Although the organization of chromatin appears static in genomes, the processes of gene activation, transcription, and chromatin remodeling drive continual changes in structure. These changes served to fine-tune the exposure of transcription factor binding sites, and clear the path for elongating RNA polymerases. Structural changes to nucleosomes include pushing nucleosomes around, partially unwrapping nucleosomes, and at its most extreme stripping histones off of DNA. These processes require histone chaperones and chromatin remodelers to both dis-assemble old nucleosomes and then re-assemble new ones.
Our studies of nucleosome dynamics were initially stimulated by studying histone variants encoded in the Drosophila genome. Alternative histone variants are conserved throughout eukaryotic evolution, and we found that one of these – the H3 histone variant H3.3 – is the exclusive substrate for a distinct nucleosome assembly system. This Replication-Independent (RI) system is dedicated to rebuilding nucleosomes in dynamic chromatin regions, particularly at transcribed genes. The RI assembly system and its’ use of H3.3 is now known to be conserved in all eukaryotes. We currently study the factors that mediate RI assembly, and the importance of RI assembly and H3.3 for chromatin function. Recent discoveries with human patients highlight the importance of H3.3: some ‘driver’ mutations in aggressive cancers map to both the H3.3 histone and in H3.3-specific assembly factors. We use Drosophila to define how these mutations affect chromatin function.