Practice question

First ribonucleotide incorporated by RNA polymerase designated +1 corresponds to transcription start site where initiation complex transitions to elongation. Typically purine adenine or guanine favored. Numbering counts downstream as +2, +3 along transcribed region, while upstream sequences receive negative values -1, -2 without zero between. Such convention allows precise localization of promoter hexamers such as bacterial -10 box centered near -10 and eukaryotic TATA around -30. Position +1 experimentally mapped by primer extension, S1 nuclease, or RNA-seq 5' end detection, fundamental for promoter characterization.