  Recent experiments have shown that stochastic effects exerted at the level of
translation contribute a substantial portion of the variation in abundance of
proteins expressed at moderate to high levels. This study analyzes
translational noise arising from fluctuations in residue-specific elongation
rates. The resulting variation has multiplicative components that lead
individual protein abundances in a population to exhibit approximately
log-normal behavior. The high variability inherent in the process leads to
parameter variation that has the features of a type of noise in biological
systems that has been characterized as extrinsic. Elongation rate variation
offers an accounting for a major component of extrinsic noise, and the analysis
provided here highlights a probability distribution that is a natural extension
of the Poisson and has broad applicability to many types of multiplicative
noise processes.
