Regulation of mRNA stability is an important control in the differential accumulation of chloroplast mRNAs that occurs in response to developmental and environmental signals. The mechanism by which differential mRNA accumulation is achieved is unknown. We have examined mRNA accumulation in a chloroplast mutant of Chlamydomonas reinhardtii previously shown to contain a single AT base-pair deletion in the psaB gene. In this mutant, steady-state levels of mRNA from psaB accumulate to a level more than twice that found in cells that have had the mutation repaired by chloroplast transformation. In vivo pulse labeling of RNA shows that increased mRNA accumulation is due to a more stable transcript. We show that inhibitors of chloroplast protein synthesis also increase mRNA accumulation from the psaB gene. The results are consistent with a link between polysome association, active synthesis and stability of psaB transcripts.

A chloroplast photosystem I reaction center mutation, at-u-g-2.3, of Chlamydomonas reinhardtii has been complemented with a wild type psaB gene to restore photosynthetic competence. The mutation was mapped in the psaB coding sequence by chloroplast transformation using subcloned restriction fragments ofpsaB. The mutation was found to be a single base pair deletion resulting in a reading frame shift and premature termination of the polypeptide. Transformants were verified by insertion of a site-directed mutation which created a new restriction enzyme site. These transformations demontrate the feasibility of insertion of site-directed mutations into the psaB gene in order to elucidate amino acid residues involved in photosystem I assembly and function.