Two distinct enhancers with different cell specificities coexist in the regulatory region of polyoma.

Two distinct nonoverlapping enhancer elements can be defined within the polyoma enhancer region. In mouse fibroblasts, element A provides a 3-fold higher enhancement of the alpha 2-collagen promoter than element B. In mouse embryonal carcinoma cells, element B shows the same efficiency as in fibroblasts, whereas that of element A decreases by a factor of 3.5. Moreover, a single point mutation (PyEC F9.1) increases the efficiency of element B in both cell types, making it superior to element A in embryonal carcinoma cells. The core of element A is located within a 35 bp region tandem duplicated in several wild-type strains, mostly homologous to a crucial repeated sequence of adenovirus E1a enhancer (Hearing and Shenk, 1983). Element B contains the consensus sequence of Weiher et al. (1982). These two homologies are precisely adjacent to the two DNAase l-hypersensitive sites present in viral chromatin.