Synthesis and degradation of unassembled polypeptides of the coupling factor of photophosphorylation CF 1 in 70S ribosome‐deficient rye leaves

The formation of polypeptides of the coupling factor CF 1 was investigated in 70S ribosome‐deficient rye leaves generated by growing the plants at a non‐permissive elevated temperature of 32°C, in order to analyse mechanisms coordinating subunit accumulation. Antibodies were raised in rabbits against total CF 1 as well as against its five individual subunits purified from chloroplast thylakoids from rye leaves. Several immunological techniques applying these antibodies (immunoprecipitation, immunoblotting, antibody affinity chromatography) were unable to detect the presence of any of the CF 1 subunits in heat‐treated 70S ribosome‐deficient leaves. After in vivo labeling with l ‐[ 35 S]methionine and subsequent immunoprecipitation, however, radioactivity was found to be incorporated into the subunits γ and δ, but not into α, β and ɛ, in 70S ribosome‐deficient leaves, demonstrating the cytoplasmic synthesis of CF 1 ‐γ and CF 1 ‐δ. Chase experiments after in vivo labeling with L‐[ 35 S]methionine indicated that the unassembled subunits γ and δ were rapidly and preferentially degraded, while they were stabilized when integrated into the complete CF 1 complex in normal green leaves from permissive growth conditions. The apparent half‐times of the unassembled subunits were 2 h for CF 1 ‐γ and 4 h for CF 1 ‐δ in 32°C‐grown leaves. Several other, stromal, plastid proteins of cytoplasmic origin were stable in 32°C‐grown leaves during the period of chase. In etiolated leaves total CF 1 , including all subunits, appeared to be less stable than in green leaves grown under permissive temperature conditions in light. Rapid degradation of the excess of unassembled subunits is regarded as an important mechanism ensuring a constant stoichiometry and apparently synchronous development of CF 1 subunits.