Histone variants are different from their canonical counterparts in structure and
are encoded by solitary genes with unique regulation to fulfill tissue or differentiation
specific functions. A single H4 variant gene (His4r or
H4r) that is located outside of the histone cluster and
gives rise to a polyA tailed messenger RNA via replication-independent expression
is preserved in Drosophila strains despite that its protein
product is identical with canonical H4. In order to reveal information on the possible
role of this alternative H4 we epitope tagged endogenous H4r and studied its spatial
and temporal expression, and revealed its genome-wide localization to chromatin at
the nucleosomal level. RNA and immunohistochemistry analysis of H4r
expressed under its cognate regulation indicate expression of the gene throughout
zygotic and larval development and presence of the protein product is evident already
in the pronuclei of fertilized eggs. In the developing nervous system a slight disequibrium
in H4r distribution is observable, cholinergic neurons are the most abundant among
H4r-expressing cells. ChIP-seq experiments revealed H4r association with regulatory
regions of genes involved in cellular stress response. The data presented here indicate
that H4r has a variant histone function.
