We first screened for uracil auxotrophs as gain-of-function
mutants that show resistance to 5-FOA (5, 11, 20). We expected
that mutants with such a dominant phenotype would
be easily isolated even when multicellular hyphae were used
for screening. The 5-FOA, a fluorinated analog of orotic acid,
is converted to 5-fluorouridine monophosphate (5-FUMP) by
two reactions catalyzed by orotate phosphoribosyltransferase
(PyrE) and orotidine-5'-phosphate decarboxylase (PyrF) (Fig.
S1). Since 5-FUMP is toxic, the wild-type strain cannot grow
in the presence of 5-FOA. Mutants of the pyrE or pyrF gene
show resistance to 5-FOA but they cannot convert orotic acid
to uridine monophosphate (UMP), which is essential for
nucleic acid synthesis. Growth of the mutants, however, can
be supported by supplementing the medium with uracil; thus,
we can isolate uracil auxotrophs as 5-FOAR mutants