The
unc45 gene of C. elegans is one of a group of muscle-affecting lethal genes. Both disruption of muscle function (Besjovic and Anderson, Genes Dev., 1988) and lack of muscle function (Waterston, EMBO, 1989 and with Williams and Barstead, unpublished) lead to a sirnilar lethal phenotype of arrested morphogenesis, and often a failure to hatch. While a viable, temperature-sensitive allele was the first
unc45 mutant isolated (Brenner, 1974), 7 lethal
unc45 alleles have been isolated using a noncomplementation screen (Venolia and Waterston, Genetics 126: 345, 1990, and Venolia unpublished). All the alleles have been isolated at a frequency consistent with gene knock- out, however, the range of phenotypes exhibited indicates that not all are null mutations. The putative null phenotype is a complete lack of muscle contraction, a block in morphogenesis at the 2-fold stage, disorganized muscle filaments, and a failure to pump and hatch. This phenotype is exhibited by the
st601,
st603,
wc2,
wc4 and wcS alleles. The other 2 lethal alleles,
st604 and wcl exhibit limited muscle contraction and development. To understand the function of this gene, the null phenotype must be more finnly established. To this end, we have attempted to isolate deletions in
unc45 using diepoxybutane mutagenesis (C.Trent, personal comrnunication). Both the
wc4 and wcS alleles were isolated in a non-complementation screen by treatment with 2 x 10-4 M DEB for 3 hrs. The wcS allele appears to be a deletion as the twofactor map distance to the dpy-l gene is reduced from 6 mu to about 1 mu, and the closely linked
vab-6 gene fails to be complemented by the wcS carrying chromosome. The embryonic phenotype of wcS homozygotes is consistent with the above description of the putative null phenotype. An additional approach to the characterization of the gene is a microscopic study of muscle structure. Using fluorescently labelled antibodies to MHC A and MHC B it has been determined that
unc45 lethal mutants synthesize the body wall myosins, and the myosins appear to aggregate in the appropriate locations (Venolia and Waterston, ibid). An electron microscopic study was undertaken to clarify this picture. Staged, two-fold embryos were hand-picked during light microscopic observation. A comparison of wild-type and
st601 embryos in this manner indicates that lethality is associated with a nearly complete failure to assemble or align thick filarnents.
st604 mutants (putative partial activity) on the other hand appear to organize filarnents in an unusually tight packing array.