FIG 7 .

Model of Nce103 regulation under changing CO₂ conditions. (A) In the 5% CO₂ environment, Sch9 is activated by phosphorylation of T570 via Pkh1/2, whereby Pkh1/2 activity is influenced by phytosphingosine (PHS) and phosphatidylinositol (PI). Moreover, a direct influence of CO₂ levels on Sch9 activation is possible. Activated Sch9 phosphorylates the transcription factor Cst6, which is subsequently not able to induce NCE103 expression. This results in very low Nce103 levels, which do not enhance CO₂ fixation. (B) Under air conditions, Sch9 is not activated by Pkh1/2 and does not inhibit Cst6. Active Cst6 induces NCE103 expression and enhances the Nce103 protein level, leading to fixation of CO₂. Furthermore, Nce103 seems to be induced by another, so far unknown mechanism.