CHARACTERIZATION OF TRANSCRIPT INITIATION SITES IN RIBOSOME-DEFICIENTBARLEY PLASTIDS

Transcription of plastid genes in higher plants is driven by two RNA p olymerases. One is encoded in the chloroplast, the other is encoded in the nucleus. RNA synthesis in ribosome-deficient plastids is performe d exclusively by the nuclear-encoded enzyme. In vitro capping was used to identify the transcriptional start sites of the genes clpP and rpl 23 in ribosome-free plastids of the barley mutant albostrians. No tran script initiation was found at sequences similar to eubacterial promot ers. Instead, transcription started near the motif 5'-YRTA-3', which i s also conserved in mitochondrial promoters of higher plants. Our data suggest that the nuclear encoded RNA polymerase is active in mature c hloroplasts and is the sole polymerase involved in transcription of rp l123.