Reduction of translating ribosomes enables Escherichia coli to maintain elongation rates during slow growth

Bacteria growing under different conditions experience a broad range of demand on the rate of protein synthesis, which profoundly affects cellular resource allocation. During fast growth, protein synthesis has long been known to be modulated by adjusting the ribosome content, with the vast majority of ribosomes engaged at a near-maximal rate of elongation. Here, we systematically characterize protein synthesis by Escherichia coli, focusing on slow-growth conditions. We establish that the translational elongation rate decreases as growth slows, exhibiting a Michaelis-Menten dependence on the abundance of the cellular translational apparatus. However, an appreciable elongation rate is maintained even towards zero growth, including the stationary phase. This maintenance, critical for timely protein synthesis in harsh environments, is accompanied by a drastic reduction in the fraction of active ribosomes. Interestingly, well-known antibiotics such as chloramphenicol also cause a substantial reduction in the pool of active ribosomes, instead of slowing down translational elongation as commonly thought.