Ribosomes bound to chloroplast membranes in Chlamydomonas reinhardtii

Abstract

The amount of chloroplast ribosomal RNAs of Chlamydomonas reinhardtii which sediment at 15,000 g is increased when cells are treated with chloramphenicol. Preparations of chloroplast membranes from chloramphenicol-treated cells contain more chloroplast ribosomal RNAs than preparations from untreated cells. The membranes from treated cells also contain more ribosome-like particles, some of which appear in polysome-like arrangements. About 50% of chloroplast ribosomes are released from membranes in vitro as subunits by 1 mM puromycin in 500 mM KCl. A portion of chloroplast ribosomal subunits is released by 500 mM KCl alone, a portion by 1 mM puromycin alone, and a portion by 1 mM puromycin in 500 mM KCl. Ribosomes are not released from isolated membranes by treatment with ribonuclease. Membranes in chloroplasts of chloramphenicol-treated cells show many ribosomes associated with membranes, some of which are present in polysome-like arrangements. This type of organization is less frequent in chloroplasts of untreated cells. Streptogramin, an inhibitor of initiation, prevents chloramphenicol from acting to permit isolation of membrane-bound ribosomes. Membrane-bound chloroplast ribosomes are probably a normal component of actively growing cells. The ability to isolate membrane-bound ribosomes from chloramphenicol-treated cells is probably due to chloramphenicol-prevented completion of nascent chains during harvesting of cells. Since chloroplasts synthesize some of their membrane proteins, and a portion of chloroplast ribosomes is bound to chloroplast membranes through nascent protein chains, it is suggested that the membrane-bound ribosomes are synthesizing membrane protein.