The histone replacement gene His4r is involved in heat stress induced chromatin rearrangement

Abstract

His4r is the only known variant of histone H4 in Drosophila. It is encoded by the His4r single-copy gene that is located outside of the histone gene cluster and expressed in a different pattern than H4, although the encoded polypeptides are identical. We generated a null mutant (His4rΔ42) which is homozygous viable and fertile without any apparent morphological defects. Heterozygous His4rΔ42 is a mild suppressor of position-effect variegation, suggesting that His4r has a role in the formation or maintenance of condensed chromatin. Under standard conditions loss of His4r has a modest effect on gene expression. Upon heat-stress the induction of the Heat shock protein (HSP) genes Hsp27 and Hsp68 is stronger in His4rΔ42 mutants with concordantly increased survival rate. Analysis of chromatin accessibility after heat shock at a Hsp27 regulatory region showed less condensed chromatin in the absence of His4r while there was no difference at the gene body. Interestingly, preconditioning before heat shock led to increased chromatin accessibility, HSP gene transcription and survival rate in control flies while it did not cause notable changes in His4rΔ42. Thus, our results suggest that His4r might play a role in fine tuning chromatin structure at inducible gene promoters upon environmental stress conditions.

Despite its sequence identity with canonical H4, Drosophila H4r product is enriched at specific chromatin regions

“…Increased resistance to heat stress of H4r knock-out flies have been described recently 5 , and we found H4r enrichment on genes involved in environmental stress responses. These observations prompted us to examine how H4r localization changes upon activation of the heat stress response and after a period of recovery time following that.…”

“…H4r was suggested to play a role in the chromatin formation at the loci of inducible genes such as heat shock genes 5 . We wondered whether there are changes in H4r expression upon heat shock and following recovery, which would further clarify the role of H4r in the expression of inducible genes.…”

“…H4r loss of function mutants show reduced viability whereas the fertility and longevity of mutants is not affected. The lack of H4r leads to minor changes in the transcriptional pattern, mostly upregulating genes 5 . Our results of H4r localization on genes associated with developmental processes and genes implicated in response to various stimuli, might be interpreted as sign of a possible role of the H4r gene to provide a H4 pool that can be involved in dynamic alteration of chromatin structure after transcriptional changes that emerges with differentiation and environmental stimuli.…”

“…Deletion of H4r does not cause a visible phenotypic change 4 , however, the loss of H4r causes reduced viability; female H4r mutants showing lower viability than males. Loss of H4r is also coupled to increased heat-stress resistance, supposedly due to the less condensed chromatin that allows a quicker and stronger response to heat stress 5 .…”

Distinct developmental phenotypes result from mutation of Set8/KMT5A and histone H4 lysine 20 in Drosophila melanogaster

“…Although this gene is non-essential (Figure 5C, D), we and others have found that His4r can partially compensate for loss of RD H4. 5,19,28,49 Therefore, we used CRISPR-Cas9 to engineer two His4r alleles (a deletion 5 , His4r Δ 5 and a K20A mutant, His4r K20A ) and we combined them with the appropriate RD histone mutant genotypes (Figure 5A). As shown in Figures 5C and D, homozygous loss of His4r in an H4 K20A background ( H4 K20A , His4r Δ/Δ ) reduces viability, but does not eliminate it, indicating that His4r expression is important for the observed viability of H4 K20A mutants but is not required.…”