ALTERED EXPRESSION OF NUCLEAR GENES ENCODING CHLOROPLAST POLYPEPTIDESIN NONPHOTOSYNTHETIC MUTANTS OF CHLAMYDOMONAS-REINHARDTII - EVIDENCE FOR POST TRANSCRIPTIONAL REGULATION

In photoautotrophic organisms it is well documented that the expressio n of nuclear genes encoding plastid proteins can be regulated at vario us levels. We present here the analysis of a non-photosynthetic strain (CC1051) of the green unicellular alga Chlamydomonas reinharntii; thi s strain carries a mutation in the newly identified Cen gene involved in the co-regulated expression of several different nuclear genes enco ding plastid proteins. We performed a differential screening strategy to isolate cDNAs corresponding to genes that are differentially expres sed in mutant and wild-type strains. Extensive hybridization experimen ts revealed that the 15 cDNA clones isolated represent five different mRNAs that fail to accumulate in the non-photosynthetic mutant. Compar ative analysis of DNA sequencing data showed that they all code for pl astid proteins. In particular, we identified genes for the chlorophyll a/b binding protein of the light-harvesting complex II (LHCII), for s ubunits II and III of photosystem I (PsaD, PsaF), for pentose-5-phosph ate 3-epimerase (PPE), an enzyme of the Calvin cycle, and for an unide ntified 7 kDa protein with a suggested lumenal location. With the exce ption of the gene for LHCII, all proteins are encoded by single-copy g enes. Evidence from run-on transcription experiments is presented show ing that expression of the above mentioned plastid proteins is affecte d at the post-transcriptional level in the mutant strain CC1051 with a defect in the Cen gene. Our results suggest that the product of the C en gene is involved in stabilization and/or processing of transcripts from nuclear genes encoding chloroplast proteins.