Schematic of ammonia assimilation in S. cerevisiae. Ammonia is transported into the cell by three transporters, Mep1p, Mep2p, and Mep3p. Three enzymes, Gdh1p, Glt1p, and Gdh3p, convert ammonia to glutamate and glutamine. Gdh1p is responsible for the majority of ammonia assimilation. Gdh1p is regulated by four transcription factors, Gat1p, Gln3p, Dal80p, and Gzf3p.