Yeast missing either of the last two steps in threonine biosynthesis (Thr1p or Thr4p) have myriad deleterious phenotypes not typically associated with amino acid auxotrophy, such as sensitivity to salts, high and low temperatures, pH, and radiation.
Actinomycins are well-known inhibitors of transcription in both prokaryotes and eukaryotes, due to their planar structure, which allow them to intercalate into GC-rich DNA.