A dynamic model for transcription elongation and sequence-dependent short pauses by RNA polymerase.

RNA polymerase is an enzyme that transcribes genes from DNA onto strands of RNA and the transcription is a processive, accurate but discontinuous process. Despite extensive structural, biochemical and biophysical studies, the transcription elongation mechanism by the RNA polymerase is still not well determined. Here a new Brownian ratchet model is presented for this transcription elongation by the RNA polymerase. The structure's conformational changes observed in the RNAP translocation cycle are incorporated into the model. Using the model, the dynamic behaviors of continuous transcription elongation between two pauses and inhibition of next nucleotide addition after misincorporation are well explained. Moreover, the sequence-dependent short pauses result from site-specific interactions of RNAP with dsDNA and/or RNA-DNA hybrid. With this model, it is demonstrated that, at a given sequence, the lifetime distribution of the short pause has the single-exponential form at saturating nucleotide concentration, which is in contrast to the multi-exponential distribution of the dwell time during the continuous transcription elongation.