A UNIQUE TRANSACTIVATION SEQUENCE MOTIF IS FOUND IN THE CARBOXYL-TERMINAL DOMAIN OF THE SINGLE-STRAND-BINDING PROTEIN FBP

The far-upstream element-binding protein (FBP) is one of several recen tly described factors which bind to a single strand of DNA in the 5' r egion of the c-myc gene. Although cotransfection of FBP increases expr ession from a far-upstream element-bearing c-myc promoter reporter, th e mechanism of this stimulation is heretofore unknown, Can a single-st rand-binding protein function as a classical transactivator, or are th ese proteins restricted to stabilizing or altering the conformation of DNA in an architectural role? Using chimeric GAL4-FBP fusion proteins we have shown that the carboxyl-terminal region (residues 448 to 644) is a potent transcriptional activation domain. This region contains t hree copies of a unique amino acid sequence motif containing tyrosine diads, Analysis of deletion mutants demonstrated that a single tyrosin e motif alone (residues 609 to 644) was capable of activating transcri ption. The activation property of the C-terminal domain is repressed b y the N-terminal 107 amino acids of FBP, These results show that FBP c ontains a transactivation domain which can function alone, suggesting that FBP contributes directly to c-myc transcription while bound to a single-strand site. Furthermore, activation is mediated by a new motif which can be negatively regulated by a repression domain of FBP.