Dissection of Genetic Pathways in C. elegans - Caenorhabditis Elegans: Molecular Genetics and Development - page 127

Another type of suppression is referred to as intragenic suppression, where a
suppressor mutation in the same gene that harbors the starting mutation reverses
its defects. For example, the suppressor may introduce a functional favorable muta-
tion into the protein, which may compensate for the reduction of activity caused by
the original deleterious mutation. Sometimes, intragenic suppressor mutations can
affect splicing, which causes a skip of the original mutation and produces a protein
with biological activity. More details on this topic can be found in a review by
Hodgkin (2005) .
6. Selection Screens
Depending on the design of the genetic screen, the isolation of mutants can be
laborious. Many screens require careful examination of nearly every single worm
for the presence or absence of a desired phenotype. For example, in the previously
mentioned screen for suppressors of an unc-4 allele, 500,000 progeny of muta-
genized worms were individually tapped on the head with a platinum pick to test if
the worms were able to move backward ( Miller et al., 1993 ). Selection screens are
designed to rapidly facilitate identification of mutants with a specific phenotype
by eliminating animals that do not carry a desired mutation. A drug screen is
one type of selection screen in which mutants resistant to a particular drug will be
readily selected because all other worms are either killed or display a specific
phenotype when lacking resistance. For example, acetylcholine, a neurotransmit-
ter, is released from synaptic vesicles at neuromuscular junctions, where it induces
muscle contraction. Acetylcholine is normally degraded by the enzyme acetylcho-
linesterase. Pesticides, such as Aldicarb, block the activity of this enzyme and
cause accumulation of acetylcholine, which ultimately kills animals, including
worms, because of excessive excitation of muscles. A screen searching for mutants
that are resistant to Aldicard (e.g., viable in its presence) identified unc-17,which
was later cloned and found to encode a broadly conserved acetylcholine trans-
porter involved in uptake of acetylcholine into synaptic vesicles ( Alfonso et al.,
1993; Brenner, 1974 ).
Inducible transgenes can also be used for selection screens, where a transgene
engineered into a strain induces a particular phenotype that when suppressed enables
easy selection of corresponding mutations. For example, GOA-1, a C. elegans
a -subunit of the major heterotrimeric G protein in the nervous system, regulates
many behaviors, including locomotion and egg laying ( Hajdu-Cronin et al., 1999 ).
To identify genes that interact with G o signaling, Hajdu-Cronin et al. (1999) gener-
ated a transgenic strain overexpressing a constitutively active GOA-1 mutant protein
under the control of a heat-shock promoter. Upon heat shock, this strain displays a
severe phenotype, paralysis. By screening for mutants that restored locomotion, they
identified two regulators of G o signaling, dgk-1 (first identified and named as sag-1
in the study), which encodes a diacylglycerol kinase ( Miller et al., 1999; Nurrish
et al., 1999 ), and eat-16, which encodes a protein orthologous to the mammalian
regulators of G protein signaling (RGS) 7 and RGS9 ( Hajdu-Cronin et al., 1999 ).