Thermoregulation of Invasion Genes in Shigella Flexneri Through the Transcriptional Activation of the Virb Gene on the Large Plasmid

Summary

Expression of several invasion (inv) genes on the large virulence plasmid of Shigella flexneri is subject to thermoregulation. The ipaBCD genes are expressed at 37 C but not at 30 C and this expression is regulated by virF through the activation of virB on the plasmid. To identify the mediator gene for the thermoregulation of the large plasmid, the transcription of virF, virB, ipa and the other two inv operons was examined by Northern dot hybridization and S1 nuclease assay methods. The results indicated that transcription of virF and virB was affected by temperature but the level of virB transcription was more strongly controlled by temperature than that of virF. On the other hand, transcription of ipa and the other two inv operons was not dependent on temerature but on virB transcription. By overexpressing the virB gene with a tac promoter, deregulation of invasion phenotype was achieved in the absence of VirF function, suggesting that the transcriptional activation of virB mediates the thermoregulation phenomenon.