In eukaryotic cells, the packaging of DNA into the nucleus is performed by histone proteins. Through the condensation of chromatin, which is formed as a complex of histones and DNA, these proteins are responsible for genome stability and the regulation of gene function. The replication dependent canonical histones are found in all cells and are involved in the regulation of affected regions in a manner dependent on their post-translational modifications. In contrast, replication-independent histone variants have specific functions in regulating gene expression and may also show cell type-specific expression.
The canonical histone genes of Drosophila melanogaster are located in a large cluster on the 2nd chromosome, containing large number of gene units. In contrast to canonical histones, histone variants are located outside the canonical histone cluster in the genome, in one or two copies, and are generally slightly different in structure from their canonical counterparts, and their specific regulation allows them to perform tissue or differentiation specific functions.
Unlike other alternative histones, the amino acid sequence of the protein product of the H4 variant (H4r) gene is the same as that of canonical H4. In order to distinguish H4r from canonical H4 and to obtain information about its role, we epitope-tagged H4r, studied its spatial and temporal expression, and revealed its localization in chromatin at the nucleosomal level. Based on immunohistochemical assays, H4r is generally expressed in embryonic and larval developmental states, but in the developing nervous system, slight differences in H4r expression are observed between cell types, a smaller population of the brain cells showing H4r accumulation that is not restrected to cell-type or differentiation stadium. ChIP-seq experiments have shown that the localization of H4r is preferentially associated with regulatory regions, especially for genes involved in cellular stress responses. Taking together, the experimental data indicate that H4r has a specific variant histone function that differs from the function of the canonical H4, with which it might contribute to the formation of transcriptional memory in non-dividing cells.