  RNA polymerase (RNAP) is an enzyme that synthesizes a messenger RNA (mRNA)
strand which is complementary to a single-stranded DNA template. From the
perspective of physicists, an RNAP is a molecular motor that utilizes chemical
energy input to move along the track formed by a ssDNA. In some circumstances,
which are described in this paper, a large number of RNAPs move simultaneously
along the same track. We refer to such collective movements of the RNAPs as
RNAP traffic because of the similarities between the collective dynamics of the
RNAPs on ssDNA track and that of vehicles in highway traffic. In this paper we
develop a theoretical model for RNAP traffic by incorporating the steric
interactions between RNAPs as well as the mechano-chemical cycle of individual
RNAPs during the elongation of the mRNA. By a combination of analytical and
numerical techniques, we calculate the rates of mRNA synthesis and the average
density profile of the RNAPs on the ssDNA track. We also suggest novel
experiments for testing our theoretical predictions.
