Practice question

Run-off transcription assay is uniquely suited to study RNA polymerase activity using linear DNA templates cut downstream of promoter, causing polymerase to transcribe and then dissociate at cut end, producing defined-length runoff transcript. By varying template truncation, promoter mutations, or adding transcription factors, investigators dissect initiation, elongation efficiency, and termination. Northern blot detects steady-state RNA, electrophoretic mobility shift assay measures DNA-protein binding, and RT-PCR quantifies RNA abundance, but none directly monitors active transcription from defined ends. Run-off system provides quantitative, controlled in vitro readout of polymerase function.