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pubmed:abstractText |
The DNA binding domain of the GAL4 transcription factor from yeast is located in the N-terminal 60 residues of the polypeptide of 881 amino acids. This domain binds 2 Zn ions, which form a binuclear cluster, Zn2C6, with 6 C residues, two of which bridge the 2 metal ions (Gardner KH et al., 1991, Biochemistry 30:11292-11302). Binding of Zn or Cd to GAL4 induces the conformation of the protein necessary to recognize the specific DNA sequence, UASG, to which GAL4 binds as a dimer. Gel retardation assays have been utilized to determine the relative affinities of the Zn2 and Zn1 forms of the N-terminal 149 residues of GAL4, GAL4(149*), for UASG DNA sequences. We show that Cd2- and Zn1GAL4(149*) bind to UASG DNA with 2-fold and 4-8-fold lower affinities than Zn2GAL4(149*), respectively. Thus, the metal species and the number of metal ions bound have measurable effects on the specific DNA binding affinity of GAL4, but these differences are small in comparison to the ratio, > 10(3) under some conditions, that characterizes the specific to nonspecific DNA binding affinities of the N-terminal fragments of GAL4. A shorter N-terminal fragment, GAL4(62*), although it continues to recognize the UASG sequence with a high degree of specificity, binds with 1,000-2,000-fold lower affinity than does Zn2GAL4(149*). Gel retardation titrations of a DNA containing 2 UASG sites with increasing concentrations of GAL4(62*) generate a series of 4 retarded bands in contrast to 2 retarded bands formed when the same DNA is titrated with GAL4(149*). These data suggest that GAL4(62*) binds to the UASG sites as individual monomers that dimerize on the DNA, whereas GAL4(149*) binds the UASG DNA cooperatively as a dimer. The approximately 10(3) lower affinity of GAL4(62*) for the UASG DNA can be accounted for by its failure to form dimers in solution. Zn2-, Zn1-, or Cd2GAL4(149*) induces differential rates of gel migration in a series of circularly permutated UASG-containing DNA restriction fragments. Analysis of the data suggests that all 3 proteins cause a 26 degrees angle of bend in the DNA when bound to 1 UASG site and 45 degrees when bound to 2 tandem UASG sites. The same assay shows that GAL4(62*) does not induce significant bending of the UASG DNA sequences. Thus, the additional subdomains found in the larger polypeptide fragment, GAL4(149*), must exert an additional force on the DNA either through direct contacts with the DNA or indirectly through altered protein conformation.
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