Figure 4.

Natural and synthetically built AND NOT (NOT IF) Boolean logic gates. An AND NOT gate generates an output when only one specific single input signal is present, but not when there is no input signal, nor a second input, nor both signals. A, Truth table and scheme of the regulatory region of the Lac operon as an AND NOT (NOT IF) gate. This AND NOT gate only generates an output when lactose is the only single input available. If Glc and lactose are available in the cell, the lac operon is OFF because the catabolite activator protein, CAP, is not bound. The same is true when Glc, but no lactose, is available. In this case, the lac repressor is bound. In the case when there is neither Glc nor lactose, the lac operon is OFF because even though CAP is bound, the lac repressor prevents transcriptional initiation. Only when there is lactose, but no Glc, available is the lac operon ON. In the absence of Glc, CAP can bind, and because of the availability of lactose, the lac repressor is not bound. Both actions are necessary for transcriptional initiation of the lac operon. (Adapted from Phillips et al., 2009.) B, An example of an AND NOT (NOT IF) gate in synthetic biology. In this synthetic system, the transactivator SCA (transactivator of the streptogramin-responsive gene regulation system) and the transrepressor PIP-KRAB are constitutively expressed along with a reporter plasmid containing a chimeric SCA- and PIP-specific promoter. The absence of SCB1 [racemic 2-(1V-hydroxy-6-methylheptyl)-3-(hydroxymethyl)butanolide] enables the binding of the transactivator SCA to its corresponding promoter region. The presence of the transrepressor pristinamycin (PI) in turn prevents the binding of PIP-KRAB to its promoter. Thus, this engineered AND NOT gate generates an output only in the presence of pristinamycin and the absence of SCB1. (Adapted from Kramer et al., 2004a.)