Organization and developmental expression of an amphibian vascular smooth muscle alpha-actin gene

A gene encoding a putative homologue of the avian and mammalian vascular smooth muscle alpha-actin was isolated from an amphibian, Rana catesbeiana, and characterized in terms of its sequence, organization, and expression pattern. To assess the expression of this gene during amphibian embryonic development, a cDNA encoding the Xenopus homologue of this mRNA was isolated and characterized by in situ hybridization. The expression of this gene was not detected in the enteric smooth muscle cells or, unlike its avian and mammalian homologues, in the somites/skeletal muscle of the Xenopus embryos/tadpoles. Its initial expression coincides with the onset of cardiac muscle differentiation and is coincidental with the expression of the cardiac alpha-actin mRNAs in the heart-forming region of the stage 26/27 embryo. As development proceeds, transcripts from this gene are expressed throughout the developing heart until the formation of the heart chambers is completed and, thereafter, its expression becomes restricted to the outflow tract of the tadpole heart. The subsequent restricted expression of this gene to the vascular system in both of these amphibians identifies it as the amphibian homologue of the avian and mammalian vascular smooth muscle alpha-actin.