The promoter of the endo A cytokeratin gene is activated by a 3' downstream enhancer.

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 alpha 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.     

A cell type-specific enhancer drives expression of the chick muscle acetylcholine receptor alpha-subunit gene

The regulation of acetylcholine receptor alpha-subunit gene expression was analyzed by transient expression assays. Using rabbit beta-globin cDNA as a reporter gene, we have confirmed that the 5'-flanking sequence of the chicken acetylcholine receptor alpha-subunit gene directs specific expression in differentiated C2C12 cells, a mouse muscle cell line, but not in undifferentiated C2C12 cells and mouse 3T3 fibroblasts. Testing chimeric plasmids containing Bal31 deletion mutants of the alpha-subunit gene upstream sequence, we found the -116 to -81 region of the alpha-subunit to be responsible for tissue- and stage-specific expression. This 36 bp fragment stimulates the activity of both alpha-subunit and SV40 promoters in a distance- and orientation-independent manner, thus fulfilling the criteria of an enhancer.