Worm Breeder's Gazette 9(3): 115

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.

Mutations in cha-1 and unc-17 Suppress the Ace-Lethal Phenotype

J. Rand and C. Johnson

The neurotransmitter acetylcholine is synthesized by choline 
acetyltransferase (ChAT), and is hydrolyzed by acetylcholinesterase (
AChE); cha-1 is the structural gene for ChAT, and ace-1, 
presumed structural genes for 
AChE classes A, B, and C, respectively. Although neither ace-1, 
s lead to any major developmental 
aberrations singly or in pairwise combination, the ace-1 
ace-3 triple mutant is lethal. Such Ace-lethal 
animals apparently undergo near-normal embryonic development, and many 
of them eventually hatch, but they are nearly paralyzed and they do 
not develop any further. In order to determine if mutations in cha-1 
could suppress the Ace-lethality, we constructed a strain homozygous 
for ace-1,  heterozygous for ace-
3 over the balancer mnC1. This strain was stable, and segregated one-
quarter quadruple mutants, i.e., ace-1 ace-3 
tes. Some (5-50%) of these quadruple mutants 
grew to maturity, and it was possible to establish a culture. The 
quadruple mutants are extremely small and hypercontracted, and can 
barely move. (In other words, we have constructed a four-gene 
synthetic dumpy.) They resemble wild-type animals treated with the 
AChE inhibitors aldicarb or lannate. They grow very slowly, sometimes 
requiring three weeks before they lay any eggs, and, in each 
generation, only a fraction of the worms which hatch reach maturity. 
Nevertheless, they are a dramatic improvement over the ace-1 
ace-3 triple mutant.
Because of our evidence that cha-1 and unc-17 are part of the same 
complex gene (Rand and Russell, Genetics 106:227, 1984), we tried to 
suppress the Ace-lethal phenotype with several different unc-17 as 
well as cha-1 alleles. We were able to establish viable quadruple 
mutant cultures using the cha-1 alleles p1152 and p1186, and the unc-
17 alleles e245, p279, and p300. However, we were not able to 
establish quadruple mutant strains using the lethal cha-1 allele m324 (
isolated by Teresa Rogalski) or the extreme unc-17 allele p1160. It 
thus appears that suppression of the Ace-lethal phenotype requires 
some residual level of functional ChAT activity.
In one sense, it seems reasonable that a defect in acetylcholine 
synthesis can partially compensate for lethality due to a deficiency 
in acetylcholine hydrolysis. The viability of quadruple mutant strains 
argues that the Ace-lethal phenotype is due to the toxic accumulation 
of acetylcholine in these animals, rather than loss of the AChE 
proteins themselves. The result that unc-17 and cha-1 mutations both 
suppress the Ace-lethal phenotype supports our model that they are 
part of the same gene, although we have not yet investigated whether 
other unc mutations might also act as suppressors. In fact, it is 
possible that other mutations which interfere with acetylcholine 
synthesis or release might also suppress Ace-lethality.Since the 
quadruple mutants lack AChE classes A, B, and C, they should prove 
useful in the study of minor AChE forms, such as class D enzyme 
reported by Stern and Russell (Neurosci. Abstr. 11:370, 1985). 
Mutagenesis of quadruple mutants has led to the isolation of 
significantly better-growing variants; the nature of the additional 
mutation(s) in these stocks responsible for the improved growth has 
not yet been analyzed.