The orf162b sequence of Rhodobacter capsulatus encodes a protein required for optimal levels of photosynthetic pigment-protein complexes

The orf162b sequence, the second open reading frame 3' of the reaction cent er (RC) EI protein gene puhA in the Rhodobacter capsulatus photosynthesis g ene cluster, is shown to be transcribed from a promoter located 5' of puhA. A nonpolar mutation of orf162b was generated by replacing most of the codi ng region with an antibiotic resistance cartridge. Although the mutant stra in initiated rapid photosynthetic growth, growth slowed progressively and c ultures often entered a pseudostationary phase. The amounts of the RC and l ight harvesting complex I (LHI) in cells obtained from such photosythetic c ultures were abnormally low, but these deficiencies were less severe when t he mutant was groan to a pseudostationary phase induced by low aeration in the absence of illumination. The orf162b mutation did not significantly aff ect the expression of a pufB::lacZ translationally in-frame gene fusion und er the control of the puf promoter, indicating normal transcription and tra nslation of RC and LHI genes. Spontaneous secondary mutations in the strain with the orf162b disruption resulted in a bypass of the photosynthetic gro wth retardation and reduced the level of light harvesting complex ZI. These results and the presence of sequences similar to orf1626 in other species indicate that the Orf162b protein is required for normal levels of the phot osynthetic apparatus in purple photosynthetic bacteria.