Mouse cytokeratin EndoA is an intermediate filament subunit of the type II cytokeratin class which initiates expression in trophectoderm cells of blastocyst during embryogenesis. To identify the regulatory elements of the endo A gene, we constructed a series of CAT expression vectors and transfected them into PYS-2 cells. We found an enhancer element locating 1 kb downstream from the endo A gene which acts on both the endo A and SV40 promoters. This enhancer consists of six direct repeated sequences with homology to the PEA3 motif in polyoma virus α enhancer core. In undifferentiated F9 embryonal carcinoma cells, expression of the construct containing the enhancer was not detected. These results indicate that one of the regulatory mechanisms of endo A gene expression is the 3′ downstream enhancer.
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