Characterisation of transcript initiation sites in ribosome-deficient barley plastids.

Transcription of plastid genes in higher plants is driven by two RNA polymerases. One is encoded in the chloroplast, the other is encoded in the nucleus. RNA synthesis in ribosome-deficient plastids is performed exclusively by the nuclear-encoded enzyme. In vitro capping was used to identify the transcriptional start sites of the genes clpP and rpl23 in ribosome-free plastids of the barley mutant albostrians. No transcript initiation was found at sequences similar to eubacterial promoters. Instead, transcription started near the motif 5'-YRTA-3', which is also conserved in mitochondrial promoters of higher plants. Our data suggest that the nuclear encoded RNA polymerase is active in mature chloroplasts and is the sole polymerase involved in transcription of rpl123.