A NUCLEAR MUTATION CONFERRING THIOSTREPTON RESISTANCE IN CHLAMYDOMONAS-REINHARDTII AFFECTS A CHLOROPLAST RIBOSOMAL-PROTEIN RELATED TO ESCHERICHIA-COLI RIBOSOMAL-PROTEIN L11

We have isolated a nuclear mutant (tsp-1) of Chlamydomonas,nomonas I e inhardtii which is resistant to thiostrepton, an antibiotic that block s bacterial protein synthesis. The tsp-1 mutant grows slowly in the pr esence or absence of thiostrepton, and its chloroplast ribosomes, alth ough resistant to the drug, are less active than chloroplast ribosomes from the wild type. Chloroplast ribosomal protein L-23 was not detect ed on stained gels or immunoblots of total large subunit proteins from tsp-1 probed with antibody to the wild-type L-23 protein from C. rein hardtii. Immunoprecipitation of proteins from pulse-labeled cells show ed that tsp-1 synthesizes small amounts of L-23 and that the mutant pr otein is stable during a 90 min chase. Therefore the tsp-1 phenotype i s best explained by assuming that the mutant protein synthesized is un able to assemble into the large subunit of the chloroplast ribosome an d hence is degraded over time. L-23 antibodies cross-react with Escher ichia coli r-protein L11, which is known to be a component of the GTPa se center of the 50S ribosomal subunit. Thiostrepton-resistant mutants of Bacillus megaterium and B. subtilis lack L11, show reduced ribosom e activity, and have slow growth rates. Similarities between the thios trepton-resistant mutants of bacteria and C. reinhardtii and the immun ological relatedness of Chlamydomonas L-23 to E. coli L11 suggest that L-23 is functionally homologous to the bacterial r-protein L11.