Distinct vascular and intestinal smooth muscle myosin heavy chain mRNAs are encoded by a single-copy gene in the chicken.

We examined whether the gizzard MHC gene is expressed in other smooth muscle tissues and, if so, whether there exist any smooth muscle MHC isoforms at the mRNA level. Northern blot analysis showed that the gizzard MHC gene was also expressed in the aorta and jejunum, but not in the pectoralis muscle or in fibroblasts. This indicates that striated muscle and non-muscle MHC isoforms are encoded in genes distinct from the smooth muscle MHC gene. Further, nuclease S1 mapping showed that the aortic smooth muscle MHC mRNA was distinct from the gizzard mRNA in the 5'-terminal coding region. Both of these mRNA species are expressed in the jejunum. These observations suggest that there exist at least two chicken smooth muscle MHC isoforms, vascular-type and intestinal-type, and that these isoforms are generated from a single-copy gene, probably by an alternative mRNA processing mechanism.