Synthesis of Vi
antigen, a capsular
polysaccharide expressed by
Salmonella typhi, is controlled by the viaA and viaB
chromosomal loci. It was previously shown that Vi
antigen expression was regulated by a system similar to the rcs regulatory system involved in colanic acid synthesis in
Escherichia coli. We have
cloned the rcsA, rcsB, and rcsC genes from S. typhi. The predicted
amino sequences of the RcsA and RcsB proteins showed a high degree of similarity to their
E. coli homologs. The
nucleotide sequence of the rcsC gene was partially determined and was shown to be homologous to that of its
E. coli counterpart. Complementation experiments indicated that rcsB and rcsC were encompassed within the viaA locus. The RcsA protein was not involved in Vi
antigen synthesis. In contrast, the RcsB protein acted as a positive regulator of Vi
polysaccharide expression. By
mRNA and
gene fusion analyses, we studied the role of RcsB and TviA, a via-B-encoded
regulatory protein characterized previously, in regulating Vi
antigen synthesis. The
transcriptional start point of tviA
mRNA was not influenced by RcsB or TviA. In the absence of RcsB or TviA protein, transcription of tviA gave rise to only a
monocistronic tviA-specific
mRNA. The presence of RcsB and TriA not only increased the amount of
monocistronic tviA-specific
mRNA but also resulted in countranscription of tviA and tviB, which is located immediately downstream of tviA on the viaB locus. In addition, TviA protein did not appear to be subject to degradation by the Lon
protease. These results strongly suggest that TviA might act in concert with RcsB at the tviA
promoter to activate transcription of the genes involved in Vi
polymer synthesis in S. typhi in a Lon-independent manner.
