Effect of coding regions on chloroplast gene expression in Chlamydomonas reinhardtii

An exogenous gene, placed between the 5'-upstream regions of the Chlamydomonas reinhardtii chloroplast genes, rbcL or psbA, and the 3'-end of the rbcL gene, do not have the same expression pattern as endogenous genes in the C. reinhardtii chloroplast. Here, we chose four chloroplast genes, rbcL, psbA, psbD and atpA, and examine the effects of chloroplast gene coding regions on gene expression in C. reinhardtii. We constructed chimeric genes composed of the promoter, 5'- and 3'-untranslated regions, varying lengths of protein coding regions of the chloroplast genes, and the bacterial beta-glucuronidase (GUS) gene (uidA) as a reporter gene, and introduced into chloroplast genomes. The transformants, which contained the rbcL-uidA and psbA-uidA chimeric genes fused to the coding region of each gene, showed high expression of uidA mRNA as compared with the previously generated transformants, RG and PG, in which uidA was only fused to the promoter and 5'-UTR of each gene. The difference in the accumulation of uidA transcripts among the transformants was the result of different rates of transcription. This result indicates that the coding region is necessary for sufficient expression of rbcL and psbA. On the other hand, the psbD and atpA coding region portions did not affect chimeric gene expression.