Figure 2. Null mutations increase fitness through varied mechanisms.

(A) In a hypothetical cellular network, E1–E4 are enzymes, M1–M5 are metabolites, S is a structural protein, R1–R3 are regulatory proteins, and H is a housekeeping protein that inhibits translation and promotes degradation of some mRNAs. Dotted lines indicate other activities of the indicated proteins. The fitness of cells depends only on the levels of S, M2, and M5. (B,C) Optimal concentrations of S, M2, and M5 in the native environment (B) and a novel environment to which the cells might need to adapt (C). Null mutations adaptive in the novel environment are marked in panel (A) with an orange ‘x’.