USE OF A MAMMALIAN INTERSPERSED REPETITIVE (MIR) ELEMENT IN THE CODING AND PROCESSING SEQUENCES OF MAMMALIAN GENES

The mammalian interspersed repetitive (MIR) element was amplified in m ammals 130 million years ago. The MIR element is at least 260 bp in le ngth and is found in similar to 105 copies in the mammalian genome. We analyzed copies of the MIR element in the DNA of various mammals to d etermine its relationship to the structure and function of genes, in a n attempt to identify specific uses of the MIR element within the mamm alian genome. We found that alternative splicing within the acetylchol ine receptor gene in humans takes place within the MIR element and res ults in the incorporation of part of the MIR element into the coding s equence of this gene. Furthermore, the polyadenylation signal (AATAAA) at the 3' end of four different mammalian genes is derived from the M IR element. These uses of the MIR element suggest that other regulator y sequences found within the mammalian genome originated from ancient transposable elements, many of which may no longer be recognizable.