THE ESSENTIAL DNA-BINDING PROTEIN SAP1 OF SCHIZOSACCHAROMYCES-POMBE CONTAINS 2 INDEPENDENT OIIGOMERIZATION INTERFACES THAT DICTATE THE RELATIVE ORIENTATION OF THE DNA-BINDING DOMAIN

The sap1 gene from Schizosaccharomyces pombe, which is essential for m ating-type switching and for growth, encodes a sequence-specific DNA-b inding protein with no homology to other known proteins. We have used a reiterative selection procedure to isolate binding sites for sap1, u sing a bacterially expressed protein and randomized double strand olig onucleotides. The sap1 homodimer preferentially selects a pentameric m otif, TA(G)CG, organized as a direct repeat and spaced by 5 nucleotide s. Removal of a C-terminal dimerization domain abolishes recognition o f the direct repeat and creates a new specificity for a DNA sequence c ontaining the same pentameric motif but organized as an inverted repea t. We present evidence that the orientation of the DNA-binding domain is controlled by two independent oligomerization interfaces. The C-ter minal dimerization domain allows a head-to-tail organization of the DN A-binding domains in solution, while an N-terminal domain is invoked i n a cooperative interaction on the DNA target between pairs of dimers.