Unstable chloroplast ribosomes in the cold-sensitive barley mutanttigrina-o 34

Abstract

When grown at 20 °C the recessive nuclear gene mutanttigrina-o 34 in barley is deficient in 70S chloroplastic ribosomes, but contains wild-type levels of 80S cytoplasmic ribosomes. When grown at 30 °C the mutant seedlings have wild-type levels of both chloroplastic and cytoplasmic ribosomes. One-dimensional polypeptide patterns of chloroplast ribosomes of the mutant grown at the permissive temperature were the same as those of the wild-type.Instability of chloroplastic ribosomes in the barley mutant grown at the restrictive temperature of 20 °C was correlated with the deficiency of chlorophylla-proteins 1,2 and 3, cytochromef, coupling factor subunits α, β, ε and two thylakoid polypeptides (31, 20 kD) of unidentified function. These proteins have been shown in other species to be synthesized on chloroplastic ribosomes. Chlorophylla/b-protein 2 synthesized on cytoplasmic ribosomes was present in wild-type amounts in the plastids of the mutant grown at the restrictive temperature. Five additional thylakoid polypeptides. (A, B, C, D, E) with as yet unknown function were also present at the restrictive temperature in normal amounts and are therefore predicted to be synthesized in the cytosol.Electron microscopical analysis revealed that the chlorophylla/b-protein 2 and other prominent thylakoid polypeptides in the mutant at 20 °C most likely are contained in the large dense grana and other abnormal membrane structures typical for the mutant.