Worm Breeder's Gazette 9(3): 42
These abstracts should not be cited in bibliographies. Material contained herein should be treated as personal communication and should be cited as such only with the consent of the author.
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.