It remains unclear if cells control the stoichiometry of the subunits of various protein complexes through tight control of protein synthesis or through differences in the degradation of excess protein. In Cell, Weismann and colleagues use ribosomal profiling to quantitatively measure absolute protein-synthesis rates and estimate absolute protein copy numbers in a cell. The authors show that proportional synthesis ('tuning' synthesis rates to subunit stoichiometry) is used for more than 90% of protein complexes in Escherichia coli, which uses polycistronic mRNAs, and in Saccharomyces cerevisiae, in which protein subunits are encoded on different mRNAs. In E. coli, proportional synthesis is achieved through translational control rather than transcriptional control. Because of the different molecular mechanisms for prokaryotic and eukaryotic protein expression, proportional synthesis might be a result of convergent evolution. This study also provides insight into the principles of expression for components of functional modules, such as enzymes of metabolic pathways and transcription factors.

Cell 157, 624–635 (2014)