Interactive control of Rhodobacter capsulatus redox-balancing systems during phototrophic metabolism

Abstract

In nonsulfur purple bacteria, redox homeostasis is achieved by the coordinate control of various oxidation-reduction balancing mechanisms during phototrophic anaerobic respiration. In this study, the ability of Rhodobacter capsulatus to maintain a balanced intracellular oxidation-reduction potential was considered; in addition, interrelationships between the control of known redox-balancing systems, the Calvin-Benson-Bassham, dinitrogenase and dimethyl sulfoxide reductase systems, were probed in strains grown under both photoheterotrophic and photoautotrophic growth conditions. By using cbb(I) (cbb form I operon)-, cbb(II)-, nifH-, and dorC-reporter gene fusions, it was demonstrated that each redox-balancing system responds to specific metabolic circumstances under phototrophic growth conditions. In specific mutant strains of R. capsulatus, expression of both the Calvin-Benson-Bassham and dinitrogenase systems was influenced by dimethyl sulfoxide respiration. Under photoheterotrophic growth conditions, coordinate control of redox-balancing systems was further manifested in ribulose 1,5-bisphosphate carboxylase/oxygenase and phosphoribulokinase deletion strains. These findings demonstrated the existence of interactive control mechanisms that govern the diverse means by which R. capsulatus maintains redox poise during photoheterotrophic and photoautotrophic growth.

FIG. 1

cbb_I::lacZ (A) and cbb_II::lacZ (B) promoter activities during photoheterotrophic growth of R. capsulatus CBB-deficient strains. β-Galactosidase activities were determined in three or four independent cultures assayed in duplicate. NG indicates no growth under photoheterotrophic growth conditions with ammonia as the nitrogen source in the absence of DMSO.

FIG. 2

nifH::lacZ promoter activity during photoheterotrophic growth of R. capsulatus with either ammonia or glutamate as the nitrogen source. β-Galactosidase activities were determined in four or five independent cultures assayed in duplicate. NG indicates no growth under photoheterotrophic growth conditions with ammonia as the nitrogen source in the absence of an ancillary electron acceptor (DMSO).

FIG. 3

dorC::lacZ promoter activity of R. capsulatus under photoheterotrophic growth conditions with or without DMSO. β-Galactosidase activities were determined in four or five independent cultures assayed in duplicate. NG indicates no growth in the absence of DMSO under conditions with ammonia as the nitrogen source.

FIG. 4

cbb_II::lacZ (A) and dorC::lacZ (B) promoter activities in wild-type R. capsulatus strain SB1003 and strain SBI/II during photoheterotrophic growth with either DMSO or TMAO as the supplied exogenous electron acceptor in the presence of either ammonia or glutamate as the nitrogen source.

FIG. 5

Mechanisms involved in adaptation of the efficiency of energy conservation to intracellular requirements under photoautotrophic (A) and photoheterotrophic (B) growth conditions. Cyclic photosynthetic electron transport and the specific redox-balancing mechanisms of the CBB, DMSOR, and nitrogenase systems contribute to redox homeostasis in R. capsulatus. NADH generated by the oxidation of carbon substrates or reverse electron flow via complex I is dissipated by the CBB system (A and B) and perhaps the nitrogenase system (B). Flux of reductant from the ubiquinone pool is transduced to the DMSOR system under phototrophic growth conditions (A and B). The dotted line indicates the influence of DMSO reduction on specific redox-balancing mechanisms under phototrophic environmental conditions (A and B).