Table 1.

Sign of epistasis in a simple CRE depends on the environment and the sign of individual mutation effects. We consider two environments, one without repressor when mutations affect only RNAP binding, and the other with high repressor concentration. In the first environment, sign of epistasis is determined only by the sign of individual mutation effects on RNAP binding, while in the second environment it is the sign of individual mutation effect on the repressor that matters. For each mutation, the signs (‘+’ and ‘-‘) represent the sign of its effect on the binding of RNAP (p) and repressor (r), respectively. ‘neg -> pos’ and ‘pos -> neg’ represent combinations that display transitions from negative to positive, or positive to negative epistasis, respectively. Certain combinations of mutations are always in negative or always in positive epistasis. The extended version of this table, which does not assume a constant ‘RNAP’ concentration in the cell, is provided in Table 1—source data 1.

DOI: http://dx.doi.org/10.7554/eLife.25192.011

			${\displaystyle p_1{r}_1}$
${\displaystyle p_2{r}_2}$		−−	+−	−+	++
	++	pos→neg neg→pos	neg→pos	pos→neg	neg→pos
	−+	pos→neg	always positive	always negative
	+−	pos→neg	always negative
	−−	neg→pos