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.