Worm Breeder's Gazette 11(5): 91
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.
In the course of studying the effects of smg mutations on unc-54 mRNA levels, we observed that four unc-54 nonsense mutations, which by all criteria are typical recessive mutations, are dominant in a smg mutant background. The four mutations e1420 (UAA at codon 614), e1419 ( UAG at codon 631), r274 (UGA at codon 761), and r308 (UGA at codon 833) ] are loosely clustered in the carboxyl-terminal region of the myosin globular head. The positions of these mutations predict the expression of truncated proteins that contain most (but not all) of the myosin head region with no attached rod segment. In smg(+) backgrounds, these nonsense mutations are recessive and accumulate very low levels of unc-54 mRNA (5-10% of the amount accumulated by wild type; see accompanying WBG article). Thus, we anticipate that these mutants (either homozygous or heterozygous) produce very low levels of the truncated myosin proteins. In smg(-) backgrounds, these mutants accumulate nearly wild-type amounts of unc- 54 mRNA. We anticipate that these strains produce large quantities of the truncated protein. We believe that the dominance of these four unc-54 alleles is due to disruptive effects that the truncated myosin proteins have on thick filament or sarcomere assembly. The strength of this smg-dependent dominance varies, depending on both the unc-54 and smg allele, but the effect can be quite striking. In the background of smg-1 through smg-5, unc-54(r274) is strongly dominant when heterozygous and lethal when homozygous. In this situation, r274 resembles the strongly dominant missense alleles of unc-54 that have been previously described (Waterston et al., J.M.B. 180:473; Bejsovec & Anderson, Cell 60:133). smg-6(r896) does not confer dominance to any of the four unc-54 alleles, while smg-6(r886) causes each of them to be dominant. Thus, we believe that r896 is a hypomorph. We have been unsuccessful in our attempts to identify the predicted premature termination polypeptides. We have looked for proteins of the predicted sizes in total protein homogenates, in immunoprecipitates using polyclonal anti-actomyosin, and in Western blots using a scallop anti-myosin antibody that recognizes head epitopes of many different myosin heavy chains. No proteins of the predicted sizes were detected. This result is disappointing, but it is not entirely unexpected. Bejsovec & Anderson (Cell 60:133) could not detect the mutant myosin of several strongly dominant unc-54 missense alleles. The mutant proteins are efficiently degraded, but a small amount of a strongly disruptive protein is sufficient to interfere with thick filament and/or sarcomere assembly. We suggest that smg-dependent dominance ('synthetic dominance', analogous to synthetic lethals) may be a common occurrence. When the truncated protein product of a nonsense or frameshift mutation has a disruptive biological activity, then smg mutations should enhance or reveal these disruptive effects. The four unc-54 mutations described above provide one example, but a number of others have been encountered. The weak dominance of dpy-5(e61) is enhanced in a smg mutant background (Hodgkin et al., Genetics 123:301). dpy-14(e188) is essentially recessive in a smg(+) background, but in a smg(-) background it is strongly dominant when heterozygous and lethal when homozygous (K. Anders, personal communication). The weak dominance of glp-1(q35) (a UGA nonsense mutation) is substantially enhanced by a smg mutation (J. Kimble, personal communication). We suggest that mutant hunts in a smg background will yield a wider variety of dominant mutations than might otherwise be obtained.