| Summary: | One of the most important weapons in the armory of Acinetobacter is its impressive genetic plasticity, facilitating rapid genetic mutations and rearrangements as well as integration of foreign determinants carried by mobile genetic elements. Of these, IS are considered one of the key forces shaping bacterial genomes and ultimately evolution. We report the identification of a novel nonautonomous IS-derived element present in multiple bacterial species from the Moraxellaceae family and its recent translocation into the hns locus in the A. baumannii ATCC 17978 genome. The latter finding adds new knowledge to only a limited number of documented examples of MITEs in the literature and underscores the plastic nature of the hns locus in A. baumannii. MITEAba12, and its predicted parent(s), may be a source of substantial adaptive evolution within environmental and clinically relevant bacterial pathogens and, thus, have broad implications for niche-specific adaptation.Insertion sequences (IS) are fundamental mediators of genome plasticity with the potential to generate phenotypic variation with significant evolutionary outcomes. Here, a recently active miniature inverted-repeat transposon element (MITE) was identified in a derivative of Acinetobacter baumannii ATCC 17978 after being subjected to stress conditions. Transposition of the novel element led to the disruption of the hns gene, resulting in a characteristic hypermotile phenotype. DNA identity shared between the terminal inverted repeats of this MITE and coresident ISAba12 elements, together with the generation of 9-bp target site duplications, provides strong evidence that ISAba12 elements were responsible for mobilization of the MITE (designated MITEAba12) within this strain. A wider genome-level survey identified MITEAba12 in 30 additional Acinetobacter genomes at various frequencies and one Moraxella osloensis genome. Ninety MITEAba12 copies could be identified, of which 40% had target site duplications, indicating recent transposition events. Elements ranged between 111 and 114 bp; 90% were 113 bp in length. Using the MITEAba12 consensus sequence, putative outward-facing Escherichia coli σ70 promoter sequences in both orientations were identified. The identification of transcripts originating from the promoter in one direction supports the proposal that the element can influence neighboring host gene transcription. The location of MITEAba12 varied significantly between and within genomes, preferentially integrating into AT-rich regions. Additionally, a copy of MITEAba12 was identified in a novel 8.5-kb composite transposon, Tn6645, in the M. osloensis CCUG 350 chromosome. Overall, this study shows that MITEAba12 is the most abundant nonautonomous element currently found in Acinetobacter.
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