Grossmann, L. C. | Search bioRxiv

bioRxiv2026

Histone H3 availability is more important for development than H3.2 versus H3.3 subtype identity

The distinct contributions of replication-dependent and replication-independent histones to development and genome function remain unclear. In this study, we investigate how the distinct protein identities of the histone H3.2 and H3.3 subtypes contribute to development and gene regulation in Drosophila. Comparing animals in which the replication-independent H3.3 genes were mutated to produce the replication-dependent H3.2 protein with those carrying deletions of the replication-independent H3.3 genes revealed that replication-independent H3.3 is essential for fertility, adult locomotor behavior, and normal longevity. However, development to adulthood does not depend on which replication-independent H3 subtype is expressed from the H3.3 loci. Moreover, replication-independent H3.3 is not required to establish or maintain global patterns of chromatin accessibility or gene expression in the adult brain. Surprisingly, we find that expression of H3.2 from the replication-dependent HisC locus is essential in post-replicative cells in the absence of replication-independent H3.3, and we uncover a critical role for the HIRA histone chaperone complex in preserving genome function when replication-independent H3.3 is deleted. We conclude that an available pool of H3 is more critical than the specific identity of H3 in the pool.

genetics↗

bioRxiv2023

Reduced histone gene copy number disrupts Drosophila Polycomb function

The chromatin of animal cells contains two types of histones: canonical histones that are expressed during S phase of the cell cycle to package the newly replicated genome, and variant histones with specialized functions that are expressed throughout the cell cycle and in non-proliferating cells. Determining whether and how canonical and variant histones cooperate to regulate genome function is integral to understanding how chromatin-based processes affect normal and pathological development. Here, we demonstrate that variant histone H3.3 is essential for Drosophila development only when canonical histone gene copy number is reduced, suggesting that coordination between canonical H3.2 and variant H3.3 expression is necessary to provide sufficient H3 protein for normal genome function. To identify genes that depend upon, or are involved in, this coordinate regulation we screened for heterozygous chromosome 3 deficiencies that impair development of flies bearing reduced H3.2 and H3.3 gene copy number. We identified two regions of chromosome 3 that conferred this phenotype, one of which contains the Polycomb gene, which is necessary for establishing domains of facultative chromatin that repress master regulator genes during development. We further found that reduction in Polycomb dosage decreases viability of animals with no H3.3 gene copies. Moreover, heterozygous Polycomb mutations result in de-repression of the Polycomb target gene Ubx and cause ectopic sex combs when either canonical or variant H3 gene copy number is also reduced. We conclude that Polycomb-mediated facultative heterochromatin function is compromised when canonical and variant H3 gene copy number falls below a critical threshold.

genetics↗