High-Resolution Structure of an Engineered Cro Monomer Shows Changes in Conformation Relative to the Native Dimer

A rationally designed, genetically engineered, monomeric form of the Cro protein from bacteriophage lambda has been crystallized and its structure determined by isomorphous replacement and refined to a resolution of 1.54 A. The structure confirms the rationale of the design but, at the same time, reveals 1-2 A shifts throughout the monomer structure relative to the previously determined structure of the dimeric wild-type protein. These changes include a 1.6 A main-chain shift in part of the beta-sheet region of the molecule relative to the alpha-helical region and a 1.1 A shift of a buried phenylalanine within the core as well as a correlated 2.2 A shift in a solvent-exposed beta-hairpin. The conformational adjustments appear to reflect an inherent flexibility of the protein that is associated with its DNA-binding function.