SM22 alpha is a 22-kDa protein of unknown function, the mRNA of which is highly and specifically expressed in smooth muscle cells (SMC). The 5' untranslated leader sequence of the rat SM22 alpha gene was found to contain two introns of 3.6 and 2.9 kb. Two transcripts of SM22 alpha exist in all SMC types examined, and genomic mapping of the gene suggests these transcripts result from different 5' transcription start points, split by the 2.9-kb intron. A small intron (102 bp), which contains an E-box consensus sequence, was found within the coding region 178 bp from the ATG start codon. The 3.6-kb intron contains 82 bp which show 98% homology at the RNA level with the rat identifier sequence (ID). Transient reporter gene assays demonstrate that a 576-bp fragment, including the ID, contains a regulatory element which may contribute to the SMC-specific expression of SM22 alpha.