XUBF, AN RNA-POLYMERASE-I TRANSCRIPTION FACTOR, BINDS CROSSOVER DNA WITH LOW SEQUENCE SPECIFICITY

Xenopus UBF (xUBF) is a transcription factor for RNA polymerase I whic h contains multiple DNA-binding motifs. These include a short basic re gion adjacent to a dimer motif plus five high-mobility-group (HMG) box es. All of these DNA-binding motifs exhibit low sequence specificity, whether assayed singly or together. In contrast, the HMG boxes recogni ze DNA structure that is formed when two double helices are crossed ov er each other. HMG box 1, in particular, requires association of two d ouble helices before it will bind and, either by itself or in the cont ext of the intact protein, will loop DNA and organize it into higher-o rder structures. We discuss how this mode of binding affects the funct ion of xUBF as a transcription factor.