Fig. 5

Proposed putative pathway for ammonium and urea uptake in S. stipitis. Ammonium (left) is internalized in the cell by facilitated diffusion via ammonium permease; to maintain the potential difference across the plasma membrane the H⁺ -ATPase Pma1 expels a proton, causing acidification of the growth medium. Urea (right) is internalized through a symport with an H⁺ ion (causing an increase in the pH of the growth medium), and it is converted into ammonium via urea amidolyase