Transcripts and sequence elements suggest differential promoter usage within the ycf3-psaAB gene cluster on mustard (Sinapis alba L.) chloroplast DNA

The mustard chloroplast DNA region spanning the ycf3 gene and part of the p saAB operon was investigated. The ycf3 gene reveals two class-II introns th at are removed during processing to give a mature 0.7-kb transcript, but no RNA editing seems to be involved. RNase protection and RT-PCR experiments suggest cotranscription of ycf3 with the downstream psaA gene, possibly fro m a NEP promoter upstream of' ycf3, whereas distinct ycf3 and psaA transcri pts are each initiated from PEP promoters. This situation is reminiscent of that for the trnK-psbA gene region. The implications for light-regulated v ersus light-independent expression of photosystem core-protein genes are di scussed.