Since its original description almost 10 years ago, the yeast two-hybrid system has been used extensively to identify protein-protein interactions from many different organisms. Simultaneously, a number of 'variations on a theme' based on the original concept have been described. In one set of variations, systems were developed to detect other macromolecular interactions: DNA-protein (one-hybrid), RNA-protein (RNA-based three-hybrid) and small molecule-protein interactions (ligand-based three-hybrid). These different versions are collectively referred to here as 'n-hybrid systems'. In another set of variations, the original configuration of the two-hybrid fusion proteins was modified to expand the range of possible protein-protein interactions that could be analyzed. For example, systems were developed to detect trimeric interactions, ligand-receptor interactions or interactions that require particular post-translational modifications. Finally, the original concept was turned upside down and 'reverse n-hybrid systems' were developed to identify mutations, peptides or small molecules that dissociate macromolecular interactions. These reagents can be used to validate, in the relevant biological systems, the potential interactions identified with the 'forward n-hybrid systems'. The powerful genetic selections of the forward and reverse n-hybrid systems are proving useful in proteomic projects aimed at generating macromolecular interaction maps.
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