The Generation of New P by the Combination of Do

,functions. A number of previous studies have described in detail how gene duplication and sequence divergence produce proteins with new properties. Gerlt and Babbitt (2001) have reviewed their work on enzymes that belong to mechanistically diverse superfamilies (those that con-serve some part of their catalytic mechanism) and to functionally distinct superfamilies (those that do not con-serve a common mechanism). Nahum and Riley (2001) de-scribed how sequence divergence in families of E. coli proteins has produced proteins with different functions. Todd et al. (2001) in a detailed study of enzymes in 31 su-perfamilies described point mutations and local structural plete descriptions of the contributions that each domain makes to the functions of the multidomain protein. Even if the proteins were characterized well enough for us to do this, a complete description would require a paper on each set of structures. Here, we determine the major con-tributions and roles made by each domain to the function of the multidomain protein, and how these are the same as, or different from, the functions found in the one-domain homologs