The small soil nematode Caenorhabditis elegans is an attractive organism for the molecular study of muscle function and development because of its anatomical simplicity and suitability for genetic and biochemical analysis (Brenner 1974; Sulston and Horvitz 1977). The body-wall musculature of C. elegans is composed of 95 cell disposed in four quadrants, which run the length of the animal beneath the cuticle. The musculature is obliquely striated, and the sarcomeres are oriented parallel to the long axis of the animal. Since these cells represent a large reaction of the animal mass, isolation of contractile proteins is comparatively simple (Epstein et al. 1974; Waterston et al. 1974, 1977a; Harris and Epstein 1977; Mackenzie and Epstein 1980). Mutants affecting the characteristic pattern of motility of C. elegans can be easily identified, and microscopic examination of these "uncoordinated," or unc strains, in the living animal by polarized light microscopy or, more carefully, by electron microscopy has led to the identification of 22 genes that produce altered muscle phenotypes (Waterston et al. 1980; Zengel and Epstein 1980). Of these, two are known to code for major structural proteins of muscle: The unc-54
gene codes for the major heavy chain of myosin (Epstein et al. 1974; MacLeod et al. 1977b), whereas the un-15 gene codes for paramyosin, the core protein of the thick filaments (Waterston et al. 1974; MacLeod et al. 1977a; Harris and Epstein 1977).