Differential rate of ribosomal protein synthesis in Escherichia coli B/r

Abstract

The differential rate of ribosomal protein synthesis, α r (ribosomal protein synthesis rate/total protein synthesis rate), was measured for Escherichia coli strain B/r growing at different steady-state rates ranging from 0.67 to 2.3 doublings/hour. For growth rates above 1.2 doublings/hour, α r was found to be proportional to the growth rate μ (doublings/h), such that α r = 0.09 μ, and the ribosome efficiency (amino acids polymerized/second per ribosome), calculated from α r, was found to be 14 to 18 amino acids/second per ribosome. With decreasing growth rates below 1.2 doublings/hour, α r was found to be increasingly greater than 0.09 μ and the ribosome efficiency gradually decreased such that at μ = 0.67, α r = 0.085, and the ribosome efficiency was reduced by 30% and was equal to 10 to 13 ammo acids/second per ribosome. These results imply that the protein to DNA ratio is constant for μ > 1.2 and equal to 4 × 10 8 to 5 × 10 8 amino acids/genome. For μ < 1.2, this ratio gradually decreases such that at μ = 0.67, protein to DNA = 3 × 10 8 to 4 × 10 8 amino acids/genome. These relationships were verified by direct measurements of the amounts of DNA, RNA and protein at different steady-state growth rates. In addition, protein accumulation was measured following a nutritional shift-up from succinate to glucose minimal medium. The results indicate that the ribosome efficiency increases by approximately 40% within the first few minutes following the shift-up.