Figure 4

Illustrations for the underlying conditions of a logic triplet. A: Aminopropylcadaverine biosynthesis in E. coli [20]. Enzymes ldcC and cadA are lysine decarboxylases, and speE is an aminopropylcadaverine synthase. The first step can be catalyzed by either ldcC or cadA depending on the reaction conditions. However, the three genes only form an if instead of a full iff logic triplet because of the presence of path 4 and 5 in Figure 4B. B: The underlying conditions for the logic triplet "c is present iff a or b is present". Compound x can be converted to compound y by either enzyme a or b, y can then be catalyzed by enzyme c to produce compound z. Dashed lines represent other reactions marked by numbers; 1, 2, and 5 mean the enzymes can catalyze other reactions, 3 and 4 mean y can be the product or substrate in other paths. The underlying conditions for the iff logic triplet are that none of the five dashed lines can be present in large number of species; that is, a, b, c, and y cannot often be involved in other type of reactions.