Slowing translation to avoid cellular depletion of active ribosomes at slow growth rates

To sustain bacterial growth, a fraction of active ribosomes must be allocated to synthesize ribosomal proteins. Two control parameters modulate the growth rate: the fraction of ribosomes allocated to ribosomal protein synthesis and the translation speed. At longer doubling times, which approach the active ribosome lifetime, the limited number of ribosomes poses a significant constraint in allocating ribosomes to ribosomal protein synthesis. We show that, to maintain slow growth while reducing the risk of depletion of the active cellular ribosome population, some bacteria must significantly reduce translation speed below its maximum, as previously observed. This change in the controls is due to a universal shot noise limitation that applies at slow growth rates in all kingdoms of life.