  How can a microorganism adapt to a variety of environmental conditions
despite there exists a limited number of signal transduction machineries? We
show that for any growing cells whose gene expression is under stochastic
fluctuations, adaptive cellular state is inevitably selected by noise, even
without specific signal transduction network for it. In general, changes in
protein concentration in a cell are given by its synthesis minus dilution and
degradation, both of which are proportional to the rate of cell growth. In an
adaptive state with a higher growth speed, both terms are large and balanced.
Under the presence of noise in gene expression, the adaptive state is less
affected by stochasticity since both the synthesis and dilution terms are
large, while for a non-adaptive state both the terms are smaller so that cells
are easily kicked out of the original state by noise. Hence, escape time from a
cellular state and the cellular growth rate are negatively correlated. This
leads to a selection of adaptive states with higher growth rates, and model
simulations confirm this selection to take place in general. The results
suggest a general form of adaptation that has never been brought to light - a
process that requires no specific machineries for sensory adaptation. The
present scheme may help explain a wide range of cellular adaptive responses
including the metabolic flux optimization for maximal cell growth.
