THE STRUCTURE OF PURR MUTANT L54M SHOWS AN ALTERNATIVE ROUTE TO DNA KINKING

The crystal structure of the purine repressor mutant L54M bound to hyp oxanthine and to the purF operator provides a stereochemical understan ding of the high DNA affinity of this hinge helix mutant. Comparison o f the PurR L54M-DNA complex to that of the wild type PurR-DNA complex reveals that these purine repressors bind and kink DNA similarly despi te significant differences in their minor groove contacts and routes t o interdigitation of the central C . G:G . C base pair step. Modeling studies, supported by genetic and biochemical data, show that the ster eochemistry of the backbone atoms of the abutting hinge helices combin ed with the rigidity of the kinked base pair step constrain the interd igitating residue to leucine or methionine for the LacI/GalR family of transcription regulators.