The role of pigments in the assembly of photosynthetic complexes in Rhodobacter capsulatus

Mutants of Rhodobacter capsulatus, blocked at different steps of bacteriochlorophyll a (BChl) synthesis between protoporphyrin IX and 2‐hydroxyethyl bacteriochlorophyllide a, were induced to synthesize the photosynthetic apparatus by lowering of oxygen tension in dark cultures. The cells were pulse‐labeled with [35S]methionine and the radioactivity chased after dilution of [35S]. The specific radioactivity in the pigment‐binding proteins of light‐harvesting and reaction center proteins of the wild‐type strain was not lowered during the chase period of three hours but in the BChl‐free mutants the label disappeared within five to thirty minutes. The polypeptides were inserted into the membrane but did not remain stably incorporated. In the mutant strain NK9 the synthesis of the carotenoid spheroidenone/spheroidene was inhibited by insertion of Tn5 in the crtI gene (phytoene desaturase), which blocked completely the formation of the light‐harvesting (LH) complex II (B800–850) but not of the LHI (B870) complex. In this mutant the polypeptides of the LH complexes were synthesized in a lower amount than in the wild‐type cells and were inserted into the membrane. The LHIIα poly‐peptide disappeared after 60 min of chase while the LHIIβ was more stable. It was concluded that the pigments are not only necessary for absorption of photons and efficient transfer of excitation energy but have a structural role by stabilizing the oligomeric LH complexes. This is in accordance with the crystal structure data of LHII complexes.