Identity and function of the variant histone H3.3 in C. elegans.

The basic element of chromatin architecture is the nucleosome, which consists of a hetero-octamer of histone proteins with two turns of DNA wrapped around it. The nucleosomes not only ensure the structural configuration of the chromatin architecture, but also participate in its functional organization, so alterations of histone proteins can greatly impact many processes in the organism. The incorporation of variant histones is one of the major ways to modulate nucleosome function. One such variant histone is H3.3, a major variant of histone H3. Its amino acid sequence is 93% identical to canonical H3 and is highly conserved among eukaryotes. Genomic studies revealed that H3.3 is incorporated into transcriptionally active as well as telomeric regions. To date, H3.3 has been shown to be involved in a diversity of processes like neuron plasticity and DNA damage response. It was also shown to be crucial for fertility and embryonic development - mice or flies lacking H3.3 are showing severe sterility and lethality phenotypes. To reconcile these diverse roles of H3.3 with its genomic distribution, and to gain mechanistic insight into its function, we turned to the simple model organism C. elegans. Surprisingly, unlike in other organisms, worms lacking H3.3 are not showing any obvious phenotype. To understand what is allowing worm viability in absence of H3.3, we are performing an RNAi synthetic lethality screen. This screen aims to identify genes the knockdown of which causes lethality or sterility for worm lacking H3.3, while not affecting wild type worms. To further characterize H3.3 in C. elegans, we inserted a gfp tag at each of the five endogens that express H3.3. We found strikingly different expression patterns for each gene, ranking from completely ubiquitous to sperm specific, and we are using ChIP-seq approach to investigate the genomic context of their incorporation into the chromatin.