The role of alternative histone usage in differentiated cell and tissue functions

Histones play an essential role in the function of both dividing and post-mitotic cells through the regulation of chromatin related processes, such as replication, repair and gene expression. Any defect that affects the amount or structure of histone proteins can lead to cell death or to failures in division. Although several degenerative diseases and cancer types have been associated with histone dysfunctions, there are still many open questions about histone functions. In the focus of our interest is the role of replication indepent (RI) histones. The His4r is the gene of an alternative form of the core histone H4 in Drosophila. The gene is located outside the canonical histone cluster, it is present in the genome in one copy as typical alternative histones are, transcribed independently to the time of DNA replication, and has a regular eukaryotic gene structure. Unlike canonical histones, His4r contains an intron and the mRNA transcribed from it is polyadenylated. Nonetheless, the amino acid sequence of His4r protein is identical to that of the canonical H4, what poses the intriguing question that what could be the functional role of His4r. In order to make possible distinquishing His4r from its identical canonical H4 counterpart we have created a transgenic Drosophila line in which the His4r gene is modified to produce His4r with a 3xFlag tag. Using this Drosophila line we analysed His4r expression in different tissues at several stages of fly development, and found that the ubiquitous expression of His4r becomes cell type-specific during neuronal development. Based on promoter analysis of His4r, and expression data from experiments of our own and others, we hypothesize that His4r may play a role in neuronal differentiation and environmental stress responses. To confirm this theory, we performed ChIP-seq experiment and found that His4r indeed showed an increased amount at inducible genes and genes controling neuronal differentiation. This genomic distribution raises the possibility that His4r may also play a role in the development of transcriptional memory. Experiments to verify this assumption are in progress.