Robustness of a gene regulatory circuit

Complex interacting systems exhibit system behavior that is often not predictable from the properties of the component parts. We have tested a particular system property, that of robustness. The behavior of a system is termed robust if that behavior is qualitatively normal in the face of substantial changes to the system components. Here we test whether the behavior of the phage λ gene regulatory circuitry is robust. This circuitry can exist in two alternative patterns of gene expression, and can switch from one regulatory state to the other. These states are stabilized by the action at the O R region of two regulatory proteins, CI and Cro, which bind with differential affinities to the O R 1 and O R 3 sites, such that each represses the synthesis of the other one. In this work, this pattern of binding was altered by making three mutant phages in which O R 1 and O R 3 were identical. These variants had the same qualitative in vivo patterns of gene expression as wild type. We conclude that the behavior of the λ circuitry is highly robust. Based on these and other results, we propose a two‐step pathway, in which robustness plays a key role, for evolution of complex regulatory circuitry.