Synthesis of pigment‐binding protein in toluene‐treated Rhodopseudomonas capsulata and in cell‐free systems

Pigment‐binding protein of the facultatively phototrophic bacterium Rhodospeudomonas capsulata could be selectively synthesized in toluene‐treated cells as well as in homologous and heterologous cell‐free translation systems by isolated polysomes. It is shown that the pigment‐binding polypeptides of the light‐harvesting complexes are encoded by messenger RNA of extreme longevity. The dependence of their synthesis on the concomitant synthesis of tetrapyrroles was demonstrated in the toluene‐treated cells. The large M r ‐28000 polypeptide of the reaction center and the M r ‐10000 pigment‐binding polypeptide of the light‐harvesting complex II were found to be synthesized by free (water‐soluble) polysomes without a cleavable ‘leader’ or ‘signal’ peptide [reviewed by W. Wickner (1979) Annu. Rev. Biochem. 48 , 23–45]. The M r ‐10000 polypeptide, as synthesized in vitro , was studied in more detail. Unlike the membrane‐assembled polypeptide in vivo it was insoluble in an organic solvent mixture (chloroform/methanol 1:1, v/v). After detergent denaturation in the presence of membrane isolated from the organism it became organic‐solvent‐soluble. Obviously the polypeptide could be induced to assume alternative conformations in which its apolar residues were either exposed to the solvent or buried within. These findings, in agreement with Wickner's hypothesis, indicate that the M r ‐10000 polypeptide may enter the lipid bilayer by a ‘membrane‐triggered’ conformational change.