THE GLOBULAR DOMAIN OF HISTONE-H1 IS SUFFICIENT TO DIRECT SPECIFIC GENE REPRESSION IN EARLY XENOPUS EMBRYOS

One molecule of a linker histone such as histone H1 is incorporated in to every metazoan nucleosome [1]. Histone H1 has three distinct struct ural domains: the positively charged amino-terminal and carboxy-termin al tails are separated by a globular domain that is similar to the win ged-helix motif found in sequence-specific DNA-binding proteins [2], T he globular domain interacts with DNA immediately contiguous to that w rapped around the core histones [3,4], whereas the tail domains are im portant for the compaction of nucleosomal arrays [5], Experiments in v ivo indicate that histone H1 does not function as a global transcripti onal repressor, but instead has more specific regulatory roles [6-9], In Xenopus, maternal stores of the B4 linker histone that are assemble d into chromatin during the early cleavage divisions are replaced by s omatic histone H1 during gastrulation [10], This transition in chromat in composition causes the repression of genes encoding oocyte-type 5S rRNAs, and restricts the competence of ectodermal cells to differentia te into mesoderm [6,9-11], Here, we demonstrate that the globular doma in of histone H1 is sufficient for di recti ng gene-specific transcrip tional repression and for restricting the mesodermal competence of emb ryonic ectoderm, We discuss our results in the context of specific str uctural roles for this domain in the nucleosome.