Enzyme Activity, Maturation and Regulation of Anaerobic Reductases in Shewanella Oneidensis MR-1

Abstract

Shewanella oneidensis MR-1 is a metal-reducing bacterium capable of using a wide range of terminal electron acceptors. These include oxygen, metal oxides and organic compounds such as dimethyl sulfoxide (DMSO) and fumarate. In addition, several nitrogen and sulfur based compounds can be used as terminal electron acceptors, including sulfite, for which the terminal reductase was recently identified as an octaheme c-type cytochrome that contains an atypical heme binding site. In this study, several additional components involved in sulfite reduction were identified. These include SirCD that form a membrane-bound electron-transferring complex with SirA, SirBI that appear to be involved in protein folding and SirEFG that is involved in the specific attachment of heme to SirA. In addition to the heme sythetase SirEF, and apo-cytochrome c chaperon SirG, an additional chaperon, CcmI, was shown to be involved in heme attachment to the atypical site of SirA. Surprisingly, CcmI was required for the maturation of the nitrite reductase, NrfA, which contains an atypical heme binding site, and plays a role in maturation of other c cytochromes required for respiration. Regulation of the many terminal reductases was also found to be complex and unlike that in other organisms studied to date. Promoter activities, transcript levels and enzyme activities of several terminal reductases were analyzed in cells grown in the presence of different electron acceptors and revealed regulation that occurs both transcriptionally and post-transcriptionally. It was found that the regulation of some reductases is more tightly regulated than others, and in addition, the availability of some electron acceptors has a more pronounced regulatory affect than others.