FIG 9.

Nitrate and nitrite efflux systems in H. polymorpha. The high-affinity nitrate transporter (Ynt1) is involved in nitrate and nitrite influx. The nitrate is reduced by nitrate reductase (NR) to nitrite, which is catalyzed to ammonium by nitrite reductase (NiR). Ssu2 and to a lesser extent Ssu1 are involved nitrate efflux. Nar1 is involved in nitrite and nitrate efflux. Nar1-dependent nitrate efflux is observed when intracellular nitrate reaches higher levels. The nitrate and nitrite influx systems allow cells to cope with intracellular nitrite in cells growing in nitrate or nitrite as the sole nitrogen source.