Alterations in the GAL4 DNA-binding Domain Can Affect Transcriptional Activation Independent of DNA Binding

Abstract

The GAL4 protein belongs to a large class of fungal transcriptional activator proteins encoding within their DNA-binding domains (DBD) six cysteines that coordinate two atoms of zinc (the Zn 2 Cys 6 domain). In an effort to characterize the interactions between the Zn 2 Cys 6 class transcriptional activator proteins and their DNAbinding sites, we have replaced in the full-length GAL4 protein small regions of the Zn 2 Cys 6 domain with the analogous regions of another Zn 2 Cys 6 protein called PPR1 an activator of pyrimidine biosynthetic genes. Alterations between the first and third cysteines abolished binding to GAL4 (upstream activation sequence of GAL (UAS G )) or PPR1 (upstream acitvation sequence of UAS) DNA-binding sites and severely reduced transcriptional activation in yeast. In contrast, alterations between the third and fourth cysteines had only minor effects on binding to UAS G but led to substantial decreases in activation in both yeast and a mammalian cell line. In the crystal structure of the GAL4 DBD-UAS G complex (Marmorstein, R., Carey, M., Ptashne, M., and Harrison, S. C. (1992) Nature 356, 408 -414), this region is facing away from the DNA, making it likely that there exists within the GAL4 DBD an accessible domain important in activation.The GAL4 protein is an 881-amino acid transcriptional activator of the GAL and MEL1 genes in Saccharomyces cerevisiae. The mechanism by which GAL4 activates transcription is conserved because GAL4 also activates transcription in plant, Drosophila, and mammalian cells if GAL4 DNA-binding sites (UAS G ) 1 are present in the promoter of appropriate reporter genes (reviewed in Ref. 1). GAL4 is a member of a large family of fungal transcriptional activators that contain within their DNA-binding domains a conserved cysteine-rich region encoding six cysteine residues that coordinate two atoms of zinc (the Zn 2 Cys 6 domain). The DNA recognition sequences to which these activators bind have been identified and all have in common a CGG or related triplet in each of the two symmetrically opposed half-sites (2-4). The sequences between the triplets as well as the spacing of the triplet sequence are highly variable, indicating that these variables may in large part determine specific recognition. Based on the extent of homology between the Zn 2 Cys 6 activators, it was hypothesized that the Cys-rich region performs a function common to all members of this class of proteins and the region immediately adjacent, which encodes divergent sequences, determines DNA binding specificity (5). We previously demonstrated that this model is essentially correct (6). All but one (Lys 23 ) of the 28 amino acids in the Zn 2 Cys 6 region of GAL4 can be replaced by the analogous sequences of PPR1, an activator of pyrimidine biosynthetic genes (7), without changing DNA binding specificity. In contrast, replacing the 14 amino acids immediately adjacent to the Zn 2 Cys 6 region resulted in switching the DNA binding specificity to that of PPR1. Similar results were obtained when hybrids of L...

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