2 OPEN COMPLEXES AND A REQUIREMENT FOR MG2+ TO OPEN THE LAMBDA-P(R) TRANSCRIPTION START SITE

Potassium permanganate (KMnO4) footprinting in the absence and presence of magnesium (Mg2+) at the lambdaP(R) promoter identified two different open complexes with Escherichia coli Esigma70 RNA polymerase (designated RP(o1) and RP(o2). The single-stranded region in RP(o1), (formed in the absence of Mg2+) was at most 12 bases long, whereas that in RP(o2) (formed in the presence of Mg2+) spanned at least 14 bases. Only in RP(o2) did the single-stranded region extend to the start point of transcription (+1, +2). These results provide a structural basis for the requirement for uptake of Mg2+ in the formation of RP(o2) from RP(o1), as deduced from kinetic studies at this promoter.