Figure 4.

Representations of a simple signal-activated subsystem and the emergence of the “subsystem incompleteness theorem.” Scheme of a signal-activated (S-A) subsystem with four basic components (an actuator/suppressor, a subsystem assembly, a feedback element, and a comparator). The output signal (response to an input stimulus) feeds into other signal-activated (S-A) systems (A). Schematic illustrating the “Incompleteness theorem” for a circuit/subsystem consisting of a docking protein DP activated by a hormone H that mediates signaling to a cell membrane-bound protein P2, a cytoplasmic protein P1, and the enhancers/promoters (EP) of two genes G1–2 (B). The network is structurally self-contained, but its operation depends on the delivery of the external signal (stimulus, H; adapted from Niklas, 2003).