Transcriptional regulation by carbon and nitrogen interactions. (a) Interactions between carbon (C) and nitrogen (N) signaling can be explained by three models, and an example(s) of each is given. Model 1, carbon and nitrogen regulation are independent and therefore are additive. Model 2, carbon and nitrogen are dependent, as both are required for an effect. Model 3, there is a dependent and independent component to carbon and nitrogen regulation. Two examples of Model 3 are shown (genes Y and Z). For gene Y, nitrogen only has an effect in the presence of carbon, while for gene Z, carbon only has an effect in the presence of nitrogen. (b) The assignment of genes W, X, Y, and Z to InterAct classes.