VSRB, A REGULATOR OF VIRULENCE GENES OF PSEUDOMONAS-SOLANACEARUM, IS HOMOLOGOUS TO SENSORS OF THE 2-COMPONENT REGULATOR FAMILY

Pseudomonas solanacearum, an important wilt pathogen of many plants, p roduces several extracellular proteins (EXPs) and extracellular polysa ccharides (EPSs) that contribute to its virulence. Using TnphoA mutage nesis, we discovered a new gene, vsrB, that when inactivated causes a major reduction in the virulence and production of an EPS. Analysis of eps=lacZ reporters showed that vsrB is required for maximal expressio n (transcription) of eps, whose products are required for production o f EPS I, a major virulence determinant. Analysis of EXPs in culture su pernatants revealed that inactivation of vsrB also causes reduced prod uction of two major EXPs, with molecular masses of 28 and 97 kDa, and a simultaneous 15-fold increase in levels of another EXP, PglA endopol ygalacturonase. The vsrB gene was cloned from a P. solanacearum genomi c library by complementation of the nonmucoid phenotype of the vsrB=Tn phoA mutant and then subcloned on a 2.4-kb DNA fragment. TnphoA fusion analysis and subcellular localization of the vsrB gene product in Esc herichia coli maxicells suggest that it is a ca. 60-kDa transmembrane protein. The nucleotide sequence of the 2.4-kb DNA fragment was determ ined, and a 638-amino-acid open reading frame was found for VsrB. A se arch of the GenBank data base found that the central part of VsrB has homology with the histidine kinase domain of sensors in the two-compon ent regulator family, while the C terminus has homology with the phosp hate receiver domain of response regulators in the same family. Geneti c analysis suggests that the receiver domain is not required for vsrB function.