Alternative routes and mutational robustness in complex regulatory networks
Introduction
Genetic changes in the smallest parts of molecular networks – genes and proteins – can affect the structure of these networks. Conversely, this structure may itself constrain properties of genes and proteins, and the kinds of mutations they can tolerate. To ask how biological networks constrain their parts has become possible with the availability of experimental data on genome-scale metabolic, transcriptional regulation, and protein interaction networks (Forster et al., 2003, Ito et al., 2001, Lee et al., 2002, Uetz et al., 2000, von Mering et al., 2002). If network structure constrains network parts, then an understanding of the evolution of genes and proteins will require an understanding of molecular networks. We here focus on one aspect of network organization, alternative pathways through a genetic network, and how such pathways affect genes in a transcriptional regulation network. The importance of alternative pathways is hinted at by systematic studies on metabolic networks, where alternative pathways of metabolite flow can make a network robust against loss-of-function mutations in enzymes (Edwards and Palsson, 2000, Segre et al., 2002). However, no comparable information exists for any regulatory network.
The evidence for alternative or ‘redundant’ pathways through regulatory gene networks is mostly anecdotal or circumstantial (Bi et al., 2000, Ho and Satoh, 2003, Kolodner et al., 2002, Lefers et al., 2001, LeRoith, 2000, Morris et al., 1995, Passalaris et al., 1999, Vance and Wilson, 2002, Wang et al., 2002a). This evidence typically comes from molecular biological studies where different regulatory pathways, sometimes involving overlapping sets of regulators, can influence the same genes or biological processes. For example, the RAD17 gene product of the yeast Saccharomyces cerevisiae is involved in detecting DNA damage during different stages of the yeast cell cycle. This protein interacts with different sets of other regulatory proteins, which are part of several redundant regulatory pathways that ensure genome stability (Kolodner et al., 2002). Another such example comes from the degradation of cholesterol in mice. Bile acids, the degradation products of cholesterol, indirectly repress the genes necessary for cholesterol degradation. They do so through several incompletely characterized alternative pathways that involve different transcription factors (Wang et al., 2002a).
Available data on genome-scale biological networks elucidated by currently available functional genomic techniques is by necessity incomplete. It may incorporate only one mode of regulation, such as transcriptional regulation; it may not contain information about all relevant regulatory molecules; and it may contain substantial experimental noise. Even with these caveats, however, such data allows a more systematic exploration of important systems – biological questions than small-scale data on individual pathways. Is the existence of alternative pathways between a regulator and its (indirect) regulatory target the exception or the rule? What are the consequences of such alternative pathways for the function and evolution of intermediate regulators, the regulatory molecules that stand between the regulator and its target? These are some of the questions we will address here.
Section snippets
Alternative regulatory pathways are not rare
We find that in regulatory networks of even moderate complexity, many pairs of regulatory molecules and their targets are connected by more than one regulatory pathway. Fig. 1a illustrates this notion with data from the transcriptional regulation network of the yeast S. cerevisiae (Lee et al., 2002). In this network, a directed edge (link) connects two genes A and B if A encodes a transcriptional regulator that can regulate the expression of B, as indicated by its binding to the regulatory
Conclusions
We show that alternative pathways are abundant in more than a dozen biochemical regulatory networks. For the transcriptional regulation network of S. cerevisiae, molecular evolution data suggest that such alternative pathways may provide robustness to mutation. Mutational robustness in a network may either be an adaptation in and by itself, or it may have emerged as a by-product of other evolutionary processes. To distinguish between these possibilities remains an important task for future
Signal transduction networks
The science signal transduction knowledge environment (http://stke.sciencemag.org/cgi/cm) contains a collection of signal transduction pathways manually assembled by experts on these networks. We analyzed the structure of all 15 signal transduction networks with more than 30 nodes that were available in this repository in May 2004. These networks are the adrenergic pathway (http://www.stke.org/cgi/cm/; CMP_8762), a network that mediates the responses of cells to epinephrine and norepinephrine;
Acknowledgments
AW would like to acknowledge support through NIH grant GM63882 to the University of New Mexico, the continued support of the Santa Fe Institute, as well as the support of the Institut des Hautes Etudes Scientifique for a sabbatical stay.
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