Table 1.

Fitness reduction in the lac system as a function of regulatory mechanism and input state

Mechanism/input state	(0,0)	(0,1)	(1,0)/(1,1)
AA	Δf2′	0	Δf1 + Δf1′
AR	0	Δf4′	Δf1
RA	Δf2 + Δf2′	Δf4	Δf1′
RR	Δf2	Δf4 + Δf4′	0

Columns correspond to input states (glucose,lactose), where 1/0 means saturating/no input. Rows correspond to the possible regulatory mechanisms. The reductions in fitness due to errors from the glucose-responsive (lactose-responsive) regulator binding site are Δf_i (Δf_i′), where i = 1…4 corresponds to the output level. The input states (glucose,lactose) = (1,0) or (1,1) are both mapped onto the same binding state because of inducer exclusion.