Transcriptional repression by a conserved intronic sequence in the nicotinic receptor α3 subunit gene

The genes encoding the nicotinic acetylcholine receptor alpha 3, alpha 5, and beta 4 subunits are genomically clustered. These genes are co-expressed in a variety of cells in the peripheral and central nervous systems. Their gene products assemble in a number of stoichiometries to generate several nicotinic receptor subtypes that have distinct pharmacological and physiological properties. Signaling through these receptors is critical for a variety of fundamental biological processes. Despite their importance, the transcriptional mechanisms underlying their coordinated expression remain to be completely elucidated. By using a bioin-formatics approach, we identified a highly conserved intronic sequence within the fifth intron of the alpha 3 subunit gene. Reporter gene analysis demonstrated that this sequence, termed "alpha 3 intron 5," inhibits the transcriptional activities of the alpha 3 and beta 4 subunit gene promoters. This repressive activity is position- and orientation-independent. Importantly, repression occurs in a cell type-specific manner, being present in cells that do not express the receptor genes or expresses them at very low levels. Electrophoretic mobility shift assays demonstrate that nuclear proteins specifically interact with alpha 3 intron 5 at two distinct sites. We propose that this intronic repressor element is important for the restricted expression patterns of the nicotinic receptor alpha 3 and beta 4 subunit genes.