Identification of a regulator that controls stationary-phase expression ofcatalase-peroxidase in Caulobacter crescentus

Expression of the catalase-peroxidase of Caulobacter crescentus, a gram-neg ative member of the or subdivision of the Proteobacteria, is 50-fold higher in stationary-phase cultures than in exponential cultures. To identify reg ulators of the starvation response, Tn5 insertion mutants were isolated wit h reduced expression of a katG::lacZ fusion on glucose starvation, One inse rtion interrupted an open reading frame encoding a protein with significant amino acid sequence identity to TipA, a helix-turn-helix transcriptional a ctivator in the response of Streptomyces lividans to the peptide antibiotic thiostrepton, and lesser sequence similarity to other helix-turn-helix reg ulators in the MerR family. The C. crescentus orthologue of tipA was named skgA (stationary-phase regulation of katG). Stationary-phase expression of katG was reduced by 70% in the skgA::Tn5 mutant, and stationary-phase resis tance to hydrogen peroxide decreased by a factor of 10. Like the wild type, the skgA mutant exhibited starvation-induced cross-resistance to heat and acid shock, entered into the helical morphology that occurs after 9 to 12 d ays in stationary phase, and during exponential growth induced katG in resp onse to hydrogen peroxide challenge. Expression of skgA increased 5- to 10- fold in late exponential phase. skgA is the first regulator of a starvation -induced stress response identified in C. crescentus. SkgA is not a global regulator of the stationary-phase stress response; its action encompasses t he oxidative stress-hydrogen peroxide response but not acid or heat respons es. Moreover, SkgA not an alternative sigma factor, like RpoS, which contro ls multiple aspects of starvation-induced cross-resistance to stress in ent eric bacteria. These observations raise the possibility that regulation of stationary-phase gene expression in this member of the alpha subdivision of the Proteobacteria is different from that in Escherichia coli and other me mbers of the gamma subdivision.