FIG 8.

Working model showing the proposed regulatory role of HrcT. Our results, together with our previous report (2), show that HrcT expression at the T3 promoter is HrpG independent. Thus, we propose that an unidentified regulator (question mark in a gray circle), possibly regulated by HrpG, which does not directly regulate the expression of hrpX (cross on a black arrow), promotes hrcT transcription at the T3 promoter. HrcT then positively regulates the expression of hrpX by binding the hrpX promoter. The transcriptional activator HrpX regulates multiple hrp operons (black-lined arrows) by binding to the PIP box promoter as indicated by asterisks above the hrp-hrc-hpa cluster. Another unknown protease-like protein (question mark in a white circle), possibly regulated by HrpX, may degrade the mRNA of hrcT or HrcT to repress the expression of hrpX. The dashed horizontal arrows represent hrp operons (A to F) consisting of individual hrp-hrc-hpa genes, which were assigned by choosing the last letter or number of a gene name under differentially shaded arrows, referring to references 2 and 3.