In vitro characterization of the tobacco rpoB promoter reveals a core sequence motif conserved between phage-type plastid and plant mitochondrial promoters.

We report here the in vitro characterization of PrpoB-345, the tobacco rpoB promoter recognized by NEP, the phage-type plastid RNA polymerase. Transcription extracts were prepared from mutant tobacco plants lacking PEP, the Escherichia coli-like plastid-encoded RNA polymerase. Systematic dissection of a approximately 1 kb fragment determined that the rpoB promoter is contained in a 15-nucleotide segment (-14 to +1) upstream of the transcription initiation site (+1). Point mutations at every nucleotide reduced transcription, except at the -5 position which was neutral. Critical for rpoB promoter function was a CRT-motif (CAT or CGT) at -8 to -6 (transcription <30%), defining it as the promoter core. The core CAT sequence is also present in the maize rpoB promoter, which is faithfully recognized by tobacco extracts. Alignment of NEP promoters identified a CATA or TATA (=YATA) sequence at the rpoB core position, also present in plant mitochondrial promoters. Furthermore, NEP and the phage T7 RNA polymerase exhibit similar sensitivity to inhibitors of transcription. These data indicate that the nuclear RpoZ gene, identified by sequence conservation with mitochondrial RNA polymerases, encodes the NEP catalytic subunit.