RUBISCO, RUBISCO ACTIVASE AND RIBULOSE-5-PHOSPHATE KINASE GENE-EXPRESSION AND POLYPEPTIDE ACCUMULATION IN A TOBACCO MUTANT DEFECTIVE IN CHLOROPLAST PROTEIN-SYNTHESIS

Expression of the genes for ribulose-1,5-bisphosphate carboxylase/oxyg enase (Rubisco; rbcS and rbcL), Rubisco activase (rca) and ribulose-5- phosphate (Ru5-P) kinase (prk) and accumulation of the polypeptides wa s examined in chlorophyllous and chlorotic sectors of the D-p1 mutant of Nicotiana tabacum. Plastids from chlorotic sectors of this variegat ed plastome mutant contained 30S and 50S ribosomal subunits, but had a bnormally low levels of plastid polysomes. Consequently, mutant plasti ds were translationally repressed, unable to synthesize plastid-encode d polypeptides including the large subunit of Rubisco despite the pres ence of the corresponding mRNAs. Transcripts of rbcS accumulated to ne ar wild type levels in chlorotic sectors, but there was little accumul ation of the Rubisco small subunit (SS) polypeptide or holoenzyme. Mes senger-RNA isolated from chlorotic sectors effectively directed the sy nthesis of Rubisco SS in vitro suggesting that posttranslational facto rs were responsible for the decrease in Rubisco SS abundance. Transcri pts of rca and prk also accumulated to near wild type levels in chloro tic sectors and a diurnal rhythm in the abundance of rca mRNA was dete cted in green and chlorotic sectors. Despite the low abundance of Rubi sco holoenzyme in chlorotic sectors, Rubisco activase and Ru5-P kinase polypeptides accumulated to significant levels. Activities of Rubisco and Ru5-P kinase paralleled protein levels, indicating that active fo rms of these enzymes were present in chlorotic sectors. The data indic ate that the developmental events governing the accumulation of Rubisc o activase and Ru5-P kinase polypeptides and the diurnal regulation of rca expression were not dependent on the attainment of photosynthetic ally competent plastids or the accumulation of Rubisco.