GENETIC CONTROL OF CHLOROPHYLL BIOSYNTHESIS IN CHLAMYDOMONAS : Analysis of Mutants at Two Loci Mediating the Conversion of Protoporphyrin-IX to Magnesium Protoporphyrin

In this report we describe two nonallelic Mendelian protoporphyrin accumulating mutants br (s)-1 and br (c)-1. Results of experiments with these mutants lead us to postulate that porphyrin biosynthesis branches into light and dark steps between protoporphyrin-IX and magnesium protoporphyrin. We hypothesize that the br (c) locus controls a dark step while the br (s) locus either controls a step in the main pathway before the branch or mediates the preparation of the magnesium ion for its insertion into protoporphyrin-IX. The br (s)-1 mutant is thought to be light sensitive because a block prior to the branch point in the porphyrin pathway prevents chlorophyll formation in either the light or the dark. The br (c)-1 mutant, which also accumulates protoporphyrin in the dark, forms chlorophyll and chloroplast lamellae when transferred to the light, showing that function of the porphyrin pathway is normal in the light.