Role of open complex instability in kinetic promoter selection by bacteriophage T7 RNA polymerase

By measuring steady-state rates of dinucleotide synthesis on double-stranded (d.s.) and partially single-stranded (p.s.s.) promoters, and topological unwinding due to open complex formation on plasmids, we have obtained evidence that open complex formation in bacteriophage T7 RNA polymerase:promoter binary complexes is thermodynamically disfavored and that the rate of collapse of the open complex is competitive with the rate of transcription initiation. It is suggested that open complex instability is a kinetic mechanism that allows T7 RNA polymerase (RNAP) to achieve promoter specificity while still allowing for efficient promoter release. Open complex instability could also provide a mechanism for modulating the K(M) for the initiating NTPs so as to allow different promoters to respond differently to physiological changes in NTP concentration.