The binding of all known linker histones, named H1a through H1e, including H1 0 and H1t, to a model chromatin complex based on a DNA fragment containing the mouse mammary tumor virus long terminal repeat promotor was systematically studied. As for the histone subtype H1b, we found a dissociation constant of 8 -16 nM to a single mononucleosome (210 base pairs), whereas the binding constant of all other subtypes varied between 2 and 4 nM. Most of the H1 histones, namely H1a, H1c, H1d/e, and H1 0 , completely aggregate polynucleosomes (1.3 kilobase pairs, 6 nucleosomes) at 270 -360 nM, corresponding to a molar ratio of six to eight H1 molecules per reconstituted nucleosome. To form aggregates with the histones H1t and H1b, however, greater amounts of protein were required. Furthermore, our results show that specific types of in vivo phosphorylation of the linker histone tails influence both the binding to mononucleosomes and the aggregation of polynucleosomes. S phase-specific phosphorylation with one to three phosphate groups at specific sites in the C terminus influences neither the binding to a mononucleosome nor the aggregation of polynucleosomes. In contrast, highly phosphorylated H1 histones with four to five phosphate groups in the C and N termini reveal a very high binding affinity to a mononucleosome but a low chromatin aggregation capability. These findings suggest that specific S phase or mitotic phosphorylation sites act independently and have distinct functional roles.H1 histones are a heterogeneous group of at least five subtypes with closely related but nonetheless different primary structures (1, 2). Two further H1 subtypes are known: the histone H1 0 , which is found in nonreplicative tissues (3, 4) and in rapidly proliferating cells (5), and the testis-specific histone variant H1t (6). The various linker histones containing a globular central region flanked by highly basic and hydrophilic C-and N-terminal tails (7,8) bind to the nucleosome and promote the organization of nucleosomes to a higher order structure (9, 10).There is evidence that histone H1 may interact differently with transcriptionally active and inactive regions of chromatin (11). Linker histones are also thought to modulate nucleosome position (12, 13) and to influence replication efficiency in vitro (14).The presence of this large number of various H1 histone subtypes and their possible posttranslational modifications, such as phosphorylation (15), make it very probable that H1 histones play numerous structural and functional roles in chromatin. Until now, no specific role for the various variants has been established although Kaludov et al. (16) showed that the mouse histone H1b binds preferentially to a regulatory sequence within a mouse H3.2 replication-dependent histone gene. Previous analysis of the structural role of H1 histones demonstrated that three subfractions of H1 histones differ in their effectiveness in condensing DNA fibers into ordered aggregates (17) and that histone subtype H1t, compared with other subtypes, dif...
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