Nonsense mutant RNAs are generally less stable than WT mRNAs. The six C. elegans smg genes are required for this nonsense-mediated mRNA decay. In smg mutants, otherwise unstable nonsense mRNAs accumulate to approximately WT levels. Nonsense-mediated mRNA decay is not essential. Null mutations in several smg genes eliminate nonsense-mediated mRNA decay, yet these animals are essentially WT. However, several other smg genes may have additional essential functions, as viable alleles of these genes are rare. Screens for essential smg genes are not saturated. Thus, we searched for new smg genes using a previously described screen (1993 Meeting Abstracts, p.255). smg mutants were isolated in a
mut-5 background, in which Tc1 is active. One such mutant (
r1131) appears to define a new smg gene: (1)
r1131 exhibits the allele-specific but not gene-specific suppression pattern of other smg mutations, (2)
r1131 complements
smg-1 smg-6 alleles, (3)
r1131 maps 0.2 m.u. right of
smg-3, (4)
unc-54 nonsense mRNAs accumulate to WT levels in
r1131 animals. We identified a polymorphic Tc1 tightly linked to and inseparable from
smg-7(
r1131). Analysis of genomic and cDNA clones surrounding this Tc1 shows that the Tc1 is inserted into an exon of a gene encoding a 1.5 kb mRNA. This mRNA is trans-spliced to SL2 and appears to be cotranscribed with the upstream gene
raf-1 (Nature, 363:133, 1993 and Andy Golden, pers. comm.). When expressed under control of the
hsp16-2 promoter, a full length cDNA clone of this gene exhibits heat-shock dependent rescue of
smg-7(
r1131), establishing that this gene is
smg-7.
smg-7 is predicted to encode a novel 50 kDa protein with two notable features: (i.) a protein:protein interaction domain known as a tetratricopeptide repeat and (ii.) a negatively charged carboxy terminus. To test whether
smg-7 is essential, we are isolating new alleles using a noncomplementation screen. Of three EMS-induced alleles isolated to date, two are recessive lethal and one is a weak viable allele. One lethal allele is a reciprocal translocation between LGIV and LGV; the other deletes >8 kb left and right of
smg-7. Less complex alleles must be isolated to unambiguosly define the smg 7 null phenotype. Characterization of new
smg-7 alleles, as well as progress in localizing SMG-7 immunologically, will be presented.