| Summary: | <p>Abstract</p> <p>Background</p> <p>The <it>Mycoplasma hominis vaa </it>gene encodes a highly variable, surface antigen involved in the adhesion to host cells. We have analysed the structure of the <it>vaa </it>locus to elucidate the genetic basis for variation of <it>vaa</it>.</p> <p>Results</p> <p>Mapping of <it>vaa </it>on existing physical maps of five <it>M. hominis </it>isolates by pulsed field gel electrophoresis revealed that <it>vaa </it>is located in a genomic region containing the majority of other characterized membrane protein genes of <it>M. hominis</it>. Sequencing of an 11 kb region containing the <it>vaa </it>locus of <it>M. hominis </it>isolate 132 showed the presence of conserved housekeeping genes at the borders of the region, <it>uvrA </it>upstream and the <it>hitABL </it>operon downstream to <it>vaa</it>. Analysis of 20 <it>M. hominis </it>isolates revealed that the <it>vaa </it>upstream region was conserved whereas the downstream region was highly variable. In isolate 132 this region contained an open reading frame (ORF) encoding a putative 160 kDa membrane protein. Homologous ORFs were present in half of the isolates, whereas this ORF, termed <it>vmp </it>(variable membrane protein), was deleted from the locus in the remaining isolates. Compellingly, the conserved upstream region and variable downstream region of <it>vaa </it>correlates with the genetic structure of <it>vaa </it>itself which consists of a conserved 5' end and a variable 3' end containing a variable number of exchangeable sequence cassettes.</p> <p>Conclusion</p> <p>Our data demonstrate that the <it>vaa </it>locus contains a divergent genetic islet, and indicate pronounced intraspecies recombination. The high variability level of the locus indicate that it is a chromosomal 'hot spot', presumably important for sustaining diversity and a high adaptation potential of <it>M. hominis</it>.</p>
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