To signal a conjunction of many inputs negative regulation is likely

Cells make many transitions from an old to a new phase of activity - between inactive and active states of an enzyme, or between phases of the cell cycle. If a cell is to survive, molecular prerequisites for functioning in the new phase should be available before a transition occurs. The cell's survival is more likely if a regulatory network gates the transition, preventing its occurrence until the prerequisites are available. Suppose a specific conjunction of inputs is required for a network, from which a single output governs the transition. Then we suggest that cells are likely to use negative regulation - a gating network based on a logical disjunction of signals for the absence of prerequisites - rather than positive regulation - a logical conjunction of signals for their presence. That is, if a logical conjunction of n prerequisites A1 AND A2 AND … AND An is needed in the new phase, a negative regulatory network is likely to enforce the corresponding logical disjunction, NOT (NOT A1 OR NOT A2 OR … OR NOT An). Five examples illustrate this conclusion. Arguments based on performance criteria support the hypothesis: negative regulation is more economical than positive regulation, because networks for computing OR can use fewer and simpler parts than those for computing AND. Negative regulation can increase reliability, because a mechanism that uses fewer, simpler parts is less likely to fail. And, a negative regulatory network can be more robust - less susceptible to errors resulting from noisy input.