CHLOROPLAST RIBOSOMAL-RNA TRANSCRIPTION FROM STRUCTURALLY DIFFERENT TANDEM PROMOTERS - AN ADDITIONAL NOVEL-TYPE PROMOTER

Identification of transcription initiation sites in the promoter regio n of the tobacco chloroplast rRNA operon has been carried out by ribon uclease protection of in vitro capped RNAs and primer extension experi ments. A promoter with typical chloroplast -10 and -35 motifs (P1) dri ves initiation of transcription from position -116 relative to the mat ure 16s rRNA sequence. In addition, we have found that a second primar y transcript starts at position -64. This proximal promoter (P2) lacks any elements similar to those reported so far in chloroplast promoter regions, and hence P2 represents a novel-type promoter. Both transcri pts are present in chloroplasts from green leaves and in non-photosynt hetic proplastids from heterotrophically cultured cells (BY2), but the ir relative amounts appear to differ. The steady state level of the P2 transcript, with respect to P1, is higher in BY2 proplastids than in leaf chloroplasts.