Posttranscriptional Regulation of Ribulose 1,5-bisphosphate Carboxylase Small Subunit Accumulation in Chlamydomonas reinhardtii

The coordinated synthesis of the subunits of ribulose 1,5-bisphosphate carboxylase was examined by analysis of the chloroplast ribosome-deficient mutant of Chlamydomonas reinhardtii, ac20crl. The absence of the chloroplast-synthesized large subunit of this enzyme from cells of this strain is a direct consequence of the lack of chloroplast ribosomes. In contrast, the absence of the cytoplasmically-synthesized small subunit of ribulose 1,5-bisphosphate carboxylase from this mutant is not understood. To discern the cause of this absence, we have compared results of in vivo radioactive labeling experiments with those of cell-free translations of RNA from ac20crl. Protein products from these experiments were identified by one-and two-dimensional electrophoretic analyses. Neither subunit, revealed either as a stained band or by fluorography of proteins radioactively labeled in vivo for 2 hours, was detected in ac20crl. Cell-free translation of polyadenylated RNA obtained from ac20crl, however, revealed wild-type levels of mRNA for the precursor to the small subunit. This messenger was found to be associated with subribosomal RNP and polysomes. We conclude that the absence of the small subunit of ribulose 1,5-bisphosphate carboxylase from ac20crl is the result of a translational or posttranslational event.