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 psaAB operon was investigated. The ycf3 gene reveals two class-II introns that 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 from a NEP promoter upstream of ycf3, whereas distinct ycf3 and psaA transcripts are each initiated from PEP promoters. This situation is reminiscent of that for the trnK-psbA gene region. The implications for light-regulated versus light-independent expression of photosystem core-protein genes are discussed.