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.

A 6.25 kbp barley plastid DNA region located between psbA and psbD-psbC were sequenced and RNAs produced from this DNA were analyzed. TrnK(UUU), rps16 and trnQ(UUG) were located upstream of psbA. These genes were transcribed from the same DNA strand as psbA and multiple RNAs hybridized to them. TrnK and rsp16 contained introns; a 504 amino acid open reading frame (ORF504) was located within the trnK intron. Between trnQ and psbD-psbC was a 2.24 kbp region encoding psbK, psbI and trnS(GCU). PsbK and psbI are encoded on the same DNA strand as psbD-psbC whereas trnS(GCU) is transcribed from the opposite strand. Two large RNAs accumulate in barley etioplasts which contain psbK, psbI, anti-sense trnS(GCU) and psbD-psbC sequences. Other RNAs encode psbK and psbI only, or psbK only. The divergent trnS(GCU) located upstream of psbD-psbC and a second divergent trnS(UGA) located downstream of psbD-psbC were both expressed. Furthermore, RNA complementary to psbK and psbI mRNA was detected, suggesting that transcription from divergent overlapping transcription units may modulate expression from this DNA region.

The psbD and psbC genes encode two polypeptides of Photosystem II. These genes are adjacent in the barley chloroplast genome and are part of a 5.7 kbp transcription unit. In dark‐grown barley, four large transcripts hybridize to psbD and psbC; two additional transcripts hybridize to psbC. Illumination of 4.5‐day‐old dark‐grown seedlings causes a decrease in the six psbD–psbC transcripts found in etioplasts and the accumulation of two different transcripts of 4.0 and 3.2 kb which hybridize to psbD and psbC. The light‐induced transcripts have a common 5′ end approximately 600 nt upstream of psbD and 3′ ends 1175 and 175 nt downstream of psbC. The shift in psbD–psbC transcript population occurs during a phase of chloroplast maturation when transcript levels and translation of chloroplast genes such as psaA–psaB and psbB decline approximately 3‐ to 5‐fold. In contrast, translation of the psbD and psbC gene products declines to a lesser extent, suggesting that the light‐induced accumulation of the 4.0 and 3.2 kb psbD–psbC transcripts is required to maintain psbD and psbC gene product translation in mature chloroplasts.

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