> simple way for you to demand retention of your URA3 plasmid. Try taking
> your transformed (Ura+) strain and selecting for a 5-fluorouracil resistant
> mutant. The result will be a strain that is deficient in the pyrimidine
> salvage pathway (5FU resistant) and is completely dependent for growth on
> the de novo pyrimidine synthesis pathway (which means that anything that
> loses the URA3 plasmid dies).
Perhaps I didn't understand the way you explained this, so I will transcribe the
reality in an easier way :
Everything which is [Ura+] is [5FOA sensitive]
Everything which is [5FOA resistant] is [Ura-] (in fact ura3- or ura5-)
So I think this is the contrary of what Colin MacDiarmid asked for... (e.g. Ura+
selection in rich media). Anyway, I don't remember the reason, but 5FOA selection is
always done on minimal media.