Near Saturation of Ribosomal L7/L12 Binding Sites with Ternary Complexes in Slowly Growing E. coli.

For Escherichia coli growing rapidly in rich medium at 37 °C, the doubling time can be as short as ~20 min and the average rate of translation (k) can be as fast as ~20 amino acids/s. For slower growth arising from poor nutrient quality or from higher growth osmolality, k decreases significantly. In earlier work from the Hwa lab, a simplified Michaelis-Menten model suggested that the decrease in k arises from a shortage of ternary complexes (TCs) under nutrient limitation and from slower diffusion of TCs under high growth osmolality. Here we present a single-molecule tracking study of the diffusion of EF-Tu in E. coli growing with doubling times in the range 62-190 min at 37 °C due to nutrient limitation, high growth osmolality, or both. The diffusive properties of EF-Tu remain quantitatively indistinguishable across all growth conditions studied. Dissection of the total population into ribosome-bound and free sub-populations, combined with copy number estimates for EF-Tu and ribosomes, indicates that in all cases ~3.7 EF-Tu copies are bound on average to each translating 70S ribosome. Thus, the four L7/L12 binding sites adjacent to the ribosomal A-site in E. coli are essentially saturated with TCs in all conditions, facilitating rapid testing of aminoacyl-tRNAs for a codon match. Evidently, the average translation rate is not limited by either the supply of cognate TCs under nutrient limitation or by the diffusion of free TCs at high osmolality. Some other step or steps must be rate limiting for translation in slow growth.