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 brs-I and bre-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 e 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 brs-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 brc-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. Granick (1948 a) first isolated a mutant of the green alga Chlorella that was blocked in chlorophyll biosynthesis and accumulated protoporphyrin-IX (PROTO) which is known to be a precursor of the iron porphyrin heme. Based on the study of this mutant and a mutant accumulating magnesium protoporphyrin (Granick, 1948 b), Granick (1948 c) then formulated a scheme in which chlorophyll and heme share a common pathway from the synthesis of 6-aminolevulinic acid (ALA) to PROTO. Magnesium is incorporated into PROTO (Granick, 1948 b) to form magnesium protoporphyrin (Mg-PROTO) which is then converted to protochlorophyllide (PCHLD) through a series of steps defined by characterizing additional mutants (Granick, 1950, 1961; Ellsworth and Aronoff, 1968 a, 1969; Aronoff et al., 1971). PCHLD is photoconverted to chlorophyllide (CHLD) in most plants (Boardman, 1966), but some other plants, including Chlamydomonas reinhardtii and Chlorella, can carry out the reduction enzymatically in the dark (Granick, 1950, 1967). Esterification of CHLD with phytoi yields chlorophyll a (Wolff and Price, 1957; Ellsworth and Aronoff, 1968 b). Analysis of porphyrin mutants isolated in the photosynthetic bacterium Rhodopseudornonas support essentially the same biosynthetic pathway up to PCHLD (Lessie and Sistrom, 1964; Lascelles and Altshuler, 1969; Richards and Lascelles, 1969). 806 THE JOURNAL OF CELL BIOLOGY • VOLUME 63, 1974 • p a g e s 8 0 6 8 2 3 on F ebuary 3, 2013 jcb.rress.org D ow nladed fom Published December 1, 1974