In vitro transcription initiation of the spinach chloroplast 16S rRNA gene at two tandem promoters.

Two potential prokaryotic promoters, P1 and P2, are characterized 164 and 114 bp upstream of the spinach chloroplast 16S rRNA gene. The strengths of these promoters, calculated according to an homology score established for E. coli RNA-polymerase, are identical. Experiments performed with a Taq I-DNA fragment, containing 16 bp of the 16S rDNA and 243 bp upstream of the gene, give evidence that in vitro, E. coli RNA-polymerase starts transcription at these two promoters. These results are based on both the size of the transcripts and their nucleotide sequences. A possible regulation by differential control of these dual promoters is suggested. S1 mapping with RNAs extracted either from green or from etiolated spinach plants, indicates that, at these two steps of plastid development, transcription in vivo starts at P1. Surprisingly only P2 appears to be conserved in the homologous sequences reported for maize, mustard and Spirodela.