John McCusker writes:
<A fur1 strain that contains a chromosomal ura3 mutation *and* URA3 on a plasmid
is <absolutely dependent for growth on the URA3 plasmid -- it dies if it loses
<plasmid. This has nothing to do with 5FOA.
In doing a synthetic lethal screen using only URA3 as the marker on the plasmid
with our gene of interest, we picked up (as an artifact) a mutant in FUR1. This
mutant failed to grow if we lost the URA3 on the plasmid - even in the presence
of uracil. However a fur1 ura3 strain can grow if supplemented with uridine
instead uracil in the media. The cell uses the URK1 gene product to convert
uridine to uridine-5'-phosphate, the downstream product of both URA3 and FUR1.
(We also picked up CAN1 as a suppressor in this screen- but that is another
vershon at mbcl.rutgers.edu