Nitrogen control in Salmonella : Regulation by the glnR and glnF gene products

The product of theglnR gene is required for nitrogen regulation of the synthesis of glutamine synthesis (Gln synthetase) [L-glutamate:ammonia ligase (ADP-forming), EC 6.3.1.2] and two periplasmic transport proteins that are subject to nitrogen control inSalmonella . Strains with mutations to loss of function of theglnR product [e.g., a strain with a Tn10 insertion or one with an ICR-induced (frameshift) mutation inglnR ] have about 3% as much Gln synthetase as a fully derepressed wild-type strain and are unable to increase synthesis of this enzyme or periplasmic transport proteins in response to nitrogen limitation. The structural gene for Gln synthetase,glnA , and those for the periplasmic transport proteins are unlinked on the chromosome; thus,glnR appears to encode a diffusible positive regulatory element. Consistent with this, the mutantglnR allele is recessive to the wild-type allele with regard to expression ofglnA (synthesis of Gln synthetase). AlthoughglnR is closely linked toglnA , strains with mutations to complete loss of function of theglnR product can be distinguished fromglnA strains by their ability to produce detectable Gln synthetase and to grow in the absence of glutamine. To demonstrate unequivocally thatglnR is distinct fromglnA , we have purified and characterized Gln synthetase from a strain with a Tn10 insertion inglnR . Because the properties of Gln synthetase from the insertion mutant, most importantly the carboxyl-terminal sequence of amino acids, are the same as those of synthetase from wild type, the Tn10 insertion cannot be inglnA (if it were, the carboxyl terminus of Gln synthetase would have to be altered); therefore we conclude that the Tn10 insertion is in a regulatory gene,glnR , which is distinct fromglnA . A model for the function of theglnR product together with the previously definedglnF product in mediating nitrogen control is discussed.