Differential Acetylation of Histone H3 at the Regulatory Region of OsDREB1b Promoter Facilitates Chromatin Remodelling and Transcription Activation during Cold Stress

The rice ortholog of DREB1, OsDREB1b, is transcriptionally induced by cold stress and over-expression of OsDREB1b results in increase tolerance towards high salt and freezing stress. This spatio-temporal expression of OsDREB1b is preceded by the change in chromatin structure at the promoter and the upstream region for gene activation. The promoter and the upstream region of OsDREB1b genes appear to be arranged into a nucleosome array. Nucleosome mapping of similar to 700bp upstream region of OsDREB1b shows two positioned nucleosomes between -610 to -258 and a weakly positioned nucleosome at the core promoter and the TSS. Upon cold stress, there is a significant change in the nucleosome arrangement at the upstream region with increase in DNasel hypersensitivity or MNase digestion in the vicinity of cis elements and TATA box at the core promoter. ChIP assays shows hyper-acetylation of histone H3K9 throughout the locus whereas region specific increase was observed in H3K14ac and H3K27ac. Moreover, there is an enrichment of RNA PolII occupancy at the promoter region during transcription activation. There is no significant change in the H3 occupancy in OsDREB1b locus negating the possibility of nucleosome loss during cold stress. Interestingly, cold induced enhanced transcript level of OsDREB1b as well as histone H3 acetylation at the upstream region was found to diminish when stressed plants were returned to normal temperature. The result indicates absolute necessity of changes in chromatin conformation for the transcription up-regulation of OsDREB1b gene in response to cold stress. The combined results show the existence of closed chromatin conformation at the upstream and promoter region of OsDREB1b in the transcription off'' state. During cold stress, changes in region specific histone modification marks promote the alteration of chromatin structure to facilitate the binding of transcription machinery for proper gene expression.