Figure 3.

Quantifying protein synthesis. The number of ribosomes translating a reading frame determines the number of footprints generated in a profiling experiment, and so counting the footprint sequences derived from a reading frame indicates the amount of the encoded protein that is being synthesized. An exemplary polycistronic bacterial transcript is shown, with two open reading frames ([ORFs] A and B) encoding a pair of proteins that assemble with a 1:3 stoichiometric ratio. To achieve this stoichiometry, ORF B is translated threefold more heavily than ORF A, leading to threefold higher ribosome density and threefold more ribosome footprints.