yellow-in-the-dark mutants of chlamydomonas lack the CHLL subunit of light-independent protochlorophyllide reductase

Light-independent protochlorophyllide reduction leading to chlorophyll form ation in the dark requires both chloroplast and nuclear gene expression in Chlamydomonas reinhardtii. Mutations in any one of the plastid (chlL, chlN, and chlB) or nuclear (y-1 to y-10) genes required for this process result in the phenotype of the yellow-in-the-dark or y mutants. Analysis of the ch it, chlN, and chlB transcript levels in both light- and dark-grown wild-typ e and y mutant cells showed that the y mutations have no effect on the tran scription of these plastid genes. Protein gel blot analysis showed that the CHLN and CHLB proteins are present in similar amounts in light- and dark-g rown wild-type cells, whereas CHLL is present only in wild-type cells grown in the dark or at light intensities less than or equal to 15 pmol m(-2) se c(-1). Analysis of chit transcript distribution on polysome profiles and ra tes of protein turnover in chloramphenicol-treated cells suggested that CHL L formation is most probably blocked at translation initiation or elongatio n. Furthermore, treatment of cells with metabolic inhibitors and uncouplers of photosynthetic electron transport showed that regulation of CHLL format ion is linked to the physiologic status of the chloroplast. Similar to wild -type cells, y mutants contain nearly identical amounts of CHLN and CHLB wh en grown in either light or darkness. However, no CHLL is present in any of the y mutants except y-7, which contains an immunoreactive CHLL smaller th an the expected size. Our findings indicate that CHLL translation is negati vely photoregulated by the energy state or redox potential within the chlor oplast in wild-type cells and that nuclear y genes are required for synthes is or accumulation of the CHLL protein.