A cell’s protein content is a major determinant of the cell’s phenotype, i.e. its observable characteristics. Therefore extensive efforts have been dedicated to studying the variation in the protein content among genetically identical cells. In an attempt to gain a new insight into the sources of this variability, we have measured the fluctuations in the protein content of a single cell over extended times and compared their statistical properties to those of protein fluctuations measured from population snapshots. Our population measurements reveal that the fluctuations in the copy- number of any highly expressed protein, measured under a broad range of conditions, exhibit a statistical properties that appear to be universal. To compare these results to single-cell fluctuations, we used recent advancements in microfluidic techniques to track single bacterial cells for many generations while monitoring their size growth, protein expression, and division events. Traces of protein copy number obtained from such measurements revealed that the temporal fluctuations in individual traces exhibit the same statistical properties as populations. This enabled us to develop a simple dynamical model that explains the universal nature of the protein distribution based on a new universal feedback mechanism that acts to stabilize the protein amount in the cell.
