Signalling System Heterogeneity in E. Coli and Shigella

The connector protein, PmrD links the PhoPQ and PmrAB two component systems and is critical for PhoPQ-regulated host defense peptide resistance in Escherichia coli. In E. coli the pmrD gene contains three putative start codons near a Shine-Dalgarno sequence and E. coli full-length in-frame PmrD is produced from the second putative start site. In S. flexneri, the pmrD gene contains a 1 bp insertion at the 5' end of the gene, that shifts the open reading frame to the first putative start site. Here, I demonstrate that the start site spacing found in Shigella flexneri does not produce PmrD and that this organism does not undergo PhoPQ regulated polymyxin B resistance. I further show that PmrD in S. sonnei is like that from E. coli and is required for high level PhoPQ-regulated polymyxin B resistance in this organism, suggesting that these species differ in this signaling network.