Light and Genetic Determinants in the Control of Specific Chloroplast Transcripts in Chlamydomonas reinhardtii

We have looked for chloroplast genes whose expression is controlled by light by comparing the abundance of specific chloroplast transcripts in light-grown and dark-grown cells of Chlamydomonas reinhardtii. In addition, we have investigated whether genetic components influence expression of such genes.Northern blot analyses of specific separated transcripts showed that in certain strains (e.g. CC-278), several chloroplast transcripts from the Hpa II 5 region of the chloroplast chromosome were more abundant in dark-grown cells than in light-grown cells. The increased abundance of these Hpa II 5-specific transcripts was dependent on the genetic background: we have isolated mutants and we have found distantly related wild type strains which do not overaccumulate these transcripts in the dark.The strains which overaccumulated the Hpa II 5-specific transcripts, and only these strains, died after several cell divisions following transfer to the dark. Overaccumulation appears to be a necessary but not sufficient prerequisite for commitment to cell death in the dark.By analogy to bacterial systems, we speculate that a critical event associated with accumulation of one of these specific transcripts involves control at the level of transcription initiation or termination-antitermination.