  We show that non-steric molecular interactions between RNA polymerase (RNAP)
motors that move simultaneously on the same DNA track determine strongly the
kinetics of transcription elongation. With a focus on the role of collisions
and cooperation, we introduce a stochastic model that allows for the exact
analytical computation of the stationary properties of transcription elongation
as a function of RNAP density, their interaction strength, nucleoside
triphosphate concentration, and rate of pyrophosphate release. Cooperative
pushing, i.e., an enhancement of the average RNAP velocity and elongation rate,
arises due to stochastic pushing. This cooperative effect cannot be explained
by steric hindrance alone but requires a sufficiently strong molecular
repulsion. It disappears beyond a critical RNAP density, above which jamming
due to collisions takes over. For strong stochastic blocking the cooperative
pushing is suppressed at low RNAP densities, but a reappears at higher
densities.
