Incorporation of the histone variant H2A.Z counteracts gene silencing mediated by H3K27 trimethylation in Fusarium fujikuroi

BACKGROUND

Fusarium fujikuroi is a pathogen of rice causing diverse disease symptoms such as 'bakanae' or stunting, most likely due to the production of various natural products (NPs) during infection. Fusaria have the genetic potential to synthesize a plethora of these compounds with often diverse bioactivity. The capability to synthesize NPs exceeds the number of those being produced by far, implying a gene regulatory network decisive to induce production. One such regulatory layer is the chromatin structure and chromatin-based modifications associated with it. One prominent example is the exchange of histones against histone variants such as the H2A variant H2A.Z. Though H2A.Z already is well studied in several model organisms, its regulatory functions are not well understood. Here, we used F. fujikuroi as a model to explore the role of the prominent histone variant FfH2A.Z in gene expression within euchromatin and facultative heterochromatin.

RESULTS

Through the combination of diverse '-omics' methods, we show the global distribution of FfH2A.Z and analyze putative crosstalks between the histone variant and two prominent histone marks, i.e., H3K4me3 and H3K27me3, important for active gene transcription and silencing, respectively. We demonstrate that, if FfH2A.Z is positioned at the + 1-nucleosome, it poises chromatin for gene transcription, also within facultative heterochromatin. Lastly, functional characterization of FfH2A.Z overexpression and depletion mutants revealed that FfH2A.Z is important for wild type-like fungal development and secondary metabolism.

CONCLUSION

In this study, we show that the histone variant FfH2A.Z is a mark of positive gene transcription and acts independently of the chromatin state most likely through the stabilization of the + 1-nucleosome. Furthermore, we demonstrate that FfH2A.Z depletion does not influence the establishment of both H3K27me3 and H3K4me3, thus indicating no crosstalk between FfH2A.Z and both histone marks. These results highlight the manifold functions of the histone variant FfH2A.Z in the phytopathogen F. fujikuroi, which are distinct regarding gene transcription and crosstalk with the two prominent histone marks H3K27me3 and H3K4me3, as proposed for other model organisms.

Actin-related protein 6 facilitates proneural protein-induced gene activation for rapid neural differentiation

Neurogenesis is initiated by basic helix-loop-helix proneural proteins. Here, we show that Actin-related protein 6 (Arp6), a core component of the H2A.Z exchange complex SWR1, interacts with proneural proteins and is crucial for efficient onset of proneural protein target gene expression. Arp6 mutants exhibit reduced transcription in sensory organ precursors (SOPs) downstream of the proneural protein patterning event. This leads to retarded differentiation and division of SOPs and smaller sensory organs. These phenotypes are also observed in proneural gene hypomorphic mutants. Proneural protein expression is not reduced in Arp6 mutants. Enhanced proneural gene expression fails to rescue retarded differentiation in Arp6 mutants, suggesting that Arp6 acts downstream of or in parallel with proneural proteins. H2A.Z mutants display Arp6-like retardation in SOPs. Transcriptomic analyses demonstrate that loss of Arp6 and H2A.Z preferentially decreases expression of proneural protein-activated genes. H2A.Z enrichment in nucleosomes around the transcription start site before neurogenesis correlates highly with greater activation of proneural protein target genes by H2A.Z. We propose that upon proneural protein binding to E-box sites, H2A.Z incorporation around the transcription start site allows rapid and efficient activation of target genes, promoting rapid neural differentiation.

An improved functional analysis of linker-mediated complex (iFALC) strategy

The functional analysis of linker-mediated complex (FALC) strategy that facilitates functional analysis of a common subunit of multi-subunit protein complexes in cells constitutes three steps; (1) a common subunit is fused to a specific subunit via recombinant DNA, (2) mutation is introduced into a portion of the common subunit of the fused protein, and (3) the mutational effect on the fused protein is evaluated by transformation and analysis of multiple appropriate gene knockout yeast strains. Conceptually, the FALC strategy is applicable to any common subunit of multi-subunit protein complexes in any cell type. However, the proximity of two subunits to fuse, preparation of multiple gene knockout cells, and utilization of yeast cells can together prevent the practical and broad usage of the FALC strategy for analyzing all multi-subunit complexes in all cell types. In this study, we analyzed histone H2B as a common subunit of histone H2A/H2B and histone variant H2A.Z/H2B dimers. The FALC strategy was improved in three ways; (i) a long linker (up to 300 amino acids) was used to fuse H2B with H2A.Z in yeast cells, (ii) the effects of the fused H2B-H2A.Z harboring mutation in the H2B portion was evaluated in H2A.Z knockout yeast strains and it was not essential to knockout two copies of H2B genes, and (iii) this occurred even in vertebrate cells possessing a dozen H2B genes. This improved FALC (iFALC) strategy reveals that vertebrate H2B-D68, corresponding to yeast H2B-D71, is critical for chromatin binding of the H2A.Z/H2B dimer, and this is evolutionarily conserved.