Chloroplast protein synthesis in Chlamydomonas reinhardtii is dispensable when cells are provided acetate as a carbon source. Mutants defective in synthesis, assembly, or function of chloroplast ribosomes are therefore conditionally viable. Positive selection of nonphotosynthetic cells on arsenate has been combined with a simple screening procedure to isolate mutants with a broad spectrum of defects in chloroplast protein synthesis. Eight new mutants deficient in chloroplast ribosomes have been isolated. Three of these have been characterized genetically and phenotypically, and compared with two previously described ribosome mutants, ac-20 and cr-1. A working model of ribosome assembly is proposed which suggests possible biochemical roles for these five Mendelian gene loci.

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