Initiation factor and ribosome levels are coordinately controlled in Escherichia coli growing at different rates

The levels of initiation factors and other translational components were compared in crude lysates of Escherichia coli grown at different rates. Cells were grown in media containing [35S]sulfate and different carbon sources, and harvested during mid-exponential phase after about 10 generations of balanced growth. Initiation factors (IF), elongation factors (EF), and a number of ribosomal proteins were identified unambiguously in gel patterns following two-dimensional polyacrylamide gel electrophoresis. The molar concentration of each protein was calculated from measurements of the radioactivity in excised gel spots and knowledge of the sulfur content of each protein. We found that the ribosomal proteins and elongation factors were present in equimolar amounts except for L7/L12 and EF-Tu which were 4-fold and 5-fold more abundant, respectively, and that the levels of each protein increased in proportion to growth rate. These results are similar to ones obtained previously by other methods, and serve to confirm the validity of our method. We found that the levels of IF2a and IF3 also were approximately proportional to growth rate. We also measured the levels of all three initiation factors using a radioimmune assay, showed that the factors are present in equimolar amounts, and confirmed that their abundance increased in parallel with ribosomes. We conclude that initiation factor levels are coordinately regulated with one another and with ribosome and elongation factor levels.