MUTANT BZIP-DNA COMPLEXES WITH 4 QUASI-IDENTICAL PROTEIN-DNA INTERFACES

The complex between the yeast transcriptional activator GCN4 and the p alindromic ATF/CREB site 5'-A4T3G2A1C0G0'T1'C2'A3'T4'-3' shows dyad s ymmetry. The basic region of GCN4 contains a segment of 18 amino acids with a partially palindromic sequence: N-LKRARNTEAARRSRARKL-C. Symme tric residues are underlined. Apart from the ATF/CREB site, GCN4 also binds well to the symmetric variants with guanine in position 4 (5'-G4 T3G2A1C0G0'T1'C2'A3'C4'-3') or thymine in position 0 (5'-A4T3G2A1T0*A 0'T1'C2'A3'T4'-3'). The half-sites of these sequences can be regarded as short pseudo-palindromes with central guanine 2/cytosine 2' base pa irs. We investigated whether the geometry of the peptide of the basic region of GCN4 could be functionally related to the pseudo-palindromic character of some target half-sites. Since inspection of the X-ray st ructures of GCN4-DNA complexes reveals that several amino acid-DNA int eractions are symmetric within the wild-type half-complexes, we introd uced mutations into a GCN4 bZip peptide that improve the symmetry of t he peptide. We found that most of the constructs retain specific DNA r ecognition. For one mutant, we conclude that it is not only capable of forming DNA complexes showing the well-known overall dyad symmetry, b ut that the protein-DNA interface of each half-complex can be divided further into two quasi-identical, quasi-symmetric substructures.