HMG-D and histone H1 interplay during chromatin assembly and early embryogenesis

MIG-D is an abundant chromosomal protein associated with condensed chromati n during the first nuclear cleavage cycles of the developing Drosophila emb ryo. We previously suggested that HMG-D might substitute for the linker his tone H1 in the preblastoderm embryo and that this substitution might result in the characteristic less compacted chromatin. We have now studied the as sociation of HMG-D with chromatin using a cell-free system for chromatin re constitution derived from Drosophila embryos. Association of HMG-D with chr omatin, like that of histone H1, increases the nucleosome spacing indicativ e of binding to the linker DNA between nucleosomes. HMG-D interacts with DN A during the early phases of nucleosome assembly but is gradually displaced as chromatin matures. By contrast, purified chromatin can be loaded with s toichiometric amounts of HMG-D, and this can be displaced upon addition of histone H1. A direct physical interaction between HMG-D and histone H1 was observed in a Far Western analysis. The competitive nature of this interact ion is reminiscent of the apparent replacement of HMG-D by H1 during mid-bl astula transition. These data are consistent with the hypothesis that HMG-D functions as a specialized linker protein prior to appearance of histone H 1.