Fig. S2.

Lock–key interactions trap genetic trajectories. Lock–key binding repressor–operator pairs constrain the trajectory depicted in Fig. 3 A and B (black trajectory). The black and cyan traces together form a valley in both (A) repression ability and (B) expression ability. The tcca operator is a low-expression operator in both environments (Fig. 1C and Fig. S5), which initially retains the repression ability at a high level. After this operator is mutated to tcga or tgca, the repression ability drops and is only restored to a higher repression ability when a new operator–repressor binding pair is formed at YQ:tgga. Cross-environmental tradeoffs can readily be observed by comparing A with B; initially, high levels of repression ability correspond to low levels of expression ability. The mutation to YK:tcca (in the cyan dashed trajectory) is neutral in repression ability according to the statistical test, and it is, therefore, considered not selectively accessible. Means are represented ± SEM. The same situation occurs for the mutation to YQ:tgga (in the cyan trajectory) in expression ability.