Histone modifications
N-terminal regions of nucleosome core complex histones
undergo various post-translational modifications. In
addition, each histone has variants encoded by different
genes. The combinations of histone variants and posttranslational
modifications can be considered a ‘histone
code’, which plays a key role in chromatin structure and
thus determines the transcriptional state and expression
level of genes. Some histone modifications, namely acetylation,
and certain phosphorylation and ubiquitination
[4,5], enhance transcription, while biotinylation and
sumoylation repress gene expression [6,7]. Trimethylation
of H3K4 activates transcription, while dimethylation
of H3K9 and H3K27 represses transcription [5]. Because
several of the histone modifications are associated with
changes in gene transcription in general, it is not surprising
that stress-induced gene regulation is associated with
histone modifications in all cases that have been investigated.
Changes in histone variants, histone modifi-
cations as well as DNA methylation are often referred
to as epigenetic regulation. However, such changes may
or may not be truly epigenetic in nature because common
epigenetics definition requires mitotic or meiotic heritability