THE DNA-BINDING SPECIFICITY OF THE BASIC REGION OF THE YEAST TRANSCRIPTIONAL ACTIVATOR GCN4 CAN BE CHANGED BY SUBSTITUTION OF A SINGLE AMINO-ACID

The X-ray structure of a GCN4 DNA complex (1) shows, that specific DNA binding of the GCN4 basic region is mediated by a complicated network of base pair and DNA backbone contacts. According to the X-ray struct ure, alanine - 14 of the basic region of GCN4 (we define the first leu cine of the leucine zipper as + 1) makes a hydrophobic contact to the methyl group of the thymine next to the center of the GCN4 binding sit e 5' ATGACTCAT 3'. We tested the DNA binding properties of the ninetee n derivatives of GCN4, which carry all possible amino acids in positio n - 14 of the basic region. Substitution of alanine - 14 of GCN4 by ei ther asparagine or cysteine changes the DNA binding specificity. Serin e in this position broadens the specificity for position 1 of the targ et, whereas other amino acids either retain or decrease GCN4 specifici ty.