THE SFIX, RFE AND METN GENES OF SALMONELLA-TYPHIMURIUM AND THEIR INVOLVEMENT IN THE HIS(C) PLEIOTROPIC RESPONSE

Two loci involved in the pleiotropic response of His' strains of Salmo nella typhimurium (sfiX and sfiY) have been characterized at the molec ular level. The sfiX gene (CS 44) has been identified as a homolog of the E. coli gene sanA, located downstream of the cytidine deaminase ge ne (cdd). The cdd-sanA (or cdd-sfiX) operon shows a highly conserved s tructure in E.coli and Salmonella. Like its E. coli homolog, the sfiX gene of S. typhimurium is required for vancomycin resistance at high t emperature. The dual effect of sfiX mutations (induction of vancomycin sensitivity and suppression of cell division inhibition) suggests a l ink between SfiX function and murein synthesis. The sfiY locus (CS 85) , contains two genes arranged in a single transcriptional unit. The up stream gene is a homolog of the E. coli gene rfe; mutations in this ge ne suppress the cell division defect of His' strains. The suppressor e ffect of rfe mutations can be reproduced by tunicamycin, suggesting th at suppression of filamentation results from an increase in the intrac ellular concentration of UDP-N-acetyl-D-glucosamine; The gene located downstream of rfe is also found in E. coli but its function is unknown . Insertions in life suppress the methionine requirement of His' strai ns of S. typhimurium by a polar effect on the downstream gene, tentati vely designated metN. Complementation with a rfe(+) clone indicates th at the rfe gene is not involved in the methionine requirement of His' strains. Thus metN expression appears to cause methionine auxotrophy i n a His(c) background.