Filamentous phage infection induces the synthesis of large amounts of an Escherichia coli protein, phage shock protein (Psp), the product of a previously undescribed gene. This induction is due to the phage gene IV protein, pIV, an integral membrane protein. The uninduced level of Psp is undetectable, but when induced by prolonged synthesis of pIV, it can become one of the most abundant proteins in the cell. Psp is also synthesized transiently in response to several stresses (heat, ethanol, and osmotic shock). High-level synthesis occurs only after extreme treatment. Unlike the members of the heat shock regulon, Psp induction does not require the heat shock sigma factor, sigma 32; some stimuli that elicit sigma 32-dependent heat shock proteins do not induce Psp synthesis. The level of Psp induction after extreme stress is even higher in sigma 32 mutant cells, which are unable to mount a normal heat shock response, suggesting that these parallel stress responses are interrelated.

The hrp cluster of the pepper and tomato pathogen Xanthomonas campestris pv. vesicatoria is required for both pathogenicity on susceptible host plants and induction of the hypersensitive reaction on resistant plants. The hrpA locus is located at the left end of the 25-kb hrp region and encodes a single 64-kDa Hrp protein, HrpA1, which belongs to the PulD superfamily of proteins involved in type II and type III protein secretion. In this study, we developed a defined medium without any plant-derived molecules that induces expression of hrpA in vitro. The hrpA transcription start site was mapped in the coding region of the hrpB8 gene, which is the last gene of the hrpB operon. The inducible hrpA promoter shows no homology to known promoter elements or other hrp loci of X. campestris pv. vesicatoria. hrpA expression was shown to be independent of the hrp regulatory gene hrpX. The amino acid sequence of the HrpA1 protein is predicted to contain an N-terminal signal sequence and no further transmembrane domains and to be rich in ␤-sheet stretches. Expression of HrpA1 in Escherichia coli cells causes induction of the psp operon like some of its counterparts, suggesting some commonality of function and that HrpA1 forms multimers. The protein product of hrpA1 was identified by using a specific polyclonal antibody. Cell fractionation studies demonstrated that the HrpA1 protein is localized in the outer membrane of X. campestris pv. vesicatoria. HrpA1 is the first component of the Hrp secretion system whose localization has been determined in the original organism.

No abstract

No abstract

No abstract