ACTIVITY OF THE CHLAMYDOMONAS CHLOROPLAST RBCL GENE PROMOTER IS ENHANCED BY A REMOTE SEQUENCE ELEMENT

The chloroplast gene rbcL encodes the large subunit of ribulose bispho sphate carboxylase. In Chlamydomonas reinhardtii, this gene is transcr ibed more actively than any other protein encoding chloroplast gene st udied to date. To delineate the rbcL gene promoter, chimeric reporter genes containing fragments of the 5' region of the rbcL gene fused to the coding sequence of the bacterial uidA gene, encoding beta-glucuron idase, were stably introduced into the chloroplast genome of Chlamydom onas by microprojectile bombardment. The relative transcription rates of endogenous and introduced genes were determined in transgenic cell lines in vivo. The basic rbcL promoter is located within the region of the gene extending from positions -18 to +63, taking position +1 as t he site of initiation of transcription. A chimeric reporter gene conta ining only the basic promoter is transcribed only 1-15% as actively as the endogenous rbcL gene, depending on the conditions under which cel ls are grown and tested. However, a chimeric gene containing rbcL sequ ences extending to position +170 or beyond is transcribed at about the same rate as the endogenous gene. Deletion of the sequence between po sitions +170 and +126, well within the protein-encoding region, reduce s the rate of transcription to that of reporter genes with the basic p romoter alone.