Abstract
An EM technique is described that allows rapid characterization of transcription in vitro. DNA is transcribed with Escherichia coli RNA polymerase in vitro, and the RNA is hybridized to its template. Measurement of the resulting transcription R-loop molecules allows accurate mapping of transcription initiation sites (promoter sites) and analysis of the direction and rate of transcription and the level of transcription from each initiation site. The 2 major early promoters pR and pL of bacteriophage .lambda. were mapped within 0.1-0.3 map units of the known positions and 3 additional sites were confirmed. Six transcription initiation sites were preliminarily mapped on plasmid pSF2124 DNA.