Positive autoregulation of vipR expression in ViaB region-encoded Vi antigen of Salmonella typhi.

vipR was the first gene in the ViaB region found to be involved in production of the Vi capsular antigen in Salmonella typhi and to be transcribed in the same direction as nine downstream genes. To investigate its function, we constructed a nonpolar vipR mutant by inserting a cat cartridge into the vipR gene on the chromosome. Inactivation of the gene decreased its transcriptional level and the levels of the downstream genes. When a functional vipR gene on a low-copy-number plasmid was introduced into the vipR mutant, the transcriptional levels of the chromosomal genes were restored. These results indicate that the vipR gene product acts as a transcriptional activator for the downstream genes. Interestingly, the vipR gene also was positively regulated by its own product. DNA-binding studies showed that protein extract that contained the VipR product specifically retarded the mobility of a 451-bp BspHI-XbaI fragment upstream of the vipR gene. In addition, complementation analysis indicated that the vipR gene was cotranscribed with a least two downstream genes, vipA and vipB. These findings suggest that the VipR product may interact with the regulatory region upstream of the vipR gene and induce operonic mRNA synthesis that includes the vipR and downstream genes.