Origin of the extrinsic noise floor in gene expression

In E. coli, the squared coefficient of variation of protein copy number, $\mathrm{CV}^{2}$, falls as $1/\langle n\rangle$ at low mean copy number $\langle n\rangle$ but saturates at a floor of approximately 0.1 for highly expressed genes (Taniguchi and co-workers, 2010). Identify the cell-to-cell variables responsible (cell size, growth rate, cell-cycle phase, ribosome and polymerase content, partitioning at division) and predict the floor value quantitatively.

Single-cell measurements that track the candidate global variables alongside protein levels and account for the floor value within experimental error.