Direct correlation between motile behavior and protein abundance in single cells.

Understanding how stochastic molecular fluctuations affect cell behavior
requires the quantification of both behavior and protein numbers in the same cells.
Here, we combine automated
microscopy with in situ hydrogel
polymerization to measure single-cell protein expression after tracking swimming behavior. We characterized the distribution of non-genetic
phenotypic diversity in Escherichia coli motility, which affects single-cell exploration. By expressing fluorescently
tagged chemotaxis proteins (CheR and CheB) at different levels, we
quantitatively mapped motile phenotype (tumble bias) to protein numbers using thousands
of single-cell measurements. Our results disagreed with established models
until we incorporated the role of CheB in receptor deamidation and the slow
fluctuations in receptor methylation. Beyond refining models, our central
finding is that changes in numbers of CheR and CheB affect the population mean tumble
bias and its variance independently. Therefore, it is possible to adjust the
degree of phenotypic diversity of a population by adjusting the global level of
expression of CheR and CheB while keeping their ratio constant, which, as shown
in previous studies, confers functional robustness to the system. Since genetic
control of protein expression is heritable, our results suggest that non-genetic
diversity in motile behavior is selectable, supporting earlier hypotheses that such
diversity confers a selective advantage.