Altered expression of nuclear genes encoding chloroplast polypeptides in non-photosynthetic mutants of Chlamydomonas reinhardtii: evidence for post-transcriptional regulation.

In photoautotrophic organisms it is well documented that the expression of nuclear genes encoding plastid proteins can be regulated at various levels. We present here the analysis of a non-photosynthetic strain (CC1051) of the green unicellular alga Chlamydomonas reinhardtii; this strain carries a mutation in the newly identified Cen gene involved in the co-regulated expression of several different nuclear genes encoding plastid proteins. We performed a differential screening strategy to isolate cDNAs corresponding to genes that are differentially expressed in mutant and wild-type strains. Extensive hybridization experiments revealed that the 15 cDNA clones isolated represent five different mRNAs that fail to accumulate in the non-photosynthetic mutant. Comparative analysis of DNA sequencing data showed that they all code for plastid proteins. In particular, we identified genes for the chlorophyll a/b binding protein of the light-harvesting complex II (LHCII), for subunits II and III of photosystem I (PsaD, PsaF), for pentose-5-phosphate 3-epimerase (PPE), an enzyme of the Calvin cycle, and for an unidentified 7 kDa protein with a suggested lumenal location. With the exception of the gene for LHCII, all proteins are encoded by single-copy genes. Evidence from run-on transcription experiments is presented showing that expression of the above mentioned plastid proteins is affected 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 Cen gene is involved in stabilization and/or processing of transcripts from nuclear genes encoding chloroplast proteins.