Relation between chloroplast genome copy number, transcription, transcript abundance and translation rate in Chlamydomonas reinhardtii

Abstract

In order to unveil possible relations between chloroplast genome copy number, transcription, transcript abundance and protein synthesis rates in Chlamydomonas reinhardtii, we conducted two sets of experiments. We first tried to establish a possible correlation between the genome copy number and the protein synthesis rates in the chloroplast. Using the drug 5-fluoro-deoxyuridine, we showed that a twentyfold reduction in chloroplast genome copy number, although differentially affecting mRNA accumulation levels, was not echoed by differences in chloroplast protein synthesis rates. This result implies that ploidy is not limiting for the expression of the chloroplast genome, at least for the duration of our experiment. We then tried to unveil the possible relation between overall chloroplast mRNA accumulation levels and protein synthesis rates for cells grown either under heterotrophic or phototrophic growth conditions. After six hours of treatment with rifampicin (an inhibitor of chloroplast transcription) chloroplast transcripts displayed various behaviors in regard to their accumulation levels. It also appeared that rifampicin had a more pronounced effect on chloroplast transcript accumulation under phototrophic conditions than under heterotrophic conditions. Anyhow, the changes in mRNA abundance in either growth condition were not reflected in changes in chloroplast protein synthesis rates. Our results thus show that no direct correlation between genome copy number, transcription, transcript accumulation levels and protein synthesis rates could be found in the chloroplast of Chlamydomonas reinhardtii, pointing to the existence of a multitude of posttranscriptional regulatory steps, that are essential for the expression of the chloroplast genome.