Worm Breeder's Gazette 12(5): 26 (February 1, 1993)

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Genetic Evidence that smg-6 is an Essential Gene

Brian Cali, Phil Anderson

Laboratory of Genetics, University of Wisconsin-Madison

The genetic properties of smg mutants suggest that most existing alleles are loss of function mutations (Genetics 123:301). However, two observations make these arguments less compelling for the existing alleles of smg-6 than for alleles of other smg genes: (1) smg-6 alleles were isolated at a relatively low frequency. Of the 54 EMS-induced smg mutationsoriginally isolated, only 2 were alleles of smg-6 (Genetics 123:301). Furthermore, none of 33 mut-2 ( r459 )-inducedsmg mutations were alleles of smg-6 .(2) smg-6 mutations have a less pronounced effect on nonsense mutant mRNA accumulation than mutations in smg-1 through smg-5 . unc-54 nonsense mutant mRNAs accumulate to only 5% of WT levels in smg(+) backgrounds. Mutations in smg-1 through smg-5 restore unc-54 nonsense mutant mRNAs levels to approximately 100% of WT. These same mutant mRNAs accumulate to only 50-60% of WT levels in smg-6 backgrounds (WBG 11(5):90).

These observations suggested that the 2 existing smg-6 alleles are hypomorphic, and that a smg-6 null mutant might have a more severe phenotype that precluded its isolation in previous screens. We theerefore undertook a non-complementation screen to isolate new alleles of smg-6 .Mutations in any of the smg genes suppress unc-54 ( r293 )(Genetics 123:301). Using suppression of unc-54 ( r293 )as a screen for new smg-6 alleles, we isolated 8 EMSintucet alleles at a frequency of 7 X 10 -4 per mutagenized gamete. This forward mutation frequency is similar to that for an average-sized gene in C. elegans. These alleles were isolated by their failure to complement smg-6 ( r896 ),and animals heteroallelic for r896 and any of the newly isolated alleles are phenotypically indistinguishable from r896 homozygotes (i.e.-hermaphrodites have protruding vulvae and males have abnormal tails). Five alleles are homozygous viable aod indistinguishable from the previously isolatet smg-6 alleles. Three alleles ( r1014 , r1059 , r1062 )are recessive lethals. r1059 is an embryonic lethal. r1062 and r1014 are early and late larval lethals, respectively. Both r1059 and r1062 fail to complement r1014 for its late larval arrest phenotype. The lethality is tightly linked to the suppression phenotype of smg-6 in all three cases (p<0. 1 m.u.).

These data indicate that smg-6 may be an essential gene, and that animals lacking smg-6 activity arrest early in development. The different arrest phenotypes of these lethal alleles suggests they represent an allelic series, r1059 being the strongest allele. We have not completely excluded the possibility that the lethality of these alleles is unrelated to the smg-6 phenotype and results from very tightly linked mutations or small deficiencies affecting nearby essential gene(s). However, as EMS is primarily a point mutagen and as the lethality associated with these alleles maps within 0.1 m.u. of smg-6 ,this possibility seems unlikely. The cloning of smg-6 and subsequent molecular analysis of these alleles will differentiate between these possibilities.