ROLE OF THE HISTONE AMINO TERMINI IN FACILITATED BINDING OF A TRANSCRIPTION FACTOR, GAL4-AH, TO NUCLEOSOME CORES

Facilitated, ''cooperative'' binding of GAL4-AH to nucleosomal DNA occ urred in response to inhibition from the core histone amino termini. T he binding of GAL4-AH (which contains the DNA-binding and dimerization domains of GAL4) to nucleosome cores containing multiple binding site s initiated at the end of a nucleosome core and proceeded in a coopera tive manner until all sites were occupied. However, following tryptic removal of the core histone amino termini, GAL4-AH binding appeared to be noncooperative, similar to binding naked DNA. Binding of GAL4-AH t o nucleosomes bearing a single GAL4 site at different positions indica ted that inhibition of GAL4 binding was largely mediated by the histon e amino termini and primarily occurred at sites well within the core a nd not near the end. When the histone amino termini were intact, bindi ng of GAL4-AH to sites near the center of a nucleosome core was greatl y enhanced by the presence of additional GAL4 dimers bound to more-acc essible positions. These data illustrate that the binding of a factor to more-accessible sites, near the end of a nucleosome, allows facilit ated binding of additional factors to the center of the nucleosome, th ereby overcoming repression from the core histone amino termini. This mechanism may contribute to the binding of multiple factors to complex promoter and enhancer elements in cellular chromatin.