-X-His104-Gly105. After evaluating the active site of the enzyme in depth using CDD, COACH and S-site tools, we have identified the catalytic triad composed of three conserved amino acid residues, His102, Val103 and Tyr108 with which ligands interacted during the catalysis process. By analyzing the MD trajectories, it is revealed that the highest degradation potential of SilA is for CIP followed by NOR and OFL. Ultimately, this study provides the possible comparative catalytic mechanism for the degradation of CIP, NOR and OFL by the SilA enzyme. Communicated by Ramaswamy H. Sarma

The nuclear effector MoHTR3 of Magnaporthe oryzae modulates host defence signalling in the biotrophic stage of rice infection.

Fungal effectors play a pivotal role in suppressing the host defence system, and their evolution is highly dynamic. By comparative sequence analysis of plant-pathogenic fungi and Magnaporthe oryzae, we identified the small secreted C2 H2 zinc finger protein MoHTR3. MoHTR3 exhibited high conservation in M. oryzae strains but low conservation among other plant-pathogenic fungi, suggesting an emerging evolutionary selection process. MoHTR3 is exclusively expressed in the biotrophic stage of fungal invasion, and the encoded protein localizes to the biotrophic interfacial complex (BIC) and the host cell nucleus. The signal peptide crucial for MoHTR3' secretion to the BIC and the protein section required for its translocation to the nucleus were both identified by a functional protein domain study. The host-nuclear localization of MoHTR3 suggests a function as a transcriptional modulator of host defence gene induction. After ΔMohtr3 infection, the expression of jasmonic acid- and ethylene-associated genes was diminished in rice, in contrast to when the MoHTR3-overexpressing strain (MoHTR3ox) was applied. The transcript levels of salicylic acid- and defence-related genes were also affected after ΔMohtr3 and MoHTR3ox application. In pathogenicity assays, ΔMohtr3 was indistinguishable from the wild type. However, MoHTR3ox-infected plants showed diminished lesion formation and hydrogen peroxide accumulation, accompanied by a decrease in susceptibility, suggesting that the MoHTR3-induced manipulation of host cells affects host-pathogen interaction. MoHTR3 emphasizes the role of the host nucleus as a critical target for the pathogen-driven manipulation of host defence mechanisms and underscores the ongoing evolution of rice blast's arms race.

Information Theory Opens New Dimensions in Experimental Studies of Animal Behaviour and Communication

Over the last 40-50 years, ethology has become increasingly quantitative and computational. However, when analysing animal behavioural sequences, researchers often need help finding an adequate model to assess certain characteristics of these sequences while using a relatively small number of parameters. In this review, I demonstrate that the information theory approaches based on Shannon entropy and Kolmogorov complexity can furnish effective tools to analyse and compare animal natural behaviours. In addition to a comparative analysis of stereotypic behavioural sequences, information theory can provide ideas for particular experiments on sophisticated animal communications. In particular, it has made it possible to discover the existence of a developed symbolic "language" in leader-scouting ant species based on the ability of these ants to transfer abstract information about remote events.

Fine mapping and causal gene identification of a novel QTL for early flowering by QTL-seq, Target-seq and RNA-seq in spring oilseed rape

A novel quantitative trait locus for early flowering in spring oilseed rape, BnaC08cqDTF, was mapped to an 86-kb region on chromosome C08, and its causal gene, CRY2, was uncovered. Days to flowering is a very important agronomic and adaptive trait of Brassica napus oilseed rape (AACC, 2n = 38). We previously identified BnaC08cqDTF as a novel candidate quantitative trait locus (QTL) for early flowering in spring oilseed rape. Here, we present fine mapping of the locus and a study of its causal gene. Initial mapping was performed by QTL sequencing of DNA pools of BC3F2 plants with extreme flowering times derived from crosses between the spring-type cv. No. 4512 (early flowering) and cv. No. 5246 (late flowering), along with fine mapping by target sequencing of the BC3F2 and BC4F2populations.Fine mapping narrowed down BnaC08cqDTF to an 86-kb region on chromosome C08. The region harbored fifteen genes. After comparative analyses of the DNA sequences for mutation between A and C syntenic regions and detected by RNA-seq and qRT-PCR between the two parents, we found that BnaC08G0010400ZS harbors an A/G nonsynonymous mutation in exon 3. This single nucleotide polymorphism (SNP) haplotypewas alsocorrelated with early flowering ina 256 accession panel. BnaC08G0010400ZS is a homolog of the AT1G04400 gene (CRY2) in Arabidopsis. The analyses of transgenic Arabidopsis verified that BnaC08G0010400ZS is responsible for early flowering. Our results contribute to a better understanding of the genetic control mechanism of early flowering in spring Brassica napus and will promote the breeding for early mature varieties.

Unraveling the Molecular Basis of Mycosporine Biosynthesis in Fungi.

The Phaffia rhodozyma UCD 67-385 genome harbors a 7873 bp cluster containing DDGS, OMT, and ATPG, encoding 2-desmethy-4-deoxygadusol synthase, O-methyl transferase, and ATP-grasp ligase, respectively, of the mycosporine glutaminol (MG) biosynthesis pathway. Homozygous deletion mutants of the entire cluster, single-gene mutants, and the Δddgs-/-;Δomt-/- and Δomt-/-;Δatpg-/- double-gene mutants did not produce mycosporines. However, Δatpg-/- accumulated the intermediate 4-deoxygadusol. Heterologous expression of the DDGS and OMT or DDGS, OMT, and ATPG cDNAs in Saccharomyces cerevisiae led to 4-deoxygadusol or MG production, respectively. Genetic integration of the complete cluster into the genome of the non-mycosporine-producing CBS 6938 wild-type strain resulted in a transgenic strain (CBS 6938_MYC) that produced MG and mycosporine glutaminol glucoside. These results indicate the function of DDGS, OMT, and ATPG in the mycosporine biosynthesis pathway. The transcription factor gene mutants Δmig1-/-, Δcyc8-/-, and Δopi1-/- showed upregulation, Δrox1-/- and Δskn7-/- showed downregulation, and Δtup6-/- and Δyap6-/- showed no effect on mycosporinogenesis in glucose-containing medium. Finally, comparative analysis of the cluster sequences in several P. rhodozyma strains and the four newly described species of the genus showed the phylogenetic relationship of the P. rhodozyma strains and their differentiation from the other species of the genus Phaffia.

Comparative performance and 16S amplicon sequencing analysis of deep and shallow cells of a full scale HFCW having sequentially decreasing depths reveals vast enhancement potential

The performance and microbial community characteristics of a full-scale horizontal flow constructed wetland (HFCW), comprising sequential cells of different depths (1.5 m to 0.8 m), treating domestic wastewater from a residential complex located at Roorkee, India was analysed. The BOD, COD, NO3-N, TN, and PO4 removal efficiencies of the system were found to be 89.9 %, 89.1 %, 72.7 %, 67.1 %, and 53.3 %, respectively. The areal removal rate constants kBOD, kTN and kTP for BOD, total nitrogen and total phosphorus removal were found to fit well with the optimized k values calculated using secondary data of 74 HSSF CWs. Comparative 16S rRNA amplicon sequencing revealed widely varying bacterial communities in the deep and shallow cells of the wetland with several communities being exclusively present in the deep cell. Abundant heterotrophic nitrifiers, aerobic denitrifiers, and phosphate accumulating bacteria demonstrate the utility of such deep HFCW systems for the combined organics and nutrients removal.

Characterization of a novel papillomavirus identified from a whale (Delphinapterus leucas) pharyngeal metagenomic library

Here, using viral metagenomic method, a novel whale papillomavirus (temporarily named wPV, GenBank accession number OP856597) was discovered in a whale (Delphinapterus leucas) pharyngeal metagenomic library. The complete genome size of wPV is 7179 bp, with GC content of 54.4% and a nucleotide composition of 23.4% A, 22.3% T, 28.4% G, and 25.9% C. The viral genome has a typical papillomavirus organization pattern, and five ORFs were predicted, including two late genes encoding L1 and L2, and three early genes encoding E1, E2, and E6. Pairwise sequence comparison and phylogenetic analysis based on the L1 gene sequence indicated that wPV may be a novel species within genus Dyodeltapapillomavirus. In addition, the E2 region of wPV was predicted to have a potential recombination event. The discovery of this novel papillomavirus increases our understanding of the viral ecology of marine mammals, providing insights into possible future infectious diseases.

RNA Editing Enzyme ADAR1 Suppresses the Mobility of Cancer Cells via ARPIN.

Deamination of adenine or cytosine in RNA, called RNA editing, is a constitutively active and common modification. The primary role of RNA editing is tagging RNA right after its synthesis so that the endogenous RNA is recognized as self and distinguished from exogenous RNA, such as viral RNA. In addition to this primary function, the direct or indirect effects on gene expression can be utilized in cancer where a high level of RNA editing activity persists. This report identified actin-related protein 2/3 complex inhibitor (ARPIN) as a target of ADAR1 in breast cancer cells. Our comparative RNA sequencing analysis in MCF7 cells revealed that the expression of ARPIN was decreased upon ADAR1 depletion with altered editing on its 3'UTR. However, the expression changes of ARPIN were not dependent on 3'UTR editing but relied on three microRNAs acting on ARPIN. As a result, we found that the migration and invasion of cancer cells were profoundly increased by ADAR1 depletion, and this cellular phenotype was reversed by the exogenous ARPIN expression. Altogether, our data suggest that ADAR1 suppresses breast cancer cell mobility via the upregulation of ARPIN.

Ligilactobacillus ubinensis sp. nov., a novel species isolated from the wild ferment of a durian fruit (Durio zibethinus).

A Gram-stain-positive, rod-shaped, non-spore-forming, catalase-negative, urease-negative, homofermentative and facultatively anaerobic strain, named WILCCON 0076T, was isolated from a wild ferment of pieces of a 'Kampung' durian fruit collected on the island of Ubin (Pulau Ubin), Singapore. The durian had fallen to the ground from a durian tree (Durio zibethinus), on which a group of long-tailed macaques had been observed picking and eating the fruits. Comparative analyses of 16S rRNA gene sequences indicated that WILCCON 0076T potentially represented a novel species within the genus Ligilactobacillus, with the most closely related type strain being Ligilactobacillus agilis DSM 20509T (16S rRNA gene sequence similarity of 97.2 %). Average nucleotide identity and digital DNA-DNA hybridization prediction values were only 86.0% and 18.9 %, respectively. On the basis of the results of a polyphasic approach that included phylogenomic, chemotaxonomic and morphological analyses, we propose a novel species with the name Ligilactobacillus ubinensis sp. nov. (type strain WILCCON 0076T=DSM 114293T=LMG 32698T).

The terrestrial isopod symbiont ‘Candidatus Hepatincola porcellionum’ is a potential nutrient scavenger related to Holosporales symbionts of protists

The order Holosporales (Alphaproteobacteria) encompasses obligate intracellular bacterial symbionts of diverse Eukaryotes. These bacteria have highly streamlined genomes and can have negative fitness effects on the host. Herein, we present a comparative analysis of the first genome sequences of ‘Ca. Hepatincola porcellionum’, a facultative symbiont occurring extracellularly in the midgut glands of terrestrial isopods. Using a combination of long-read and short-read sequencing, we obtained the complete circular genomes of two Hepatincola strains and an additional metagenome-assembled draft genome. Phylogenomic analysis validated its phylogenetic position as an early-branching family-level clade relative to all other established Holosporales families associated with protists. A 16S rRNA gene survey revealed that this new family encompasses diverse bacteria associated with both marine and terrestrial host species, which expands the host range of Holosporales bacteria from protists to several phyla of the Ecdysozoa (Arthropoda and Priapulida). Hepatincola has a highly streamlined genome with reduced metabolic and biosynthetic capacities as well as a large repertoire of transmembrane transporters. This suggests that this symbiont is rather a nutrient scavenger than a nutrient provider for the host, likely benefitting from a nutrient-rich environment to import all necessary metabolites and precursors. Hepatincola further possesses a different set of bacterial secretion systems compared to protist-associated Holosporales, suggesting different host-symbiont interactions depending on the host organism.

MRPS36 provides a structural link in the eukaryotic 2-oxoglutarate dehydrogenase complex.

The tricarboxylic acid cycle is the central pathway of energy production in eukaryotic cells and plays a key part in aerobic respiration throughout all kingdoms of life. One of the pivotal enzymes in this cycle is 2-oxoglutarate dehydrogenase complex (OGDHC), which generates NADH by oxidative decarboxylation of 2-oxoglutarate to succinyl-CoA. OGDHC is a megadalton protein complex originally thought to be assembled from three catalytically active subunits (E1o, E2o, E3). In fungi and animals, however, the protein MRPS36 has more recently been proposed as a putative additional component. Based on extensive cross-linking mass spectrometry data supported by phylogenetic analyses, we provide evidence that MRPS36 is an important member of the eukaryotic OGDHC, with no prokaryotic orthologues. Comparative sequence analysis and computational structure predictions reveal that, in contrast with bacteria and archaea, eukaryotic E2o does not contain the peripheral subunit-binding domain (PSBD), for which we propose that MRPS36 evolved as an E3 adaptor protein, functionally replacing the PSBD. We further provide a refined structural model of the complete eukaryotic OGDHC of approximately 3.45 MDa with novel mechanistic insights.

Molecular detection and genetic characterization of bovine hepacivirus identified in ticks collected from cattle in Harbin, northeastern China.

Bovine hepacivirus (BovHepV) is a member of the genus Hepacivirus of the family Flaviviridae, which can cause acute or persistent infections in cattle. Currently, BovHepV strains identified in cattle populations worldwide can be classified into two genotypes with eight subtypes in genotype 1. BovHepV has been identified in a wide geographic area in China. Interestingly, the viral RNA of BovHepV has also been detected in ticks in Guangdong province, China. In this study, Rhipicephalus microplus tick samples were collected in Heilongjiang province, northeastern China, and BovHepV was screened with an overall positive rate of 10.9%. Sequence comparison and phylogenetic analysis showed that the BovHepV strains detected in this study belong to the subtype G. This is the first report about the detection of BovHepV in ticks in Heilongjiang province, China, which expands our knowledge that ticks may be a transmission vector of BovHepV.

Chitinophaga horti sp. nov., Isolated from Garden Soil

A strictly aerobic Gram-negative bacterium, designated R8T, isolated from garden soil in South Korea was subjected to a taxonomic study. The cells were non-spore-forming, oxidase-positive and catalase-negative, and non-motile rods (without flagella). Growth was observed between 10°C and 40°C (optimum, 30°C) and between pH 6.0 and 9.0 (optimum, pH 7.0) and in the presence of 0%-1.5% (w/v) NaCl (optimum, 0%). The G + C content of the genomic DNA was 49.9% and the major isoprenoid quinone was found to be menaquinone-7. The major fatty acids of strain R8T were iso-C15:0, C16:1 ω5c, and summed feature 3 (comprising iso-C15:0 2-OH and/or C16:1 ω7c/ω6c). Phosphatidylethanolamine was identified as a major polar lipid. Comparative 16S rRNA gene sequence analysis showed that strain R8T had the highest 16S rRNA gene sequence similarity of 98.3% with Chitinophaga sedimenti TFL-3T. Phylogenetic analyses using 16S rRNA gene sequences and concatenated 92 marker protein sequences revealed that strain R8T formed a robust phylogenetic lineage with C. sedimenti within the genus Chitinophaga. Average nucleotide identity and digital DNA-DNA hybridization values of strain R8T to Chitinophaga species were less than 77.9% and 21.1%, respectively. The phenotypic, phylogenetic, and chemotaxonomic properties support that strain R8T represents a novel species of the genus Chitinophaga, for which the name Chitinophaga horti sp. nov. is proposed. The type strain is R8T (= KACC 19895T = JCM 33215T).

Structural Analysis of the A Mating Type Locus and Development of the Mating Type Marker of Agaricus bisporus var. bisporus

Karyotyping in Agaricus bisporus is crucial for both the isolation of homokaryotic strains and the confirmation of dikaryon establishment. For the verification of the karyotype, the A mating type loci of two homokaryotic strains, H39 and H97, were analyzed through comparative sequence analysis. The two loci showed major differences in two sequence regions designated as Region 1 and Region 2. H97 had a putative DNA transposon in Region 1 that had target site duplications (TSDs), terminal inverted repeats (TIRs), and a loop sequence, in contrast to H39, which only had the insertional target sequence. Homologous sequences of the transposon were discovered in the two different chromosomes of H97 and in one of H39, all of which have different TSDs but share high sequence homology in TIR. Region 2 shared three consensus sequences between H97 and H39. However, it was only from H97 that a large insertional sequence of unknown origin was discovered between the first and second consensus sequences. The difference in length in Region 1, employed for the verification of the A mating type, resulted in the successful verification of mating types in the heterokaryotic and homokaryotic strains. This length difference enables the discrimination between homo- and heterokaryotic spores by PCR. The present study suggests that the A mating type locus in A. bisporus H97 has evolved through transposon insertion, allowing the discrimination of the mating type, and thus the nuclear type, between A. bisporus H97 and H39.

Isolation, identification and optimization of exopolysaccharide-based bioflocculant synthesis by soil bacteria Bacillus atrophaeus NR-12

Bioflocculants are polysaccharide-based, biodegradable macromolecules synthesized by soil microorganisms. A bacterium prodicing an extracellular bioflocculant wasisolated from the salty and dry soil and identified as Bacillus atrophaeus. Effects of culture conditions including carbon sources, nitrogen sources, pH, shaker speed on bioflocculant production were studied. Isolate comparative analysis of 16S rRNA sequence identified the isolate to have 99% similarity to Bacillus atrophaeus and it was deposited in the GenBank as Bacillus atrophaeus NR- 2 with accession number OP679007. The strain was characterized by ability to grow and synthesize the bioflocculant in alkaline conditions – pH 11. It was supposed that the strain can overcome adverse effects of environment – alkalinity and salinity by enhanced production of bioflocculant in stress. In the optimized medium (sucrose and tryptone used as C/N sources) the strain produced 4.76 mL bioflocculant.

Isolation, identification and optimization of exopolysaccharide-based bioflocculant synthesis by soil bacteria Bacillus atrophaeus NR-12

Bioflocculants are polysaccharide-based, biodegradable macromolecules synthesized by soil microorganisms. A bacterium prodicing an extracellular bioflocculant wasisolated from the salty and dry soil and identified as Bacillus atrophaeus. Effects of culture conditions including carbon sources, nitrogen sources, pH, shaker speed on bioflocculant production were studied. Isolate comparative analysis of 16S rRNA sequence identified the isolate to have 99% similarity to Bacillus atrophaeus and it was deposited in the GenBank as Bacillus atrophaeus NR- 2 with accession number OP679007. The strain was characterized by ability to grow and synthesize the bioflocculant in alkaline conditions – pH 11. It was supposed that the strain can overcome adverse effects of environment – alkalinity and salinity by enhanced production of bioflocculant in stress. In the optimized medium (sucrose and tryptone used as C/N sources) the strain produced 4.76 mL bioflocculant.

Quantification of the Diversity in Gene Structures Using the Principles of Polarization Mapping.

Results of computational analysis and visualization of differences in gene structures using polarization coding are presented. A two-dimensional phase screen, where each element of which corresponds to a specific basic nucleotide (adenine, cytosine, guanine, or thymine), displays the analyzed nucleotide sequence. Readout of the screen with a coherent beam characterized by a given polarization state forms a diffracted light field with a local polarization structure that is unique for the analyzed nucleotide sequence. This unique structure is described by spatial distributions of local values of the Stokes vector components. Analysis of these distributions allows the comparison of nucleotide sequences for different strains of pathogenic microorganisms and frequency analysis of the sequences. The possibilities of this polarization-based technique are illustrated by the model data obtained from a comparative analysis of the spike protein gene sequences for three different model variants (Wuhan, Delta, and Omicron) of the SARS-CoV-2 virus. Various modifications of polarization encoding and analysis of gene structures and a possibility for instrumental implementation of the proposed method are discussed.

CYSRT1: An Antimicrobial Epidermal Protein that Can Interact with Late Cornified Envelope Proteins

Late cornified envelope (LCE) proteins are small cationic epidermal proteins with antimicrobial properties, and the combined deletion of LCE3B and LCE3C genes is a risk factor for psoriasis that affects skin microbiome composition. In a yeast two-hybrid screen, we identified CYSRT1 as an interacting partner of members of all LCE groups except LCE6. These interactions were confirmed in a mammalian cell system by coimmunoprecipitation. CYSRT1 is a protein of unknown function that is specifically expressed in cutaneous and oral epithelia and spatially colocalizes with LCE proteins in the upper layers of the suprabasal epidermis. Constitutive CYSRT1 expression is present in fully differentiated epidermis and can be further induced invivo by disruption of the skin barrier upon stratum corneum removal. Transcriptional regulation correlates to keratinocyte terminal differentiation but not to skin bacteria exposure. Similar to LCEs, CYSRT1 was found to have antibacterial activity against Pseudomonas aeruginosa. Comparative gene sequence analysis and protein amino acid alignment indicate that CYSRT1 is highly conserved among vertebrates and has putative antimicrobial activity. To summarize, we identified CYSRT1 in the outer skin layer, where it colocalizes with LCE proteins and contributes to the constitutive epidermal antimicrobial host defense repertoire.

Dynamic state of plasmid genomic architectures resulting from XerC/D-mediated site-specific recombination in Acinetobacter baumannii Rep_3 superfamily resistance plasmids carrying blaOXA-58 - and TnaphA6-resistance modules.

The acquisition of bla OXA genes encoding different carbapenem-hydrolyzing class-D β-lactamases (CHDL) represents a main determinant of carbapenem resistance in the nosocomial pathogen Acinetobacter baumannii. The blaOXA-58 gene, in particular, is generally embedded in similar resistance modules (RM) carried by plasmids unique to the Acinetobacter genus lacking self-transferability. The ample variations in the immediate genomic contexts in which blaOXA-58 -containing RMs are inserted among these plasmids, and the almost invariable presence at their borders of non-identical 28-bp sequences potentially recognized by the host XerC and XerD tyrosine recombinases (pXerC/D-like sites), suggested an involvement of these sites in the lateral mobilization of the gene structures they encircle. However, whether and how these pXerC/D sites participate in this process is only beginning to be understood. Here, we used a series of experimental approaches to analyze the contribution of pXerC/D-mediated site-specific recombination to the generation of structural diversity between resistance plasmids carrying pXerC/D-bounded bla OXA-58- and TnaphA6-containing RM harbored by two phylogenetically- and epidemiologically-closely related A. baumannii strains of our collection, Ab242 and Ab825, during adaptation to the hospital environment. Our analysis disclosed the existence of different bona fide pairs of recombinationally-active pXerC/D sites in these plasmids, some mediating reversible intramolecular inversions and others reversible plasmid fusions/resolutions. All of the identified recombinationally-active pairs shared identical GGTGTA sequences at the cr spacer separating the XerC- and XerD-binding regions. The fusion of two Ab825 plasmids mediated by a pair of recombinationally-active pXerC/D sites displaying sequence differences at the cr spacer could be inferred on the basis of sequence comparison analysis, but no evidence of reversibility could be obtained in this case. The reversible plasmid genome rearrangements mediated by recombinationally-active pairs of pXerC/D sites reported here probably represents an ancient mechanism of generating structural diversity in the Acinetobacter plasmid pool. This recursive process could facilitate a rapid adaptation of an eventual bacterial host to changing environments, and has certainly contributed to the evolution of Acinetobacter plasmids and the capture and dissemination of bla OXA-58 genes among Acinetobacter and non-Acinetobacter populations co-residing in the hospital niche.