EXPRESSION OF THE SMOOTH-MUSCLE MYOSIN HEAVY-CHAIN GENE IS REGULATED BY A NEGATIVE-ACTING GC-RICH ELEMENT LOCATED BETWEEN 2 POSITIVE-ACTINGSERUM RESPONSE FACTOR-BINDING ELEMENTS

To identify cis- and trans-acting factors that regulate smooth muscle- specific gene expression, we studied the smooth muscle myosin heavy ch ain gene, a rigorous marker of differentiated smooth muscle. A compari son of smooth muscle myosin heavy chain promoter sequences from multip le species revealed the presence of a highly conserved 227-base pair d omain (nucleotides -1321 to -1095 in rat). Results of a deletion analy sis of a 4.3-kilobase pair segment of the rat promoter (nucleotides -4 220 to +88) demonstrated that this domain was necessary for maximal tr anscriptional activity in smooth muscle cells. Gel-shift analysis and site-directed mutagenesis demonstrated that one true CArG and another CArG-like element contained within this domain were both recognized by the serum response factor and were both required for the positive act ivity attributable to this domain. Additional studies demonstrated tha t mutation of a GC-rich sequence within the 227-base pair conserved do main resulted in a nearly 100% increase in transcriptional activity. G el-shift analysis showed that this GC rich repressor element was recog nized by both Spl and Sp3. These data demonstrate that transcriptional control of the smooth muscle myosin heavy chain gene is highly comple x, involving both negative and positive regulatory elements, including CArG sequences found in the promoters of multiple smooth muscle diffe rentiation marker genes.