Identification of the gene coding for the Endo B murine cytokeratin and its methylated, stable inactive state in mouse nonepithelial cells.

The Endo B type-I keratin intermediate filament protein is first expressed at the 4- to 8-cell stage of mouse development. In the adult, its expression is restricted to a variety of simple epithelial cell types. To investigate the mechanisms responsible for the restricted expression of Endo B, the gene coding for Endo B has been identified from among the five different Endo B genes found in the mouse genome by Southern hybridization analysis and cloning all or part of four of the genes. Nuclear run-on experiments demonstrate that Endo B expression is regulated at the level of transcription. The 5' end of the active gene, designated Endo beta 1, was found to be highly methylated and in a relatively nuclease-resistant chromatin conformation in fibroblasts and myoblasts that do not express Endo B, but undermethylated and relatively sensitive to nuclease digestion in endodermal cells or F9 embryonal carcinoma cells. The inactive state of the Endo B beta 1 gene in fibroblast appears to be very stable, because somatic cell hybrids formed by the fusion of HeLa cells, which express the homologous human protein, keratin 18, and mouse fibroblasts, continue to express keratin 18 but do not activate Endo B expression. Similarly, the fusion of mouse endodermal cells and fibroblasts results in hybrids that do not extinguish Endo B expression. These results suggest that Endo B transcription is limited by two different mechanisms. In somatic cells such as fibroblasts or myoblasts, expression may be restricted by methylation and a stable, nonpermissive transcriptional state. However, in embryonal carcinoma cells, the Endo B beta 1 gene is undermethylated and in a relatively nuclease-sensitive conformation, but it is restricted by an additional, negative regulatory mechanism.