Virulence genes regulated at the transcriptional level by Ca2+ in Yersinia pestis include structural genes for outer membrane proteins.

Yersinia pestis, the causative agent of plague, has a virulence determinant called the low-Ca2+ response (Lcr+ phenotype) that confers on the bacterium Ca2+ dependence for growth at 37 degrees C and expression of V antigen. This virulence determinant is common to the three species of Yersinia and is mediated by Lcr plasmids (called pCD in Y. pestis). In this study, we generated insertions of Mu dI1(Ap lac) in pCD1 of Y. pestis KIM, screened for cells showing transcriptional regulation by Ca2+, and obtained inserts that define at least four pCD1 genes. Their patterns of transcription under different growth conditions closely paralleled the pattern of expression of the V antigen. We tested for expression of Lcr-specific yersinial outer membrane proteins (Yops) by the pCD1::Mu dI1(Ap lac) plasmids. Four of the inserts each eliminated expression of a different Yop; one of these Yops was unique to Y. pestis. Two of the insertions affecting Yops caused avirulence, and one caused strongly decreased virulence of Y. pestis in mice. These data indicate that Yops, like the V antigen, are virulence attributes regulated in the low-Ca2+ response.