b43*: An Enhancer Displaying Neural-Restricted Activity Is Located in the 3*-Untranslated Exon of the Rat Nicotinic Acetylcholine Receptor b4 Gene

Members of a neuronal nicotinic acetylcholine receptor subunit gene cluster ordered b4, a3, a5 in the vertebrate genome are expressed in highly restricted patterns in the PNS and CNS. Nothing is known, however, about the regulatory elements that control transcription of these genes in selected neuronal cell populations. We report here a novel enhancer, designated b439, that is positioned in the b4 39-untranslated exon. It is composed of two nearly identical 37 bp direct repeats that are separated by 6 bp. Multimerization of the enhancer upstream of the a3 minimal promoter results in synergistic activation. Analysis in different cell types, including three neural lines and primary keratinocytes, shows that b439 is preferentially active in the neural line PC12, which expresses all members of the cluster. Mobility shift assays reveal a cell-type-specific complex, which forms with the first repeat of the enhancer and PC12 extracts. Complexes co-migrating with the PC12 cell complex are not detected with extracts from other lines, which suggests that PC12 cells contain a differentially expressed factor that may be important for the restricted activity of b439. The cell-type-specific activity of the b439 enhancer suggests that it is important for regulating restricted expression patterns of one or more clustered neuronal acetylcholine receptor genes. Its location within the b4 gene may be a selective pressure for maintaining tight linkage of clustered neuronal nAchR genes.