  We consider a Markovian model for the kinetics of RNA Polymerase (RNAP) which
provides a physical explanation for the phenomenon of cooperative pushing
during transcription elongation observed in biochemical experiments on
Escherichia coli and yeast RNAP. To study how backtracking of RNAP affects
cooperative pushing we incorporate into this model backward (upstream) RNAP
moves. With a rigorous mathematical treatment of the model we derive conditions
on the mutual static and kinetic interactions between RNAP under which
backtracking preserves cooperative pushing. This is achieved by exact
computation of several key properties in the steady state of this model,
including the distribution of headway between two RNAP along the DNA template
and the average RNAP velocity and flux.
