HYBRID TRYPSINIZED NUCLEOSOMAL ARRAYS - IDENTIFICATION OF MULTIPLE FUNCTIONAL ROLES OF THE H2A H2B AND H3/H4 N-TERMINI IN CHROMATIN FIBER COMPACTION/

A defined 12-mer nucleosomal array in solution exists in a complex equ ilibrium between an unfolded 29S conformation, a 40S folding intermedi ate, an extensively folded 55S conformation, and soluble oligomeric st ates formed from cooperative intermolecular association of individual 12-mer arrays. Proteolytic removal of all of the core histone N-termin al tail domains previously has been shown to abolish both salt-depende nt nucleosomal array folding and oligomerization. To elucidate the ind ividual contributions of the H2A/H2B and H3/H4 tail domains to nucleos omal array condensation, ''hybrid'' trypsinized nucleosomal arrays hav e been assembled from tandemly repeated 5S rDNA and either trypsinized H3/H4 tetramers and intact H2A/H2B dimers or trypsinized H2A/H2B dime rs and intact H3/H4 tetramers. Neither of the hybrid trypsinized array s formed either the 40S or the 55S folded conformations in 2 mM MgCl2. In greater than or equal to 4 mM MgCl2, however, both fully trypsiniz ed arrays and each hybrid trypsinized array formed the 40S folding int ermediate, but not the 55S conformation. In contrast to folding, each hybrid trypsinized nucleosomal array oligomerized completely in MgCl2. These studies have identified three mechanistically distinct function s performed by the core histone N-termini during salt-dependent conden sation of nucleosomal arrays. The complexity of tail domain function i n chromatin is discussed in the context of a competitive interaction m odel in which the core histone N-termini provide direct mechanistic li nks between the structure and function of the chromatin fiber.