Integration host factor is required for RpoN-dependent hrpL gene expression and controls motility by positively regulating rsmB sRNA in Erwinia amylovora

Erwinia amylovora requires an hrp-type III secretion system (T3SS) to cause disease. It has been reported that HrpL, the master regulator of T3SS, is transcriptionally regulated by sigma factor 54 (RpoN), YhbH, and HrpS. In this study, the role of integration host factor (IHF) in regulating hrpL and T3SS gene expression was investigated. IHF is a nucleoid-associated protein that regulates gene expression by influencing nucleoid structure and DNA bending. Our results showed that both ihfA and ihfB mutants of E. amylovora did not induce necrotic lesions on pear fruits. Growth of both mutants was greatly reduced, and expression of the hrpL and T3SS genes was significantly down-regulated as compared with those of the wild type. In addition, expression of the ihfA, but not the ihfB gene, was under auto-suppression by IHF. Furthermore, both ihfA and ihfB mutants were hypermotile, due to significantly reduced expression of small RNA (sRNA) rsmB. Electrophoresis mobility shift assay further confirmed that IHF binds to the promoters of the hrpL and ihfA genes, as well as the rsmB sRNA gene. These results indicate that IHF is required for RpoN-dependent hrpL gene expression and virulence, and controls motility by positively regulating the rsmB sRNA in E. amylovora.