Effects of constitutive gene expression on the dynamics of random boolean networks

We investigate the effects of constitutive gene activation upon the dynamics of random Boolean networks using a suite of models. We find that constitutive activity leads to simpler state spaces, with fewer and larger basins of attraction. The major difference in patterns of basin number and distribution is seen between networks with no activation and those with a single constitutively active node. Increasing the proportion of constitutively active node intensifies, but does not qualitatively change, the observed patterns. The proportion of genes constitutively active interacts in a nonlinear way with other network parameters, such as average connectivity and proportion of inhibitory links. We conclude that constitutive gene activation has a fundamental effect on network behavior that has been overlooked in previous random Boolean network studies. It acts to constrain the start states of the networks, and the states which can be reached during development and differentiation, and we hypothesize that constitutive activation and repression of genes may help to guide the process whereby a single genetic regulatory network produces a range of different cell types.