A redox-responsive pathway for aerobic regulation of photosynthesis gene expression in Rhodobacter sphaeroides 2.4.1

Abstract

To further understand the proposed signal transduction pathway involving the presumed redox proteins RdxBH and cbb3 cytochrome oxidase in Rhodobacter sphaeroides 2.4.1, a series of mutants lacking components of both the Prr two-component activation system and the cbb3-type cytochrome oxidase or RdxBH were constructed. We report that under highly aerobic conditions, aberrant photosynthesis gene expression and spectral complex formation typical of cbb3- or RdxBH-deficient mutants were no longer observed when either prrA (encoding the response regulator of the Prr system) or prrB (encoding the presumed sensor kinase) was also deleted. These double-mutant strains are phenotypically identical to single-mutant PrrA and PrrB strains, suggesting that the signal(s) originating from the cbb3 terminal oxidase affects downstream puc and puf operon expression by acting exclusively through the Prr system. When the same double-mutant strains were examined under anaerobic dark dimethyl sulfoxide growth conditions, photosynthesis gene expression was obligatorily linked to the two-component activation system. However, photosynthesis gene expression under the same growth conditions was significantly higher in the cbb3 mutant strain when compared to that in the wild type. Similarly, under anaerobic photosynthetic conditions the high levels of the oxidized carotenoid, spheroidenone, which accumulate in cbb3-deficient mutants were nearly restored to normal in a PrrB- CcoP- double mutant. This observation, together with previously published results, suggests that the regulation of the CrtA-catalyzed reaction possesses both transcriptional and posttranscriptional regulatory effectors. We propose that the cbb3 cytochrome oxidase, which by definition can interact with external oxygen, serves to control the activity of the Prr two-component activation system under both aerobic and anaerobic conditions. Although independent from the cbb3 oxidase, the RdxBH proteins are also required for normal functioning of the Prr two-component activation system and are therefore believed to lie between the cbb3 oxidase in this oxygen-sensing, redox signaling pathway and the Prr activation system.

FIG. 1

Spectral complex formation under aerobic (A) and anaerobic (B) conditions of growth. Cells were grown as described in Materials and Methods and harvested at OD₆₀₀ values of approximately 0.2 and 0.5, respectively. The amounts of light-harvesting complexes were expressed as nanomoles of spectral complex per milligram of crude membrane protein. Experiments were performed in duplicate, and standard deviations were ≤15% (Table 2). Filled bars represent LHII (B800-850 light-harvesting complex); open bars represent LHI (B875 light-harvesting complex).

FIG. 2

β-Galactosidase values under aerobic (A) and anaerobic (B) conditions of growth. Strains contain the puf::lacZ and puc::lacZ transcriptional fusions in pUI1663 and pCF200Km, respectively, in trans. Experiments were performed in duplicate, and standard deviations were ≤20%. Filled bars represent the puc::lacZ fusion; open bars represent the puf::lacZ fusion. β-Galactosidase values are expressed in micromoles per minute per milligram of protein.