STRUCTURE AND EXPRESSION OF A SMOOTH-MUSCLE CELL-SPECIFIC GENE, SM22-ALPHA

SM22 alpha is expressed exclusively in smooth muscle-containing tissue s of adult animals and is one of the earliest markers of differentiate d smooth muscle cells (SMCs). To examine the molecular mechanisms that regulate SMC-specific gene expression, Foe have isolated and structur ally characterized the murine SM22 alpha gene. SM22 alpha is a 6.2-kil obase single copy gene composed of five exons. SM22 alpha mRNA is expr essed at high levels in the aorta, uterus, lung, and intestine, and in primary cultures of rat aortic SMCs, and the SMC line, A7r5. ln contr ast to genes encoding SMC contractile proteins, SM22 alpha gene expres sion is not decreased in proliferating SMCs. Transient transfection ex periments demonstrated that 441 base pairs of SM22 alpha 5'-flanking s equence was necessary and sufficient to program high level transcripti on of a luciferase reporter gene in both primary rat aortic SMCs and A 7r5 cells. DNA sequence analyses revealed that the 441-base pair promo ter contains two CArG/SRF boxes, a CACC box, and one potential MEF-2 b inding site, cis-acting elements which are each important regulators o f striated muscle transcription. Taken together, these studies have id entified the murine SM22 alpha promoter as an excellent model system f or studies of developmentally regulated, Lineage-specific gene express ion in SMCs.