Electron microscopic analysis of transcription of a ribosomal RNA operon of E. coli.

Transcription in vitro of the E. coli ribosomal RNA operon, rrnE, was analysed by electron microscopy. The transcription initiation sites of the two rrnE promoters in tandem, P1 and P2, were mapped and the transcription from both sites was compared. The first and the second transcription initiation site are about equally used when all nucleotides are present at 200 microM. Lowering the concentration of the second promoter's start nucleotide CTP to 3 microM reduces the use of the P2 site sharply. At all CTP concentrations used the nascent RNA chains from P1 are in the average longer than those from P2 after a fixed transcription time. Most probably, this difference is caused by a longer average interval before formation of the productive complex with the second promoter.