Isolation and characterization of the 5'-flanking sequence of the human ocular lens MIP gene

The MIP (major intrinsic protein) gene, a member of an ancient family of membrane channel genes, encodes the predominant fiber cell membrane protein of the ocular lens. Its specific expression in the lens fibers is temporally and spatially regulated during development. To study the regulation of expression of MIP and delineate the regulatory elements underlying its tissue specificity and ontogenic profile, we have cloned 2840 bp of the human MIP 5'-flanking sequence. The human MIP 5'-flanking sequence contains three complete Alu repetitive elements in tandem at position between nt -1699 and -2684 (nt -1699/-2684). These Alu elements appear to have had a complex evolutionary history with insertions at different times. We have fused DNA fragments containing MIP 5'-flanking sequences to the bacterial cat reporter gene encoding chloramphenicol acetyltransferase and assayed them in primary cultures of chicken lens cells. We have mapped two negative regulatory regions in the human MIP 5'-flanking sequences -1564/-1696 and -948/-1000. We demonstrated that the human MIP 5'-flanking sequence -253/+42 contains a functional promoter in lens cells but is inactive in kidney epithelial cells or mouse fibroblasts, suggesting that this sequence contains regulatory elements responsible for the lens-specific expression of MIP.