4 PROMOTERS IN THE RICE PLASTID PSBK-PSBI-PSBD-PSBC OPERON

The transcriptional organization of the plastid psbK-psbI-psbD-psbC ge ne cluster in rice seedlings was investigated. In total, seven classes of overlapping transcripts (RNAs a-g) based on the 5 ' ends were dedu ced and mapped in detail. Light induced the accumulation of class d RN As, which were not detected in the dark-grown plants, and enhanced the accumulation of all other transcripts. An in vitro transcription assa y demonstrated that there are four promoters residing in the gene clus ter, which drive four overlapping transcription units to produce RNAs of classes a, c, d and f. An in vitro capping assay identified these t ranscription initiation sites, which coincide with the mapped 5 ' term ini of the respective RNAs. The heterogenous 5 ' termini of class d RN As were proved to arise from transcriptional initiation mediated by th e light-responsive promoter residing between psbI and psbD. It is sugg ested that RNAs of class b, e and g are processed from precursor trans cripts by some plastid nucleolytic activities.