MUTATIONS THAT INCREASE EXPRESSION OF THE RPOS GENE AND DECREASE ITS DEPENDENCE ON HFQ FUNCTION IN SALMONELLA-TYPHIMURIUM

The RpoS transcription factor (also called sigma(S) or sigma(38)) is r equired for the expression of a number of stationary-phase and osmotic ally inducible genes in enteric bacteria. RpoS is also a virulence fac tor for several pathogenic species, including Salmonella typhimurium. The activity of RpoS is regulated in response to many different signal s, at the levels of both synthesis and proteolysis. Previous work with rpoS-lac protein fusions has suggested that translation of rpoS requi res hfq function, The product of the hfq gene, host factor I (HF-I), i s a ribosome-associated, site-specific RNA-binding protein originally characterized for its role in replication of the RNA bacteriophage Q b eta of Escherichia coil, In this study, the role of HF-I was explored by isolating suppressor mutations that map to the region directly upst ream of rpoS, These mutations increase rpoS-lac expression in the abse nce of HF-I and also confer substantial independence from HF-I. DNA se quence analysis of the mutants suggests a model in which the RNA secon dary structure near the ribosome binding site of the rpoS mRNA plays a n important role in limiting expression in the wild type, Genetic test s of the model confirm its predictions, at least in part, It seems lik ely that the mutations analyzed here activate a suppression pathway th at bypasses the normal HF-I-dependent route of rpoS expression; howeve r, it is also possible that some of them identify a sequence element w ith an inhibitory function that is directly counteracted by HF-I.