| Pathogenicity islands are considered to be important agents of lateral gene transfer, responsible for the movement of large blocks of genes between bacterial species. This is believed to be an extremely important mechanism of spread for the dispersal of antibiotic resistance and toxin-encoding genes. In SaPI1, we have a pathogenicity island whose excision, replication, and encapsidation has been demonstrated, in partnership with phage 80alpha. The molecular mechanisms underlying this mobility are largely unknown.; In this work, we identified several phage-like genes encoded by SaPI1, and hypothesized that they play roles in the mobilization of SaPI1, by &phis;80alpha. We investigated the transcriptional activity of SaPI1 during its replication cycle, and showed an increase in expression of 5 of the 6 transcripts investigated, during the replication of SaPI1. We knocked out three SaPI1 genes, rep, hth2, and int, and assayed for the ability of the mutants to excise, replicate, and be transferred to a new host, relative to wild-type SaPI1. We have shown that the Rep protein of SaPI1 has both origin-binding and helicase activities, and is required for the replication of SaPI1. Deletion of the rep gene appears to have little effect on the transcriptional activity of other SaPI1 genes during SaPI1 replication. We have also determined that the hth2 gene is required, SaPI1 replication being severely impaired in its absence. Deletion of hth2 results in down regulated transcription of operons 1, 2, and 3 of SaPI1 during replication. In addition, we have extended the previous observations of the role of int, demonstrating that it is required for excision of the SaPI1 DNA from its host, and also for integration of the transferred SaPI1 DNA into the chromosome of its new host. Deletion of the int gene results in downregulation, by an unknown mechanism, of transcription of operons 1, 2, and 3 during SaPI1 replication.; These results have demonstrated the importance of several SaPI1 genes for replication of the element, and begun to reveal the transcriptional events underlying this process. This will provide a basis for future investigations of the interactions between pathogenicity islands and their helper phages. |
