Histone dynamics and roles of histone acetyltransferases during cold-induced gene regulation in Arabidopsis

In Arabidopsis, CBF transcription factors bind to and activate certain cold-regulated (COR) gene promoters during cold acclimation. Consistent with the prevailing model that histone acetylation and nucleosomal depletion correspond with transcriptionally active genes, we now report that H3 acetylation increases and nucleosome occupancy decreases at COR gene promoters upon cold acclimation. Overexpression of CBF1 resulted in a constitutive increase in H3 acetylation and decrease in nucleosome occupancy, consistent with the constitutive activation of COR gene expression. Overexpression of a truncated form of CBF2 lacking its transcriptional activation domain resulted in a cold-stimulated increase in H3 acetylation, but no change in nucleosomal occupancy or COR gene expression, indicating that histone acetylation is congruent with but not sufficient for cold-activation of COR gene expression. Plants homozygous for T-DNA disruption alleles of GCN5 (encoding a histone acetyltransferase) or ADA2b (a GCN5-interacting protein) show diminished expression of COR genes during cold acclimation. Contrary to expectations, H3 acetylation at COR gene promoters was stimulated upon cold acclimation in ada2b and gcn5 plants as in wild type plants, but the decrease in nucleosome occupancy was diminished. Thus, GCN5 is not the HAT responsible for histone acetylation at COR gene promoters during cold acclimation. Several other HAT mutant plants were also tested; although some do affect COR gene expression, none affected histone acetylation. Therefore, H3 acetylation at the COR gene promoters is not solely dependent on any of the HATs tested.