UBIQUITOUS EXPRESSION OF THE 43-KDA AND 44-KDA FORMS OF TRANSCRIPTIONFACTOR USF IN MAMMALIAN-CELLS

USF is a helix-loop-helix transcription factor that, like Myc, recogni zes the DNA binding motif CACGTG. Two different forms of USF, characte rized by apparent molecular weights of 43,000 and 44,000, were origina lly identified in HeLa cells by biochemical analysis. Clones for the 4 3-kDa USF were first characterized, but only partial clones for the hu man 44-kDa USF (USF2, or FIP) have been reported. Here we describe a c omplete cDNA for the 44-kDa USF from murine cells. Analysis of this cl one has revealed that the various USF family members are quite diverge nt in their N-terminal amino acid sequences, while a high degree of co nservation characterizes their dimerization and DNA-binding domains. I nterestingly, the 3' noncoding region of the 44-kDa USF cDNAs displaye d an unusual degree of conservation between human and mouse. In vitro transcription/translation experiments indicated a possible role for th is region in translational regulation. Alternative splicing forms of t he 44-kDa USF messages exist in both mouse and human. Examination of t he tissue and cell-type distribution of USF by Northern blot and gel r etardation assays revealed that while expression of both the 43- and 4 4-kDa USF species is ubiquitous, different ratios of USF homo- and het erodimers are found in different cells.