The chloroplast ATP synthase in Chlamydomonas reinhardtii. II. Biochemical studies on its biogenesis using mutants defective in photophosphorylation.

We have carried out an analysis of the synthesis, cellular accumulation, and membrane binding of the chloroplast-encoded subunits of the ATP synthase (alpha, beta, epsilon, I, III, and IV) in several mutants of Chlamydomonas reinhardtii defective in photophosphorylation. These data gave some insight on the putative genetic lesion in each mutant and allowed some characterization of the assembly and stabilization of the ATP synthase complex in the thylakoid membranes. Four chloroplast mutants are likely to be altered in chloroplast structural genes coding for coupling factor (CF) 1 subunits beta and epsilon and for CF0 subunits I and IV. A fifth chloroplast mutant and three nuclear mutants were altered in genes regulating either transcription or translation of chloroplast genes coding for CF1 subunits alpha and beta and CF0 subunits III and IV. Evidence is presented (i) for a control of the rate of synthesis of subunit beta by subunit alpha in the absence of ATP synthase assembly and (ii) for an interaction between alpha and beta subunits in the stroma of the chloroplast which protects alpha subunits from proteolytic degradation. The role of several chloroplast-encoded subunits of CF0 and CF1 in the stabilization of partially assembled ATP synthase is discussed. We conclude that in the absence of ATP synthase assembly, CF0 cannot accumulate in the thylakoid membranes, whereas alpha and beta subunits, presumably engaged in soluble CF1, can accumulate in the stroma of the chloroplast.