DIFFERENTIAL REGULATION OF CHLOROPLAST GENE-EXPRESSION IN CHLAMYDOMONAS-REINHARDTII DURING PHOTOACCLIMATION - LIGHT STRESS TRANSIENTLY SUPPRESSES SYNTHESIS OF THE RUBISCO LSU PROTEIN WHILE ENHANCING SYNTHESIS OF THE PS-II D1 PROTEIN

Transfer of Chlamydomonas reinhardtii cells grown photoautotrophically in low light to higher light intensities has a dramatic transient eff ect on the differential expression of the two major chloroplast encode d photosynthetic proteins. Synthesis of the D1 protein of Photosystem II increases more than 10-fold during the first six hours in high ligh t (HL), whereas synthesis of the large subunit (LSU) of Rubisco drops dramatically within 15 min and only gradually resumes at about 6 h. Sy nthesis of the chloroplast-encoded ATP synthase beta subunit, the nucl ear-encoded Rubisco small subunit and the nuclear-encoded beta-tubulin is not noticeably affected. Up regulation of psbA mRNA translation ac counts for a substantial fraction of the increased Di synthesis, since accumulation of psbA mRNA increases 4.2- and 6.3-fold less than D1 sy nthesis at 6 and 18 h in HL. Down-regulation of LSU synthesis is not c orrelated with a reduction in the steady-state level of the rbcL trans cript. Primer extension mapping of the 5' ends of the rbcL mRNAs revea ls transcripts with start points located at -93 and -168 relative to t he first translated ATG. Transfer of low light (LL)-grown cells to HL temporarily decreases the ratio of the -93 to -168 transcripts, but th is ratio normalizes after 6 h in HL, coincident with the recovery in t he synthesis of LSU. These several distinct effects of temporary Light stress were correlated with a rapid, sustained increase in the reduct ion state of Q(A), a transient decline in photosynthetic efficiency, a less rapid drop in total chlorophyll content and a delay in cell divi sion.