CHARACTERIZATION OF THE RCSA AND RCSB GENES FROM SALMONELLA-TYPHI - RCSB THROUGH TVIA IS INVOLVED IN REGULATION OF VI-ANTIGEN SYNTHESIS

Synthesis of Vi antigen, a capsular polysaccharide expressed by Salmon ella typhi, is controlled by the viaA and viaB chromosomal loci. It wa s previously shown that Vi antigen expression was regulated by a syste m similar to the res regulatory system involved in colanic acid synthe sis in Escherichia coil. We have cloned the rcsA, rcsB, and resC genes from S. typhi. The predicted amino sequences of the RcsA and RcsB pro teins showed a high degree of similarity to their E. coli homologs. Th e nucleotide sequence of the rcsC gene was partially determined and wa s shown to be homologous to that of its E. coli counterpart. Complemen tation experiments indicated that rcsB and rcsC were encompassed withi n the viaA locus. The RcsA protein was not involved in Vi antigen synt hesis. In contrast, the RcsB protein acted as a positive regulator of Vi polysaccharide expression. By mRNA and gene fusion analyses, we stu died the role of RcsB and TviA, a viaB-encoded regulatory protein char acterized previously, in regulating Vi antigen synthesis. The transcri ptional start point of tviA mRNA was not influenced by RcsB or TviA. I n the absence of RcsB or TviA protein, transcription of tviA gave rise to only a monocistronic tviA-specific mRNA. The presence of RcsB and TviA not only increased the amount of monocistronic tviA-specific mRNA but also resulted in cotranscription of tviA and tviB, which is locat ed immediately downstream of tviA on the viaB locus. In addition, TviA protein did not appear to be subject to degradation by the Lon protea se. These results strongly suggest that TviA might act in concert with RcsB at the tviA promoter to activate transcription of the genes invo lved in Vi polymer synthesis in S. typhi in a Lon-independent manner.