The response to stationary-phase stress conditions in Escherichia coli: role and regulation of the glutamic acid decarboxylase system

Inducible bacterial amino acid decarboxylases are expressed at the end of a ctive cell division to counteract acidification of the extracellular enviro nment during fermentative growth. It has been proposed that acid resistance in some enteric bacteria strictly relies on a glutamic acid-dependent syst em. The Escherichia coil chromosome contains distinct genes encoding two bi ochemically identical isoforms of glutamic acid decarboxylase, GadA and Gad B, The gadC gene, located downstream of gadB, has been proposed to encode a putative antiporter implicated in the export of gamma-aminobutyrate, the g lutamic acid decarboxylation product. In the present work, we provide in vi vo evidence that gadC is cc-transcribed with gadB and that the functional g lutamic acid-dependent system requires the activities of both GadA/B and Ga dC, We also found that expression of gad genes is positively regulated by a cidic shock, salt stress and stationary growth phase. Mutations in hns, the gene for the histone-like protein H-NS, cause derepressed expression of th e gad genes, whereas the rpoS mutation abrogates gad transcription even in the hns background. According to our results, the master regulators H-NS an d RpoS are hierarchically involved in the transcriptional control of gad ex pression: H-NS prevents gad expression during the exponential growth wherea s the alternative sigma factor RpoS relieves H-NS repression during the sta tionary phase, directly or indirectly accounting for transcription of gad g enes.