Carbon Dioxide Metabolism and its Regulation in Nonsulfur Purple Photosynthetic Bacteria

Abstract

Nonsulfur purple (NSP) photosynthetic bacteria are able to photosynthetically metabolize and grow at the expense of a wide range of reduced and oxidized organic compounds as well as using inorganic carbon dioxide under anaerobic conditions. In addition, these organisms, for the most part, are able to oxidize these compounds under aerobic conditions as well. Thus, NSP bacteria are widely dispersed and survive in diverse environments. Carbon dioxide reduction is important for both photoheterotrophic and photoautotrophic metabolism, and CO2 serves as an essential electron acceptor for the maintenance of cellular redox homeostasis when highly reduced organic carbon is used as the electron donor for growth. Since earlier reviews, much progress towards elucidating molecular mechanisms governing carbon dioxide assimilation has been made, primarily in two representative species, Rhodobacter (Rba.) sphaeroides and Rba. capsulatus. These studies established the importance of the main transcriptional regulator, CbbR, and its interaction with specific promoter sequences. Furthermore, the redox-sensitive two-component global regulator system, RegAB/PrrAB, was shown to be important for controlling cbb operon gene expression and appeared to be linked to the regulation of redox balancing mechanisms. A further level of complexity is found in Rhodopseudomonas (Rps.) palustris, where a unique three-protein two-component system, in addition to CbbR, was recently shown to contribute to the regulation of CO2 fixation. This chapter will focus on recent progress made in understanding the mechanism of regulation of CO2 metabolism in NSP bacteria.