Worm Breeder's Gazette 11(1): 54

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.

smg Mutations Affect Accumulation of unc-54 Nonsense and Frameshift mRNAs

Rock Pulak and Phil Anderson

Mutations in any of the six smg loci are allele-specific suppressors 
of a variety of mutations in unrelated genes (Papp et al., Pulak & 
Anderson, 1989 Meeting Abstracts).  The smg-suppressible allele unc-54(
r293) contains a small deletion of the unc-54 polyadenylation site.  
unc-54(r293) accumulates a small amount of an aberrantly large mRNA (
<10% wild-type levels).  unc-54(r293); smg double mutants accumulate 
approximately wild-type amounts of the same large mRNA (Pulak & 
Anderson, 1989 Meeting Abstracts).  We have tested whether smg 
mutations also affect accumulation of unc-54 mRNAs that contain 
frameshift or nonsense mutations.  In many organisms, such translation 
terminating mutations accumulate reduced amounts of the corresponding 
unc-54(e190) is a 401 base pair out-of-frame deletion in the central 
region of the gene (Dibb et al., 1985).  unc-54 mRNA is barely 
detectable in e190 (at most a few percent of wild-type levels).  unc-
54(e1300) is an amber mutation near the 3' end of the gene (Dibb et al.
, 1985).  e1300 accumulates a reduced but easily detected amount of 
unc-54 mRNA (approximately 20-30% of wild-type levels).  smg mutations 
do not phenotypically suppress e190 or e1300, but they dramatically 
affect the amount of accumulated mRNA.  Both unc-54(e190); smg and unc-
54(e1300); smg double mutants accumulate approximately wild-type 
amounts of unc-54 mRNA.  Thus, function of the wild-type smg genes is 
required for instability of these mutant mRNAs.  We are testing other 
sequenced mutations to determine whether smg mutations affect mRNA 
accumulation in all types of nonsense mutations, in all regions of the 
gene, and in all genes.  (If they do, smg mutations might be valuable 
for identifying clones of genes for which amber alleles are available.)

Two lines of evidence suggest that smg mutations slightly elevate 
expression of the wild-type unc-54 gene.  The first line of evidence 
is a genetic test.  In animals heterozygous for the semidominant 
mutation unc-105(n490), function mutations 
have a heterozygous effect on the phenotype.  This allows us to 
distinguish animals that contain one or two wild-type copies of unc-54 (
representing a two-fold difference in wild-type gene expression).  We 
use this phenotypic difference to test whether smg mutations affect 
expression of a single copy of the wild-type gene.  They do, but the 
effect is not as strong as adding a second wild-type gene.  The second 
line of evidence involves Northern blots.  smg mutants (whether or not 
they contain unc-54 mutations) consistently accumulate slightly more 
unc-54 message than N2.  The increase, however, is less than two-fold.