Figure 7. Schematic diagram showing the proposed function of Gaf1 in the nitrogen-signaling pathways in S. pombe.

Nitrogen starvation causes induction of gaf1⁺ expression, and Gaf1, in turn, represses the expression of ste11⁺ via direct interaction with its promoter. It has been previously known that nitrogen starvation leads to the activation of Rst2 via the cAMP-dependent PKA pathway [14], [15] as well as Atf1-Pcr1 via the Sty1 MAPK pathway [3], [5], [6], [14]–[16], [47]–[49], consequently resulting in induction of ste11⁺ expression. In addition, phosphodiesterase is most likely stimulated by PKA activity to create a feedback mechanism [50]. The pathway addressed in this study is shown in thick lines, and other paths previously determined are shown in thin lines. Activation and inhibition are indicated by arrows and crossing bars, respectively. Dotted lines indicate pathways remained to be fully determined.