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lethality; (2) a dominant visible marker (e.g., GFP) driven by a ubiquitous promoter
to indicate the presence of the transgenes. The progeny of this transgenic strain will
be comprised of two populations: the marker-positive worms (transgene
+
) and the
marker-negative worms (transgene
), the frequency of each being dependent on the
frequency of stochastic loss of the exchromosomal array. If a synthetic lethal muta-
tion were to be introduced into one of these transgene-containing worms, only
progeny containing the transgene would be viable, as the marker-negative population
would be all dead due to the absence of an extrachromosomal array expressing the
wild-type protein. Conversely, both transgene
+
and transgene
progeny would be
present and viable with nonlethal enhancers.
Fig. 3
Synthetic lethality screen using extrachromosomal arrays. The starting strain (P0) with an initial
mutation (S) and a rescuing array containing a dominant visible marker (Ex) is mutagenized. The progeny
(F1) carrying the visible marker (green) are cloned onto individual plates. As an F1 worm is heterozygous
for a recessive mutation, only 25% of the progeny (F2) will be homozygous. Thus, several F2 progeny
carrying the marker from each F1 plate are then cloned onto individual plates (here, we show four
F2 worms per F1). The progeny (F3) of each cloned F2 are inspected for the presence of the marker.
For an F2 that is homozygous for a synthetic lethal mutation (Lm), all viable F3 progeny will be marker-
positive (green) because the progeny lacking the marker (white) will be dead (dotted box). In the case of
F2 worms derived from an F1 worm carrying a nonlethal mutation (nLm), the progeny F3 of each F2 worm
will be both marker-positive (green) and marker-negative (white). This screening process is also known as
an F2 clonal screen. (See color plate.)
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