LCRG, A SECRETED PROTEIN INVOLVED IN NEGATIVE REGULATION OF THE LOW-CALCIUM RESPONSE IN YERSINIA-PESTIS

The purpose of this study was to define the function of LcrG, the prod uct of the first gene in the lcrGVHyopBD operon of the low-Ca2+-respon se (LCR) virulence plasmid of Yersinia pestis. We created a Y. pestis strain having an in-frame deletion in lcrG. This nonpolar mutant had a n abnormal LCR growth phenotype: it was unable to grow at 37-degrees-C in the presence of 2.5 mM Ca2+ (''Ca2+ blind'') but was able to grow at 37-degrees-C when 18 mM ATP was present. At 37-degrees-C it failed to downregulate the expression and secretion of its truncated product (LcrG), V antigen, and YopM. All of these mutant properties were compl emented by plasmids carrying normal lcrG. However, a nonpolar lcrE mut ation and an lcrH mutation (both also causing a Ca 2+-blind phenotype) were not complemented in this way. The Y. pestis parent strain expres sed LcrG at 37-degrees-C in the presence and absence of Ca2+ and trans ported it to the medium when Ca2+ was absent. We identified two LCR-re gulated loci, lcrD and yscDEF, required for this transport. Complement ation analysis of the Y. pestis lcrR strain previously shown to lack t he expression of LcrG showed that the loss of LcrG but not of LcrR cau sed the Ca2+-blind phenotype of that mutant. Taken together, the resul ts show that LcrG is a negative regulator of the LCR, perhaps function ing in Ca 2+ sensing along with LcrE.