Perfect adaptation achieved by transport limitations governs the inorganic phosphate response in S. cerevisiae.
Cells cope with and adapt to ever-changing environmental conditions. Sophisticated regulatory networks allow cells to adjust to these fluctuating environments. One such archetypal system is the S. cerevisiae Pho regulon. When external inorganic phosphate (Pi) concentration is low, the Pho regulon activates, expressing genes that scavenge external and internal Pi. However, the precise mechanism controlling this regulon remains elusive. We conducted a systems analysis of the Pho regulon on the single cell level under well-controlled environmental conditions. This analysis identified a robust, perfectly adapted Pho regulon state in intermediate Pi conditions, and we discovered a hitherto unknown intermediate nuclear localization state of the transcriptional master regulator Pho4p. The existence of an intermediate nuclear Pho4p state unifies and resolves outstanding incongruities associated with the Pho regulon, explains the observed programmatic states of the Pho regulon, and improves our general understanding of how nature evolves and controls sophisticated gene regulatory networks. We further propose that robustness and perfect adaptation are not achieved through complex network-centric control, but by simple transport biophysics. The ubiquity of multi-transporter systems suggests that similar mechanisms could govern the function of other regulatory networks as well.