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We previously reported that the rbcL transcript level was significantly increased by light signal [Lee and Sim (1995)]. To investigate whether or not the accumu-lation of rbcL mRNA by light signal is caused by an increase of transcription activity of the rbcL gene, run-on transcription assays were performed. The results indicated that the accumulation of rbcL mRNA is due to an increase in the transcriptional activity of the rbcL gene by light. An electrophoretic mobility shift assay (EMSA) was also carried out to elucidate the specific binding proteins that interact with the rbcL promoter region. As a result of EMSA, a plastid pro-tein designated as the rbcL promoter binding (RLPB) protein factor was detected in the plastid extract of light-grown seedlings, but not in that of dark-adapted seedlings before harvesting. It was also ascertained in this study that the promoter core region for the RLPB protein factor to bind is between -3 and -32 nucleotide sequences from the transcription initiation site of the rbcL gene. These results suggest that the stimulation of rbcL transcription by light is, in part, due to the in-crease in binding of the RLPB protein factor to the rbcL promoter.