THE TRANSLATIONAL PLACEMENT OF NUCLEOSOME CORES IN-VITRO DETERMINES THE ACCESS OF THE TRANSACTING FACTOR SUGF1 TO DNA

The sea urchin G-string binding factor (suGF1) is one of several prote ins that bind sequence-specifically to oligo(dG . dC) motifs, frequent ly present upstream of eukaryotic genes. In this study we investigate the interaction of suGF1, purified to near homogeneity, with its oligo (dG . dC) binding site in a reconstituted nucleosome core in vitro. We show that the in vitro reconstitution of a 214 bp fragment containing a suGF1 binding site results in the appearance of five distinct nucle osome core species. These species contain the histone octamer in an id entical rotational setting but in different translational frames. The resulting different nucleosomal locations of the suGF1 binding site in the five core species are shown to modulate the ability of suGF1 to b ind to nucleosomal DNA, even though the rotational setting of the DNA in the nucleosome cores maximally exposes the suGF1 binding site, We p ropose that a direct protein-protein steric clash between suGF1 and th e histone octamer is the most likely determinant in modulating the bin ding of suGF1 to its nucleosomally wrapped binding site. This result s uggests that in vivo suGF1, like TBP, NF1 and heat shock factor, may r equire a complementary nucleosome disrupting activity or that suGF1 bi nds to free nascent replicated DNA prior to nucleosome deposition.