Light-dependent changes in psbD and psbC transcripts of barley chloroplasts: accumulation of two transcripts...

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.