The chloroplast psbK operon from mustard (Sinapis alba L.): multiple transcripts during seedling development and evidence for divergent overlapping transcription.

The mustard chloroplasts genes psbK and psbI are co-transcribed, giving rise to precursor transcripts of several size classes, which are processed to the monocistronic mature RNAs. The psbK and psbI coding regions are flanked by the two tRNA genes trnS-GCU and trnQ-UUG on the opposite DNA strand. Transcript mapping indicates that the (primary) psbK-psbI transcript overlaps the complete trnS-GCU and trnQ-UUG transcripts. The transcription start site of the psbK operon appears to overlap that of the rps16 gene. During seedling development, the psbK and psbI precursors and mature transcripts all become detectable between 30 and 48 h after sowing and then remain at constant levels without much difference either in light or in darkness.