Department of Plant Microbiology and Pathology, University of Missouri at Columbia, 108 Waters Hall, Columbia, MO 65211 U.S.A.
In Erwinia carotovora subsp. carotovora (Ecc) strain 71 (Ecc71), HrpLEcc, an alternate sigma factor of the extracytoplasmic function subfamily, plays a central role in the expression of the hrp (hypersensitive reaction and pathogenicity) regulon. We document here that sigma-54 (RpoN) is required for full expression of hrpLEcc and that HrpS, in conjunction with sigma-54, activates hrpLEcc transcription. We also made the novel observation that integration host factor is required for the activation of the hrpLEcc promoter. Our findings reveal that the RsmA/rsmB RNA-mediated post-transcriptional system, known to control ex-tracellular enzyme and harpin production, affects hrpLEcc expression as well. For example, hrpLEcc RNA levels are barely detected in an RsmB- strain. Conversely, hrpLEcc mRNA levels are much higher in RsmA- bacteria than in the RsmA+ parent. This effect is due to RsmA-promoted decay of hrpLEcc RNA. Moreover, the following regulators known to control the production of either RsmA, rsmB RNA, or both also affect hrpLEcc expression: GacA (response regulator of a two-component system), KdgR (an IclI type repressor), HexA (a LysR type repressor), RsmC (a putative transcriptional adapter). Based upon the data now available for Ecc and extrapolating from the evidence in other systems, we propose a tentative model that depicts the Hrp regulatory system of Ecc and explains the basis for coregulation of extracellular enzyme production and expression of the Hrp regulon.