Regulation of hydroperoxidase (catalase) expression in Escherichia coli

As part of its adaptive response to oxidative stress, Escherichia coli produces two inducible hydroperoxidases called HPI and HPII. Upon exposure to sublethal levels of hydrogen peroxide, HPI expression is induced at the transcriptional level by OxyR, a member of the LysR family of autoregulators. OxyR, functioning as both a sensor and transducer, contains a critical redox-sensitive Cys residue that is oxidized by hydrogen peroxide. This is thought to induce a conformational change in the tertiary structure of the OxyR tetramer altering its DNA-binding specificity and resulting in an increase in the transcription of katG and several other OxyR-dependent genes. In contrast, synthesis of the HPII enzyme is not induced by hydrogen peroxide. Expression of both HPI and HPII is growth phase-dependent levels of HPI and HPII are 10-fold higher in stationary phase than exponential phase cultures. These growth phase-dependent increases are largely dependent on RpoS, a stationary phase specific sigma factor that is itself subject to complex transcriptional and post-transcriptional controls. Several metabolic signals have been proposed to activate the RpoS regulon including hyperosmolarity, weak acids, homoserine lactone and UDP-glucose. Since both HPI and HPII are members of the RpoS regulon, elucidation of the mechanism of regulation of RpoS should contribute to our general understanding of hydroperoxidase regulation.