Backbone dynamics of sequence specific recognition and binding by the yeast Pho4 bHLH domain probed by NMR

Backbone dynamics of the basic/helix-loop-helix domain of Pho4 from Saccharomyces cerevisae have been probed by NMR techniques, in the absence of DNA, nonspecifically bound to DNA and bound to cognate DNA. Alpha proton chemical shift indices and nuclear Overhauser effect patterns were used to elucidate the secondary structure in these states. These secondary structures are compared to the co-crystal complex of Pho4 bound to a cognate DNA sequence (Shimizu T. Toumoto A, Ihara K, Shimizu M, Kyogou Y, Ogawa N, Oshima Y, Hakoshima T, 1997, EMBO J 15: 4689-4697). The dynamic information provides insight into the nature of this DNA binding domain as it progresses from free in solution to a specifically bound DNA complex. Relative to the unbound form, we show that formation of either the nonspecific and cognate DNA bound complexes involves a large change in conformation and backbone dynamics of the basic region. The nonspecific and cognate complexes, however, have nearly identical secondary structure and backbone dynamics. We also present evidence for conformational flexibility at a highly conserved glutamate basic region residue. These results are discussed in relation to the mechanism of sequence specific recognition and binding.