Before revision:  We explicitly calculate the resources required for a canonical signaling motif, a push-pull network . We arrive at an analytical expression that shows that storing the state of the bound receptor over time requires at least one readout molecule to store the state and at least 4 kT of energy to store it reliably. Thus, each of the resources -- time, protein copy numbers, and energy -- limits sensing, and it is the limiting resource which places the fundamental limit on sensing .
After revision:  We explicitly calculate the resources required for a canonical signaling motif, a push-pull network , this requires i) time and receptors; ii) downstream molecules; iii) energy (fuel turnover) to drive modification. These three resource classes cannot compensate each other, and it is the limiting resource which places the fundamental limit on sensing .