  Individual mRNAs are translated by multiple ribosomes that initiate
translation with a few seconds interval. The ribosome speed is codon dependant,
and ribosome queuing has been suggested to explain specific data for
translation of some mRNAs in vivo. By modelling the stochastic translation
process as a traffic problem, we here analyze conditions and consequences of
collisions and queuing. The model allowed us to determine the on-rate (0.8 to
1.1 initiations per sec) and the time (1 sec) the preceding ribosome occludes
initiation for Escherichia coli lacZ mRNA in vivo. We find that ribosome
collisions and queues are inevitable consequences of a stochastic translation
mechanism that reduce the translation efficiency substantially on natural
mRNAs. The cells minimize collisions by having its mRNAs being unstable and by
a highly selected codon usage in the start of the mRNA. The cost of mRNA
breakdown is offset by the concomitant increase in translational efficiency.
