In most animals, the start of embryogenesis requires specific histones. In Drosophila
linker histone variant BigH1 is present in early embryos. To uncover the specific
role of this alternative linker histone at early embryogenesis, we established fly
lines in which domains of BigH1 have been replaced partially or completely with that
of H1. Analysis of the resulting Drosophila lines revealed that at normal temperature
somatic H1 can substitute the alternative linker histone, but at low temperature the
globular and C-terminal domains of BigH1 are essential for embryogenesis. In the presence
of BigH1 nucleosome stability increases and core histone incorporation into nucleosomes
is more rapid, while nucleosome spacing is unchanged. Chromatin formation in the presence
of BigH1 permits the fast-paced nuclear divisions of the early embryo. We propose
a model which explains how this specific linker histone ensures the rapid nucleosome
reassembly required during quick replication cycles at the start of embryogenesis.
