Worm Breeder's Gazette 10(2): 31

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.

Suppressors of a Splice Site Defective unc-54 Mutation

Brian Carr and Philip Anderson

r661 is an allele of unc-54 generated by imprecise excision of Tc1 
from its 'hotspot' of insertion within the gene.  unc-54(r661) 
homozygotes are very slow, have low brood sizes, and contain only 
about 20% of the wild-type amount of myosin heavy chain B.  The DNA 
sequences of r661 and other imprecise excisions from the hotspot 
suggest that the defect in r661 is due to altered mRNA splicing.  The 
insertional hotspot is immediately adjacent to the 5' splice site of 
IVS-3.  Imprecise excision yields mutations in and around this splice 
site, and we believe that r661 owes its phenotype to an insufficient 
amount of properly spliced mRNA.  r661 contains two closely spaced 5' 
splice sites which may compete with each other for splicing; only one 
of these sites yields functional mRNA [Eide and Anderson WBG 2:40; MCB 
8:737-748 (1988)].
We have isolated two, unlinked, recessive suppressors of unc-54(r661)
following EMS mutagenesis.  Both suppressors, designated here as Sup(
r662) and Sup(r663), improve the motility of unc-54(r661), but the 
suppressed animals are not wild-type.  Neither suppressor shows a 
distinct phenotype other than suppression of unc-54(r661).unc-54(r661) 
accumulates approximately 5-8 fold less unc-54 mRNA than wild-type.  
The transcript is normal in size, indicating that unspliced (IVS-3 
containing) mRNA does not accumulate in these animals.  unc-54(r661);
Sup(r662)  and unc-54(r661);Sup(r663)  strains accumulate 2-3 fold 
more unc-54 mRNA than unc-54(r661) alone.   Therefore, these 
suppressors may improve the motility of unc-54(r661) by increasing the 
steady state level of unc-54 message.  We do not know yet whether this 
increase in message level occurs by altering the specificity of 
splicing, the efficiency of splicing, or by some other mechanism.  We 
will use primer extension sequencing to identify the spliced products 
of unc-54(r661) and its suppressors.
unc-54(r661);Sup(r662) hermaphrodites have a low-penetrance, 
protruding vulva (p-vul) phenotype.  This suggested that Sup(r662) 
might be related to the 'morpho-mab' class of suppressors  (mab-1, 11, 
13, 14, 15, 16), which also have a p-vul phenotype.  Such suppressors 
suppress specific alleles ofseveral genes, including unc-54(r293), 
09), and lin-29(n546) [J.  Hodgkin, WBG 10(1):112 (
1987); J.  Hodgkin et. al, this issue].   We tested the ability of Sup(
r662) and Sup(r663) to suppress unc-54(r293).    Neither suppressor 
suppressed unc-54(r293).  Furthermore, none of the Mabs listed above 
suppressed  unc-54(r661).    Therefore,  Sup(r662)  and Sup(r663)  are 
distinct from the 'morpho-mab' class of suppressors.