Histone variants leading to altered nucleosome structure, dynamics and DNA accessibility
occur frequently, albeit rarely for H4. We carried out a comprehensive in silico scrutiny
of fungal genomes, which revealed the presence of a novel H4 variant (H4E) in the
ascomycetes, throughout the Pezizomycotina, in basal species of the Taphrinomycotina
and also in the Glomeromycota. The coding cognate genes show a specific intron/exon
organization, different from H4 canonical genes. H4Es diverge from canonical H4s mainly
in the N- and C-terminal extensions, showing marked differences in the distribution
and number of Lys and Arg residues, which may result in novel post-translational modifications.
In Aspergillus nidulans (Pezizomycotina, Eurotiomycetes) the H4E variant protein level
is low in mycelia. However, the encoding gene is well expressed at 37°C under nitrogen
starvation. H4E localizes to the nucleus and interacts with H3, but its absence or
overexpression does not result in any detectable phenotype. Deletion of only one of
the of the two canonical H4 genes results in a strikingly impaired growth phenotype,
which indicates that H4E cannot replace this canonical histone. Thus, an H4 variant
is present throughout a whole subphylum of the ascomycetes, but with hitherto no experimentally
detectable function.
