| Abstrakt: |
ABSTRACTChromatin immunoprecipitation assays were employed to assess the kinetics of transcription factor assembly and histone modifications that occur during gamma interferon (IFN-γ) induction of CIITA gene expression. CIITA is the master regulator of major histocompatibility complex class II transcription. Promoter IV (PIV), the major IFN-γ responsive promoter for CIITA expression, requires both STAT1 and IFN regulatory factor 1 (IRF-1) for induction by IFN-γ. STAT1 binding to PIV was detected first and was accompanied by a modest acetylation of histones H3 and H4 that were associated with the region. Despite these changes, which occurred within 30 min of IFN-γ treatment, CIITA mRNA was not detected until IRF-1 protein was synthesized and bound to its site, a process that required >120 min. In contrast to these events, fetal trophoblast-like cell lines, which are refractory to CIITA induction by IFN-γ, failed to assemble the above factors or modify their chromatin, suggesting that accessibility to the promoter is blocked. Bisulfite sequencing of PIV showed strong hypermethylation of PIV, providing a link between methylation, chromatin structure, and factor binding. Together, this analysis provides a kinetic view of the activation of the CIITA gene in response to IFN-γ and shows that regulatory factor assembly, chromatin modification, and gene expression proceed in discrete steps. |
