Two E-boxes are the focal point of muscle-specific skeletal muscle type 1 Na+ channel gene expression

Abstract

We have characterized a group of cis-regulatory elements that control muscle-specific expression of the rat skeletal muscle type 1 sodium channel (SkM1) gene. These elements are located within a 3.1-kilobase fragment that encompasses the 5'-flanking region, first exon, and part of the first intron of SkM1. We sequenced the region between -1062 and +311 and determined the start sites of transcription; multiple sites were identified between +1 and +30. The basal promoter (-65/+11) lacks cell-type specificity, while an upstream repressor (-174/-65) confers muscle-specific expression. A positive element (+49/+254) increases muscle-specific expression. Within these broad elements, two E boxes play a pivotal role. One E box at -31/-26 within the promoter, acting in part through its ability to bind the myogenic basic helix-loop-helix proteins, recruits additional factor(s) that bind elsewhere within the SkM1 sequence to control positive expression of the gene. A second E box at -90/-85 within the repressor controls negative regulation of the gene and acts through a different complex of proteins. Several of these cis- regulatory elements share both sequence and functional similarities with cis- regulatory elements of the acetylcholine receptor δ-subunit; the different arrangement of these elements may contribute to unique expression patterns for the two genes.