PROMOTER SUBSTITUTION AND DELETION ANALYSIS OF UPSTREAM REGION REQUIRED FOR RPOS TRANSLATIONAL REGULATION

The RpoS sigma factor of enteric bacteria is required for the increase d expression of a number of genes that are induced during nutrient lim itation and growth into stationary phase and in response to high osmol arity, RpoS is also a virulence factor for several pathogenic species, including Salmonella typhimurium. The activity of RpoS is regulated a t both the level of synthesis and protein turnover. Here we investigat e the posttranscriptional control of RpoS synthesis by using rpoS-lac protein and operon fusions, Substitution of the native rpoS promoters with the tac or lac UV5 promoters allowed essentially normal regulatio n after growth into stationary phase in rich medium or after osmotic c hallenge. Regulation of these fusions required the function of hfq, en coding the RNA-binding protein host factor I (HF-I), Short deletions f rom the 5' end of the rpoS transcript did not affect regulation very m uch; however, a larger deletion mutation that still retains 220 bp ups tream of the rpoS ATG codon, including a proposed antisense element in hibitory for rpoS translation, was no longer regulated by HF-I. Severa l models for regulation of rpoS expression by HF-I are discussed.