Of genes affecting thick filaments in the body wall muscles of C.
unc-45 III is one of the most difficult in which to isolate mutations. Onry 5 alleles have been isolated in general screens, compared to hundreds of alleles of
unc-54, the major myosin ( myoB) gene. The 5
unc-45 alleles are all temperature sensitive (ts), recessive, and cause a reduction in thick filament number and organization at 25 C. It has been shown that
unc-54(0);
unc-45(ts) mutants are no worse off than
unc-54 null mutants, suggest that
unc-45 acts at least in part through unc - 54 A precomplementation screen was performed to look for non-ts alleles of
unc-45. aphrodites were mutagenized with EMS and mated with--
unc-45(
e286) males. Slow cross progeny were picked at 20 C and examined for Dpy and Slow progeny. Screening 18,000 cross progeny yielded 4
unc-45 isolates, 3 of which are homozygous lethal and one of which is ts. This EMS induced mutation frequency of 1/4,500 genes is comparable to the general EMS induced forward mutation frequency of 1/2,000 genes (Brenner, 1974), and suggests that the lethal alleles may be loss of function alleles. The
unc45(st601) allele has been the most completely characterized: it is fully recessive to wild-type and ts alleles, but is not itself ts. Recombination in the
st601 to
dpy-1 interval yielded a map distance of 10, compared to the published
e286-
dpy-1 distance of 6.6 +/-.9 (More extensive data puts the
e286-
dpy-1 value at 7.4mu). The finding of lethal alleles of
unc-45 suggests that
unc-45 interacts with more than the
unc-54 gene product, myoB. Mutants totally-lacking myoB are viable, having a few thick filaments composed of myoA. Thus, the failure by
unc-45 to organize myoB into thick filaments should not, by itself, be fatal. All 3 lethal alleles (
st601,
st603,
st604) have been examined for their terminal phenotype. The embryo achieves 2-fold length with a slight overlap of head and tail. When the early 2-fold stage is reached, the embryos are capable of small movements, and the pharynx develops, but the embryos have almost never been observed to pump and they mostly fail to hatch. This phenotype is similar to the recently reported
myo3(st378) hypomorphic phenotype, except that
myo-3(
st378) animals do pump and hatch ( waterston WBG 9:2, see Fire and Waterston this issue).
myo-3 encodes the minor body wall muscle myosin, myo A. The similarity in phenotype of the
myo-3(
st378) homozygotes and the
unc45(st601) homozygotes suggests that
unc-45 is essential for myoA action, in addition to its interaction with myoB. The failure of the
unc-45 lethal mutants to pump, might also suggest a role for
unc-45 in pharyngeal muscle thick filament organization.