ARC Special Research Centre for Functional and Applied Genomics, Institute for Molecular Bioscience, University of Queensland, Brisbane, Qld 4072, Australia
Department of Biology and Biochemistry, University of Bath, Bath BA27AY, UK
Physics Department, University of Queensland, Brisbane, Qld 4072, Australia
Current address: Novartis Institute for Biomedical Research, Basel 4056, Switzerland
Two models for the growth of regulatory networks are presented. Both predict that the number of transcriptional regulators will scale quadratically with total gene number.
Abstract
Background
Increased biological complexity is generally associated with the addition of new genetic information, which must be integrated into the existing regulatory network that operates within the cell. General arguments on network control, as well as several recent genomic observations, indicate that regulatory gene number grows disproportionally fast with increasing genome size.
Results
We present two models for the growth of regulatory networks. Both predict that the number of transcriptional regulators will scale quadratically with total gene number. This appears to be in good quantitative agreement with genomic data from 89 fully sequenced prokaryotes. Moreover, the empirical curve predicts that any new non-regulatory gene will be accompanied by more than one additional regulator beyond a genome size of about 20,000 genes, within a factor of two of the observed ceiling.
Conclusions
Our analysis places transcriptional regulatory networks in the class of accelerating networks. We suggest that prokaryotic complexity may have been limited throughout evolution by regulatory overhead, and conversely that complex eukaryotes must have bypassed this constraint by novel strategies.