A classic way to isolate thymidylate synthase negative mutants of bacteria is to treat a growing culture with thymidine and trimethoprim. Most of the cells are killed, and the survivors are greatly enriched in thymidylate synthase-negative mutants.
(a) What phenotype would allow you to identify these mutants?
(b) What is the biochemical rationale for the selection? (That is, why are the mutants not killed under these conditions?)
(c) How would the procedure need to be modified to select mammalian cell mutants defective in thymidylate synthase?