Cellular perception of growth rate and the mechanistic origin of bacterial growth laws

Bacteria organize many activities according to their grow rate via the ppGpp signaling system. Yet it is not clear how this signaling system "knows" how fast cells grow. Through quantitative experiments, we show that ppGpp responds inversely to the rate of translational elongation in E. coli. Together with its roles in inhibiting ribosome biogenesis and activity, ppGpp closes a key regulatory circuit that enables the cell to perceive and control the rate of its growth across conditions. The celebrated linear growth law relating the ribosome content and growth rate emerges as a consequence of keeping a supply of ribosome reserves while maintaining elongation rate in slow growth conditions. Further analysis suggests the elongation rate itself is detected by sensing the ratio of dwelling and translocating ribosomes, a strategy employed to collapse the complex, high-dimensional dynamics of the molecular processes underlying cell growth to perceive the physiological state of the whole.