Rapid splicing and stepwise processing of a transcript from the psbB operon in tobacco chloroplasts: determination of the intron sites in petB and petD.

Expression of the psbB gene cluster in tobacco chloroplasts has been studied. This cluster contains the genes for the 51 kDa chlorophyll a apoprotein (psbB) and the 10 kDa phosphoprotein (psbH) of the photosystem II, and cytochrome b6 (petB) and subunit IV (petD) of the cytochrome b/f complex in this order. Northern blot hybridization and reverse transcription analyses have revealed that petB and petD contain single introns and the psbB gene cluster is transcribed as a single polycistronic unit. The primary transcript seems to be spliced very rapidly and then processed into several small RNA species. The exact splice sites have been located by cDNA sequencing. The transcriptional initiation site of the psbB operon has been determined by S1 mapping with in vitro capped chloroplast RNA. The stepwise processing of chloroplast RNA precursors is discussed.