Molecular analysis of the unc-54 myosin heavy-chain gene of Caenorhabditis elegans

Abstract

We have identified a strain of Caenorhabditis elegans in which the transposable element Tcl is genetically active. Most spontaneous mutations affecting the unc-54 myo-sin heavy chain gene of C. ekgans variety Bergerac are due to insertions of Tcl within unc-54. The Bergerac genome contains an unusually high number of Tcl elements, but this is not responsible for transpositional activity. Another variety of C. ekgans, strain DH424, contains an equally high number of Tcl elements, but transpositions are not detected. Tcl insertion mutations are genetically unstable. They revert to unc-54+ in both germ-line and somatic cells. Germ-line revertants are wild type and contain precise or nearly precise excisions of Tcl. Somatic revertants are genetic mosaics; they contain small patches of revertant muscle tissue in otherwise mutant animals. The pattern of mosaicism often allows us to know when and where during muscle development the excisions occur. Somatic reversion can be over 1000-fold more frequent than germ-line reversion. Most, if not all, organisms contain within their genomes multiple copies of discrete DNA sequences that are capable of transposition to many different chromosomal sites (for review, see ref. 1). These DNA sequences usually range in size from several hundred to several thousand base pairs and collectively are termed transposable genetic elements, or transposons. Transposons are responsible for a variety of genetic phenomena in these organisms, but an understanding of the biological roles of transposons in nature is only beginning to emerge. One important feature of transposable elements is their ability to induce mutations. Insertion of a transposon into (or nearby) a structural gene can disrupt that gene and lead to a mutant phenotype. Mutations of this type were first recognized in maize and systematically described by McClintock (ref. 2; for review, see ref. 3). They since have been documented in many different organisms. Spontaneous mutations (those that arise in the absence of any known mutagenic treatment) are often caused by transposon insertion. This is true for organisms as diverse as E. coli, maize, yeast, Drosophila, and mice (4-14). We have described the gene structures of 65 spontaneous mutations affecting the unc-54 gene of Caenorhabditis elegans, variety Bristol (15). unc-54 encodes one of two myosin heavy chain genes expressed in body-wall muscle cells (16). We were surprised to find that none of these spontaneous mutations is due to transposon insertion within unc-54. Yet, the nematode strain used for these studies (C. elegans variety Bristol) contains within its genome-25 dispersed copies of a DNA sequence, designated Tcl, whose structure is typical for that of a transposable element (17, 18). Two other nematode strains (C. elegans varieties Bergerac and DH424) contain >250 dispersed copies of Tcl in their genome (17, 18). We thought that Tcl might be transpositionally active in these strains because of its high copy number. The Bergerac strain contains a mutator system that exhi...

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FGF and PI3 kinase signaling pathways antagonistically modulate sex muscle differentiation inC. elegans

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