Summary: Erythromycin-resistant mutants mapping to the Mendelian -M2 locus of were cold-sensitive for growth at 15 °C. The phenotypes erythromycin-resistance and cold-sensitivity could not be separated in crosses.

Mutants at the -M2 locus were previously found to have chloroplast ribosomes with altered [C]erythromycin binding capacity (Mets & Bogorad, 1971). When grown at 25 °C, haploid strains containing -M2 alleles alone or in combination with other loci had fewer chloroplast ribosomes than the wild type. A further reduction occurred when -M2 strains were shifted to 15 °C. Diploids containing both -M2 and its wild-type allele contained normal amounts of chloroplast ribosomes at 25 and 15°C, and were cold-resistant and antibiotic-sensitive.

The inability of chloroplast ribosome-deficient -M2 strains to grow at 15 °C in the presence of a reduced carbon source indicates that chloroplast protein synthesis is essential for cellular growth even when photosynthesis is not essential.


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