Identification of a Regulator That Controls Stationary-Phase Expression of Catalase-Peroxidase in Caulobacter crescentus

Expression of the catalase-peroxidase of Caulobacter crescentus, a gram-negative member of the α subdivision of the Proteobacteria, is 50-fold higher in stationary-phase cultures than in exponential cultures. To identify regulators of the starvation response, Tn5 insertion mutants were isolated with reduced expression of a katG::lacZ fusion on glucose starvation. One insertion interrupted an open reading frame encoding a protein with significant amino acid sequence identity to TipA, a helix-turn-helix transcriptional activator in the response of Streptomyces lividans to the peptide antibiotic thiostrepton, and lesser sequence similarity to other helix-turn-helix regulators 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 resistance 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 days in stationary phase, and during exponential growth induced katG in response 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 the oxidative stress-hydrogen peroxide response but not acid or heat responses. Moreover, SkgA is not an alternative ς factor, like RpoS, which controls multiple aspects of starvation-induced cross-resistance to stress in enteric bacteria. These observations raise the possibility that regulation of stationary-phase gene expression in this member of the α subdivision of the Proteobacteria is different from that in Escherichia coli and other members of the γ subdivision.

新月柄杆菌（Caulobacter crescentus）是变形菌门（Proteobacteria）α亚群的革兰氏阴性菌，其过氧化氢酶-过氧化物酶（catalase-peroxidase）的表达在静止期培养物中是指数期培养物的50倍。为鉴定饥饿应答的调控因子，研究人员在葡萄糖饥饿条件下分离得到katG::lacZ融合基因表达水平降低的Tn5插入突变体。其中一个插入突变破坏了一个开放阅读框（open reading frame，ORF），其编码的蛋白与变铅青链霉菌（Streptomyces lividans）对肽类抗生素硫链丝菌素（thiostrepton）应答中的螺旋-转角-螺旋（helix-turn-helix）转录激活因子TipA具有显著的氨基酸序列同源性，与MerR家族其他螺旋-转角-螺旋调控因子的序列相似性则相对较低。新月柄杆菌中tipA的直系同源基因被命名为skgA（katG的静止期调控因子，stationary-phase regulation of katG）。在skgA::Tn5突变体中，katG的静止期表达量下降70%，菌株对过氧化氢的静止期抗性降低至原水平的1/10。与野生型菌株相比，skgA突变体仍可表现出饥饿诱导的热激与酸激交叉抗性，能够进入在静止期培养9~12天后出现的螺旋形态，且在指数生长期仍可通过过氧化氢刺激诱导katG的表达。skgA的表达在指数生长后期上调5~10倍。skgA是首个在新月柄杆菌中被鉴定的饥饿诱导应激应答调控因子。SkgA并非静止期应激应答的全局调控因子：其调控范围仅涵盖氧化应激-过氧化氢应答，而非酸激或热激应答。此外，SkgA并非诸如RpoS这类的替代性σ因子——RpoS可调控肠杆菌中多种饥饿诱导的应激交叉抗性。上述研究结果提示，在变形菌门α亚群的该类细菌中，静止期基因表达的调控机制与大肠杆菌（Escherichia coli）及变形菌门γ亚群的其他成员存在差异。