PROTEIN THAT BINDS TO THE DISTAL, BUT NOT TO THE PROXIMAL, CCAAT OF THE HUMAN THYMIDINE KINASE GENE PROMOTER

Mobility shift assays were used to examine protein binding to the huma n TK gene CCAAT boxes. Similar protein binding patterns were observed with probes containing either the proximal or distal CCAAT. However, p robes containing both CCAAT boxes in which one of the CCAAT boxes was inactivated by mutation did not demonstrate identical binding patterns . One of the complexes formed with the longer probes was only observed when the distal CCAAT was intact. This species was not formed with pr obes that only contained an intact proximal CCAAT, and its formation c ould only be competed by oligonucleotides containing the distal CCAAT motif. This observation reveals the existence of a protein that can bi nd to the distal, but not to the proximal, CCAAT of the human TK promo ter. This protein may account for the previous observation that the tw o CCAAT motifs are not functionally equivalent. The protein that binds to the distal, but not to the proximal, CCAAT (DTK-CBP) was also pres ent in two human cell lines. Significantly more DTK-CBP was present in nuclear extracts of HepG2 and WI38 cells than in TK(-)ts13 cells. How ever, this protein was not observed in three different murine cell lin es and one primary culture. Its abundance in some human cell lines sug gests it might modulate the expression of human TK mRNA in cells that express this protein. (C) 1995 Wiley-Liss, Inc.