Active transcription from a promoter positioned within the coding region of a divergently oriented gene: the tobacco chloroplast rpl32 gene.

A new transcription unit has been identified and characterized in the small single-copy region of tobacco chloroplast DNA. A primary transcript (1550 nucleotides) spanning the entire transcription unit contains no significant open reading frames (ORFs), other than ORF55, recently identified as the gene encoding the ribosomal protein CL32 (rpl32). The leader sequence extends 1101 nucleotides from the rpl32 initiation codon. Primer extension and in vitro capping experiments in combination with ribonuclease protection assays, revealed a promoter situated more than 322 bp inside the coding region of ndhF, which is divergently oriented with respect to rpl32. A canonical Pribnow-box is found just upstream of the transcription start site, but a typical -35 motif was not detected. This is the first internal divergent promoter to be characterized in the chloroplast genome.