Methylation status and chromatin structure of an early response gene (ornithine decarboxylase) in resting and stimulated NIH-3T3 fibroblasts.

The early response gene ornithine decarboxylase (odc) is indispensable for normal and malignant cell growth. Although DNA methylation is generally associated with chromatin condensation and gene inactivation, the odc gene is heavily methylated at CCGG-sequences in animal cell lines. In this work we analyzed the chromatin structure and the DNA methylation status at the CpG-rich promoter sequences at the odc locus in mouse 3T3 fibroblasts. We show that the proximal promoter region of the odc locus is not hypermethylated, while the distal promoter sequences appear to have a few methylated CCGG-sites and display methylation polymorphism. Furthermore, it was found that the 5' promoter region of odc is constitutively more sensitive to micrococcal nuclease than the coding and 3' regions of the odc gene. Stimulation of the cells with serum resulted in an appearance of a DNase I sensitive site at the promoter region. The chromatin structure of the mid-coding and 3' regions of the odc gene also underwent structural changes that were accompanied by the rapid accumulation of odc mRNA. Such changes were not detected in the chromatin structure of glyceraldehyde-3-phosphate dehydrogenase (gadph) gene, whose expression remains invariant upon serum stimulation. These data suggest that the chromatin structure may play an important role in the rapid transcriptional activation of odc and other immediate early genes during serum stimulation.