In vitro transcripts from the rrn B ribosomal RNA cistron originate from two tandem promoters.

The functional structure of the promoter region of a bacterial ribosomal operon is analyzed by in vitro transcription of linear DNA fragments derived from the hybrid Col EI plasmid pGG1. This plasmid contains the promoter region of the rrn B ribosomal cistron present in the lambdarifd18. When transcripts arising from this promoter region are terminated by restriction endonuclease cleavage of DNA, two RNA chains are resolved by gel electrophoresis that differ in length by about 100 bases. Evidence is presented indicating that these two transcripts arise from different initiation sites, each directing tandem transcription on the sense strand of the early portion of the ribosomal cistron.