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are unable to crawl backwards. To identify gene products that directly interact with
UNC-4,
Miller et al. (1993)
screened for mutants that suppressed a hypomorphic
temperature sensitive allele of unc-4 and successfully identified a gf mutation in a
gene, unc-37, which was found to encode a Groucho-like protein forming a complex
with UNC-4 to regulate transcription (
Winnier et al., 1999
). Importantly, this muta-
tion in unc-37 was not able to suppress unc-4 null alleles, suggesting that this
suppression was dependent on UNC-4 activity.
Suppression of a Hypermorphic Allele
Suppression of hypermorphic alleles, that is, gf mutations, yields a spectrum of
alleles in genes with attributes opposite to those identified in suppression of lf
mutations (
Fig. 4
iii). This type of suppression is a powerful way to identify compo-
nents of a biological pathway. A good example comes from the highly conserved
RTK/Ras/MAPK pathway, which is involved in vulval induction in C. elegans (see
Fig. 1
). The worm Ras gene, let-60, is a crucial component of this pathway. Gain-of-
function mutations in let-60 cause a multiple-vulva (Muv) phenotype. A group of
key components acting downstream of let-60, including lin-45 (the worm Raf gene)
(
Hsu et al., 2002
), mpk-1 (the worm ERK gene) (
Wu and Han, 1994
), mek-2 (the
worm MEK gene) (
Wu et al., 1995
), ksr-1 (a C. elegans kinase suppressor of Ras)
(
Sundaram and Han, 1995
), and sur-2 (
Singh and Han, 1995
), were all identified in
screens for suppressors of the Muv phenotype caused by a gf allele of let-60.
Suppression of Engineered Gain-of-Function (gf) Alleles
gf mutations used for suppressor screens are not limited to genetically defined
alleles. By engineering into worms transgenes (''artificial gf alleles'') expressing
mutant proteins, one can produce a phenotype that facilitates selection and identi-
fication of suppressors. For instance,
Zheng et al. (2004)
devised a suppressor screen
hunting for genes interacting with an important class of neurotransmitter receptor,
ionotopic glutamate receptors (iGluRs), that mediate most excitatory synaptic sig-
naling between neurons (
Zheng et al., 2004
). The screen was performed in a trans-
genic strain engineered to express a non-N-methyl-d-aspartate (NMDA) type iono-
tropic glutamate receptor (GLR-1) subunit containing a dominant mutation.
Expression of this transgene under the control of the glr-1 promoter resulted in a
hyper-reversal phenotype, that is, transgenic animals show a higher frequency of
reversing direction during movement than wild-type worms. By searching for muta-
tions suppressing this phenotype, Zheng and colleagues identified a gene encoding a
type I transmembrane protein, SOL-1, that can bind to GLR-1 and participate in the
gating of non-NMDA iGluRs.
Suppressors of Engineered Pathological Processes
Greater than 40% of human-disease-related genes have clear C. elegans orthologs
(
Culetto and Sattelle, 2000
). C. elegans has been used to model human diseases,
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