Worm Breeder's Gazette 11(5): 90

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 Gene Products Degrade Aberrant mRNA

Rock Pulak and Phil Anderson

We previously described the effect of smg mutations on the 
accumulation of two unc-54 mutant mRNAs (WBG 11(1):54).  Both of these 
mutations (an out-of-frame deletion and an amber mutation) accumulate 
reduced levels of unc-54 mRNA in smg(+) backgrounds and approximately 
wild-type levels of unc-54 mRNA in smg mutant backgrounds.  To better 
characterize the effect of smg mutations on mRNA accumulation, we 
analyzed a number of additional nonsense mutations affecting unc-54.  
Using RNase protection assays, we measured the steady-state level of 
mRNA in 14 sequenced unc-54 nonsense mutations in both smg(+) and smg(-
) backgrounds.  All three classes of nonsense mutations were 
represented (UAA, UGA, and UAG).  The unc-54 gene is 1966 codons in 
length; the nonsense mutations we analyzed are distributed along the 
length of the gene from codon 420 (unc-54(r316)) to codon 1906 (unc-54(
e1300)).  All fourteen nonsense mutations accumulate low levels of unc-
54 mRNA in smg(+) backgrounds.  For twelve of the fourteen, the amount 
of steady-state mRNA is 5-10% of wild type.  The two exceptions are 
the two mutations closest to the unc-54 3' end.  e1328 (codon 1863) 
and e1300 (codon 1906) accumulate 20-30% of wild-type levels of unc-54 
We constructed all unc-54(nonsense);smg double mutants, using one 
allele each of smg-1 through smg-6.  We have analyzed most of the 84 
double mutant combinations.  In no case is the unc-54 mutation 
phenotypically suppressed by the smg mutation, but in all cases we 
observe a striking effect on the steady-state levels of unc-54 mRNA.  
Near wildtype amounts of unc-54 mRNA are present in all strains.  The 
amounts of unc-54 mRNA average about 82% of wild type for smg-1 
through smg-5, and about 46% of wild type for smg-6.  Based on genetic 
observations described in an accompanying WBG article (this issue, 
Pulak et al.), we suspect that smg-6(r896), the allele used, is a weak 
allele of smg-6.  
We conclude that all unc-54 nonsense mutations accumulate low levels 
of mRNA.  This effect is not uniform along the entire length of unc-54.
Nonsense mutations near the unc-54 3' end are less affected and 
accumulate greater amounts of mRNA.  In smg mutant backgrounds, the 
mRNA levels are nearly wild type.  smg-6 is an exception only in the 
degree to which it affects the message level.  
These data suggest that the smg gene system may be involved in a 
cell surveillance process that minimizes the potentially disruptive 
effects of errors of transcription or mRNA processing.  mRNAs that are 
recognized as being aberrant (due to premature translation termination)
are degraded by the wild-type products of the smg genes.  This system 
may protect cells from potentially toxic effects of truncated gene 
products (see accompanying WBG article).