Upon stimulation with alpha/beta interferon, rat cells synthesize three Mx proteins. Sequence analysis of corresponding cDNAs reveals that these three proteins are derived from three distinct genes. One of the rat cDNAs is termed Mxl because it is most closely related to the mouse Mxl cDNA and because it codes for a nuclear protein that, like the mouse Mxl protein, inhibits influenza virus growth. However, this protein differs from mouse Mxl protein, in that it also inhibits vesicular stomatitis virus (VSV), a rhabdovirus. A second rat cDNA is more closely related to the mouse Mx2 cDNA and directs the synthesis of a cytoplasmic protein that inhibits VSV but not influenza virus. The third rat cDNA codes for a cytoplasmic protein that differs from the second one in only eight positions and has no detectable activity against either virus. These results indicate that rat Mx proteins have antiviral specificities not anticipated from the analysis of the murine Mxl protein.

Sequences complementary to muscle poly(A)+RNA were cloned in the plasmid pBR322 and the resulting colonies were screened by colony hybridization with labeled cDNA derived from skeletal muscle and smooth muscle (gizzard). The skeletal musclespecific clones were further screened by RNA blotting hybridization for a muscle mRNA having the size expected for a putative type M creatine kinase (M-CK) mRNA. The remaining clones with the expected hybridization properties were finally characterized by hybrid-selected translation, and a cloned sequence was shown to contain DNA hybridizing to mRNA that could be translated into M-CK. This plasmid, pMCKI, was further characterized by restriction mapping. Blot analysis of total cell RNA from differentiating myogenic cell cultures showed accumulation of M-CK mRNA in cultures older than 42 hr but not in young little-differentiated cultures.During the development of myogenic cells, mononucleated cells fuse to form myotubes which become contractile at the time when well-organized myofibrils, the characteristic organelles of cross-striated muscle, appear in the cytoplasm of fully differentiated cells. The myofibrils are composed of a set of myofibrillar contractile proteins that are produced only in terminally differentiated muscle cells. In the course of differentiation, these proteins replace the cytoplasmic contractile isoproteins, which are expressed in nonmuscle cells and in the nonterminally differentiated myogenic progenitor cells. Hence, isoprotein "switches" occur in differentiating myogenic cells and have been described for some contractile proteins (1, 2) as well as for enzymes such as creatine kinase (CK) (3-5) that have an important role in energy metabolism in muscle.In early stages of differentiation of cultured-myogenic cells as well as in embryonic muscle, BB-CK is accumulated but, as differentiation reaches its terminal phases, isoenzymes containing the muscle-specific CK subunit (M-CK) progressively appear (3-5) until, in adult muscle, the latter subunit replaces the type B CK subunit (B-CK) previously present (5). In (Feldbach, Switzerland), and "Gene-Screen" membranes were obtained from New England Nuclear (Dreieich,

No abstract