NORMAL GENOMIC

• Hach A, Hon T, Zhang L
• The coiled coil dimerization element of the yeast transcriptional activator Hap1, a Gal4 family member, is dispensable for DNA binding but differentially affects transcriptional activation.
• J Biol Chem. 2000; 275: 248-54
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• The heme activator protein Hap1 is a member of the yeast Gal4 family, which consists of transcription factors with a conserved Zn(2)Cys(6) cluster that recognizes a CGG triplet. Many members of the Gal4 family contain a coiled coil dimerization element and bind symmetrically to DNA as homodimers. However, Hap1 possesses two unique properties. First, Hap1 binds asymmetrically to a direct repeat of two CGG triplets. Second, Hap1 binds to two classes of DNA elements, UAS1/CYC1 and UAS/CYC7, and permits differential transcriptional activation at these sites. Here we determined the residues of the Hap1 dimerization domain critical for DNA binding and differential transcriptional activation. We found that the Hap1 dimerization domain is composed of functionally redundant elements that can substitute each other in DNA binding and transcriptional activation. Remarkably, deletion of the coiled coil dimerization element did not severely diminish DNA binding and transcriptional activation at UAS1/CYC1 but completely abolished transcriptional activation at UAS/CYC7. Furthermore, Ala substitutions in the dimerization element selectively diminished transcriptional activation at UAS/CYC7. These results strongly suggest that the coiled coil dimerization element is responsible for differential transcriptional activation at UAS1/CYC1 and UAS/CYC7 and for making contacts with a putative coactivator or part of the transcription machinery.

• Stark MR, Escher D, Johnson AD
• A trans-acting peptide activates the yeast a1 repressor by raising its DNA-binding affinity.
• EMBO J. 1999; 18: 1621-9
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• The cooperative binding of gene regulatory proteins to DNA is a common feature of transcriptional control in both prokaryotes and eukaryotes. It is generally viewed as a simple energy coupling, through protein-protein interactions, of two or more DNA-binding proteins. In this paper, we show that the simple view does not account for the cooperative DNA binding of a1 and alpha2, two homeodomain proteins from budding yeast. Rather, we show through the use of chimeric proteins and synthetic peptides that, upon heterodimerization, alpha2 instructs a1 to bind DNA. This change is induced by contact with a peptide contributed by alpha2, and this contact converts a1 from a weak to a strong DNA-binding protein. This explains, in part, how high DNA-binding specificity is achieved only when the two gene regulatory proteins conjoin. We also provide evidence that features of the a1-alpha2 interaction can serve as a model for other examples of protein-protein interactions, including that between the herpes virus transcriptional activator VP16 and the mammalian homeodomain-containing protein Oct-l.

• Allain FH et al.
• Solution structure of the HMG protein NHP6A and its interaction with DNA reveals the structural determinants for non-sequence-specific binding.
• EMBO J. 1999; 18: 2563-79
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• NHP6A is a chromatin-associated protein from Saccharomyces cerevisiae belonging to the HMG1/2 family of non-specific DNA binding proteins. NHP6A has only one HMG DNA binding domain and forms relatively stable complexes with DNA. We have determined the solution structure of NHP6A and constructed an NMR-based model structure of the DNA complex. The free NHP6A folds into an L-shaped three alpha-helix structure, and contains an unstructured 17 amino acid basic tail N-terminal to the HMG box. Intermolecular NOEs assigned between NHP6A and a 15 bp 13C,15N-labeled DNA duplex containing the SRY recognition sequence have positioned the NHP6A HMG domain onto the minor groove of the DNA at a site that is shifted by 1 bp and in reverse orientation from that found in the SRY-DNA complex. In the model structure of the NHP6A-DNA complex, the N-terminal basic tail is wrapped around the major groove in a manner mimicking the C-terminal tail of LEF1. The DNA in the complex is severely distorted and contains two adjacent kinks where side chains of methionine and phenylalanine that are important for bending are inserted. The NHP6A-DNA model structure provides insight into how this class of architectural DNA binding proteins may select preferential binding sites.

• Suckow M, Kisters-Woike B, Hollenberg CP
• A novel feature of DNA recognition: a mutant Gcn4p bZip peptide with dual DNA binding specificities dependent of half-site spacing.
• J Mol Biol. 1999; 286: 983-7
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• Homodimeric DNA-binding proteins with relaxed half-site spacing requirements for their DNA targets have been described. As an example, the yeast transcriptional activator Gcn4p binds in vitro equally well to the AP1 site (5'A4T3G2A1C0T1'C2'A3'T4'3') and the ATF/CREB site (5'A4T3G2A1C0G0'T1'C2'A3'T4'3'), which have identical but differently spaced half-site blocks. We describe a novel feature for the bZip class of DNA-binding proteins. The N-14 mutant of a Gcn4p-derived bZip peptide shows a diametrically opposed base-pair recognition specificity depending on the half-site spacing of its DNA target: on pseudo-palindromic, AP1 site-like binding sites, guanine is required in position 2 for proper binding; in contrast, on palindromic, ATF/CREB site-like targets, position 2 must be cytosine to prevent a loss of binding. Modeling studies suggest that the different base-pair requirements on differently spaced DNA targets are due to minimal alterations of the distances between the relevant atoms of the N-14 side-chain and the corresponding target groups on the DNA. Although the N-14 peptide does not have a natural counterpart, its behavior hints at the possibility that dual binding modi dependent on half-site spacing may occur also for natural homodimeric DNA-binding proteins.

• Kim YG, Smith J, Durgesha M, Chandrasegaran S
• Chimeric restriction enzyme: Gal4 fusion to FokI cleavage domain.
• Biol Chem. 1998; 379: 489-95
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• Gal4, a yeast protein, activates transcription of genes required for metabolism of galactose and melibiose. It binds as a dimer to a consensus palindromic 17-base pair DNA sequence. It is a member of the third family of proteins that contain zinc-mediated peptide loops that interact specifically with nucleic acids. Gal4 has a very distinctive zinc coordination profile and mode of DNA-binding. Here, we report the creation of a novel site-specific endonuclease by linking the N-terminal 147 amino acids of Gal4 to the cleavage domain of FokI endonuclease. The fusion protein is active and under optimal conditions, binds to a 17 bp consensus DNA site and cleaves near this site. As expected, the cleavage occurs on either side of the consensus binding site(s).

• Vuidepot AL, Bontems F, Gervais M, Guiard B, Shechter E, Lallemand JY
• NMR analysis of CYP1(HAP1) DNA binding domain-CYC1 upstream activation sequence interactions: recognition of a CGG trinucleotide and of an additional thymine 5 bp downstream by the zinc cluster and the N-terminal extremity of the protein.
• Nucleic Acids Res. 1997; 25: 3042-50
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• The DNA binding domain of the yeast transcriptional activator CYP1(HAP1) contains a zinc-cluster structure. The structures of the DNA binding domain-DNA complexes of two other zinc-cluster proteins (GAL4 and PPR1) have been studied by X-ray crystallography. Their binding domains present, besides the zinc cluster, a short linker peptide and a dimerization element. They recognize, as homodimers, two rotationally symmetric CGG trinucleotides, the linker peptide and the dimerization element playing a crucial role in binding specificity. Surprisingly, CYP1 recognizes degenerate forms of a direct repeat, CGGnnnTAnCGGnnnTA, and the role of its linker is under discussion. To better understand the binding specificity of CYP1, we have studied, by NMR, the interaction between the CYP1(55-126) peptide and two DNA fragments derived from the CYC1 upstream activation sequence 1B. Our data indicate that CYP1(55-126) interacts with a CGG and with a thymine 5 bp downstream. The CGG trinucleotide is recognized by the zinc cluster in the major groove, as for GAL4 and PPR1, and the thymine is bound in the minor groove by the N-terminal region, which possesses a basic stretch of arginyl and lysyl residues. This suggests that the CYP1(55-126) N-terminal region could play a role in the affinity and/or specificity of the interaction with its DNA targets, in contrast to GAL4 and PPR1.

• Shore D
• Telomeres. Different means to common ends.
• Nature. 1997; 385: 676-7

• Reardon BJ, Winters RS, Gordon D, Winter E
• A peptide motif that recognizes A.T tracts in DNA.
• Proc Natl Acad Sci U S A. 1993; 90: 11327-31
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• The DAT1 gene of Saccharomyces cerevisiae encodes a DNA binding protein that specifically interacts with nonalternating oligo(A).oligo(T) tracts (A.T tracts). Deletion analysis of DAT1 coding information showed that the amino-terminal 36 residues are sufficient for specific DNA binding activity. Furthermore, a 35-residue synthetic peptide corresponding to amino acids 2-36 bound to A.T tracts with an equilibrium dissociation constant of 4 x 10(-10) M. Within this region the pentad Gly-Arg-Lys-Pro-Gly is repeated three times. Mutational analysis revealed that the Arg side chains are required for high-affinity binding, whereas the other pentad side chains are dispensable. Chemical interference experiments showed that the DAT1 protein interacts with the minor groove of the double helix. The data suggest that the pentad arginines interact in a cooperative manner with a repeated minor groove feature of A.T tract DNA to achieve high-affinity recognition. Amino acid similarities with other DNA binding proteins suggest that the DAT1 protein pentad represents a specialized example of a widespread motif used by proteins to recognize A.T base pairs.

• Gadhavi PL, Raine AR, Alefounder PR, Laue ED
• Complete assignment of the 1H NMR spectrum and secondary structure of the DNA binding domain of GAL4.
• FEBS Lett. 1990; 276: 49-53
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• Complete 1H NMR resonance assignments are presented for the cysteine rich region of the DNA binding domain of the yeast transcriptional activator GAL4. The protein contains short helical regions between Asp-12 and Leu-19 and between Lys-30 and Trp-36. It is clearly distinct from the C2H2 class of zinc finger protein typified by the Xenopus laevis transcription factor (TF)IIIA. We also find that the first SP(X)(X) sequence, a recently proposed DNA binding motif (residues 41 to 44), appears to be tightly packed against the metal binding domain.

• Carey M, Kakidani H, Leatherwood J, Mostashari F, Ptashne M
• An amino-terminal fragment of GAL4 binds DNA as a dimer.
• J Mol Biol. 1989; 209: 423-32
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• GAL4 is a yeast transcriptional activator protein that binds to specific 2-fold rotationally symmetric sites on DNA and stimulates transcription of the genes required for galactose catabolism. The DNA binding region of the protein is located within the first 74 amino acids and contains a "zinc finger" sequence motif. We show that a polypeptide comprising the first 147 amino acids of GAL4, designated GAL4 (1-147), binds DNA as a dimer in vitro. Although a protein containing only the first 74 amino acids, designated GAL4 (1-74), binds DNA specifically, its affinity is reduced relative to GAL4 (1-147). Addition of the strong dimerization domain of lambda repressor to GAL4 (1-74) generates a protein that binds as tightly as GAL4 (1-147). GAL4 (1-147) makes rotationally symmetric contacts with its recognition site when assayed by DNase I, exonuclease III and hydroxyl radical footprinting and by phosphate ethylation interference. Binding of GAL4 (1-147) in vitro requires either zinc or cadmium.