Fig. 2. The rate of mRNA translation is maximal at a cytoplasmic concentration of~1x.

a Titration of mRNA concentration for eGFP expression. The indicated concentration (2.5 µg/mL) was chosen for the experiments in (b–e). Data from n = 3 independent experiments. Data are presented as mean values ± SEM. b eGFP expression as a function of time for various dilutions of a 1× extract. c Translation rate as a function of cytoplasmic concentration. These are the directly-measured data from experiments where the eGFP mRNA concentration was kept constant and the translation machinery was proportional to the cytoplasmic concentration. Data are from n = 6 independent experiments for dilution from 1× extracts and n = 7 independent experiments for dilution from 2× retentates. Data are normalized relative to the translation rates at a cytoplasmic concentration of 1×. Means and standard errors are overlaid on the individual data points. In this and the subsequent panels, the darker green represents data from diluting 2× retentates and the lighter green from diluting 1× extract. d Inferred translation rates for the situation where the mRNA concentration as well as the ribosome concentration is proportional to the cytoplasmic concentration. The rates from (c) were multiplied by the relative cytoplasmic concentrations. Data are from n = 6 independent experiments for dilution from 1× extracts and n = 7 independent experiments for dilution from 2× retentates. Data are presented as mean values ± SEM. e Inferred translation rates for the situation where both the mRNA concentration and the ribosome concentration are kept constant at all dilutions. This represents an estimate of the apparent bimolecular rate constant for translation. The rates from (c) were divided by the relative cytoplasmic concentrations. Data are from n = 6 independent experiments for dilution from 1× extracts and n = 7 independent experiments for dilution from 2× retentates. Data are presented as mean values ± SEM. f TCA-precipitable ³⁵S incorporation as a function of time for translation from endogenous mRNAs. Various dilutions of a 1× extract are shown. CHX denotes a 1× extract treated with 100 µg/mL cycloheximide. g Inferred translation rates for the situation where mRNA concentration is kept constant and ribosome concentration is proportional to the cytoplasmic concentration. The rates from (h) were divided by the relative cytoplasmic concentration. The gray data points are from CHX (100 µg/mL)-treated 1× extracts. Data are from n = 3 independent experiments for dilution from 1× extracts and n = 3 independent experiments for dilution from 2× retentates. Data are presented as mean values ± SEM. h Translation rate as a function of cytoplasmic concentration. These are the directly measured data from experiments where the ³⁵S concentration was kept constant but both the (endogenous) mRNA concentration and translational machinery were proportional to the cytoplasmic concentration. Data are from n = 3 independent experiments for dilution from 1× extracts and n = 3 independent experiments for dilution from 2× retentates. Data are normalized relative to the translation rates at a cytoplasmic concentration of 1×. Means and standard errors are overlaid on the individual data points. i Inferred translation rates for the situation where both the mRNA concentration and the ribosome concentration are kept constant at all dilutions. The rates from (h) were divided twice by the relative cytoplasmic concentrations (i.e., by the relative concentration squared). Data are from n = 3 independent experiments for dilution from 1× extracts and n = 3 independent experiments for dilution from 2× retentates. Data are presented as mean values ± SEM. Source data for panels (a, d, and g) are provided as a Source Data file.