Regulation by the modulation of gene expression variability.

Classically, transcription is regulated so that the average expression per cell changes, often with a distribution that extends across the population. Roggiani and Goulian (M. Roggiani and M. Goulian, J. Bacteriol. 197:1976-1987, 2015, doi:http://dx.doi.org/10.1128/JB.00074-15) have shown that this is what happens when the torCAD operon of Escherichia coli is induced anaerobically by the addition of trimethylamine-N-oxide (TMAO). However, when the same inducer is added to aerobically growing cells, only a subset of the cells respond, although the mean expression per cell is similar to that obtained anaerobically. Thus, in the presence of oxygen, the variance but not the expression mean is altered. The regulation of gene expression variance appears to be due to noise in the phosphorelay that governs torCAD transcription.