Light-induced switch in barley psbD-psbC promoter utilization: a novel mechanism regulating chloroplast gene expression.

The synthesis of reaction center protein D2 and mRNAs which encode this protein are differentially maintained at high levels in mature barley chloroplasts. To understand the differential maintenance of psbD mRNA abundance, we have studied the transcription and the RNAs produced from the psbD-psbC operon in plastids of light and dark-grown barley seedlings. Ten psbD-psbC RNAs synthesized in dark-grown barley share four different 5'-ends, two of which arise by transcription initiation, and one of which is generated by 5'-processing of longer psbD-psbC transcripts. Illumination of dark-grown barley causes the decline of these ten transcripts, and the accumulation of two different psbD-psbC RNAs. Capping assays, in vitro transcription and RNA processing experiments and treatment of plants with tagetitoxin (a selective inhibitor of chloroplast transcription), indicate that the light-induced transcripts arise by transcription initiation. Run-on transcription and RNA quantitation experiments provide evidence that both light-induced transcription and RNA stability play roles in the accumulation of the light-induced RNAs. These data document a novel mechanism for regulating plastid gene expression involving a light-induced switch in psbD-psbC promoter utilization.