Enhancer and promoter activity in the JH to IGHM intron of the Pekin duck, Anas platyrhynchos

A transcriptional enhancer, Eμ, was defined in the IGH locus of the Pekin duck, Anas platyrhynchos. Regions of DNA from the JH to IGHM intron were cloned into reporter constructs containing the SV40 promoter and transiently transfected into chicken B and T lymphocytes. A strong transcriptional activity, of several hundred-fold greater than that of a reporter construct with the promoter alone, was localized to a 281 bp region that contains 2 E-box motifs, CAGCTG. This fragment showed enhancer activity in both orientations and was active in chicken B cells but not in T cells. When the activity of the enhancer was tested in constructs without a promoter, it showed high transcriptional activity in the forward orientation, but much less activity (by two orders of magnitude) when tested in the reverse orientation. This suggests that the fragment contains not only enhancer activity but may contain promoter activity analogous to that of the Iμ promoter described in mammals. Thus it appears that the location, but not the fine structure, of the Eμ enhancer was established before the evolutionary divergence of the avian and mammalian lineages some 300 Myr ago.