Protein-protein interactions mapped by artificial proteases: where sigma factors bind to RNA polymerase

Interactions between proteins are important to understand but difficult to study. Conjugating a protein to a small artificial protease endows it with the ability to cut other proteins where it binds to them. Analysing the sit es cut on the target proteins leads to new understanding of the structure o f each complex. The binding of a factors to a common region on RNA polymera se provides an example.