Hidden specificity in an apparently nonspecific RNA-binding protein

A novel high-throughput sequencing kinetics approach is used to measure functional binding of the apparently nonspecific RNA-binding protein C5 to all possible sequence variants in its substrate binding site; C5 binds different substrate variants with affinities varying widely, and with a similar affinity distribution to that of highly specific nucleic-acid-binding proteins, but it does not bind its physiological RNA targets with the highest affinity.