HMG-D, THE DROSOPHILA-MELANOGASTER HOMOLOG OF HMG-1 PROTEIN, IS ASSOCIATED WITH EARLY EMBRYONIC CHROMATIN IN THE ABSENCE OF HISTONE H1

We show that HMG-D, an abundant chromosomal protein, is associated wit h condensed chromatin structures during the first six nuclear cleavage cycles of the developing Drosophila embryo and that histone H1 is abs ent from these same structures. As H1 accumulates from nuclear divisio n 7 onwards, the nuclei become more compact and transcriptionally acti ve. This compaction is paralleled by a reduction in size of mitotic ch romatin. In addition, we find a striking correlation between the switc h in HMG-D:H1 ratios and the changes that occur between nuclear cycles 8 and 13 that are collectively termed the mid-blastula transition. Th is transition is characterized by an increase in the nuclear cycle tim es, a change in the nucleo-cytoplasmic ratio, and a 5- to 20-fold decr ease in nuclear volume. We propose that this is a direct consequence o f a re-organization of chromatin from a less condensed state with HMG- D to a more condensed state with H1. We argue that HMG-D, either by it self or in conjunction with other chromosomal proteins, induces a cond ensed state of chromatin that is distinct from, and less compact than the H1-containing 30 nm fibre and that this state of chromatin could f acilitate rapid nuclear cycles.