Abstract
Microbial cytochrome c peroxidases (Ccp) have been studied for 75 years, but their physiological roles are unclear. Ccps are located in the periplasms of bacteria and the mitochondrial intermembrane spaces of fungi. In this study, Ccp is demonstrated to be a significant degrader of hydrogen peroxide in anoxic Escherichia coli Intriguingly, ccp transcription requires both the presence of H(2)O(2) and the absence of O(2) Experiments show that Ccp lacks enough activity to shield the cytoplasm from exogenous H(2)O(2) However, it receives electrons from the quinone pool, and its flux rate approximates flow to other anaerobic electron acceptors. Indeed, Ccp enabled E. coli to grow on a nonfermentable carbon source when H(2)O(2) was supplied. Salmonella behaved similarly. This role rationalizes ccp repression in oxic environments. We speculate that micromolar H(2)O(2) is created both biologically and abiotically at natural oxic/anoxic interfaces. The OxyR response appears to exploit this H(2)O(2) as a terminal oxidant while simultaneously defending the cell against its toxicity.