A common regulatory motif, where a heterodimeric transcriptional regulator positively autoregulates only one of its components, is found to have particular properties that enable precise and robust control of cellular responses to environmental stimuli, providing an explanation for the prevalence of this motif in evolved regulatory networks.
Many important biological systems rely on regulation by dimers of proteins which upregulate the transcription of numerous targets, including one, and only one, of the dimer pair. This is termed asymmetric self-upregulation.ASymmetric Self-UpREgulated (ASSURE) networks confer rapid induction of their targets and their network behaviors are robust to parameter variation—both features appear to have contributed to the prevalence of the network across widely different biological systems.Likely evolutionary precursors to ASSURE networks are symmetrically self-upregulated network mediated by homodimers. In silico and experimental studies demonstrate that the ASSURE network confers a competitive advantage over its symmetrical counterpart.
Positive feedback is a common mechanism enabling biological systems to respond to stimuli in a switch-like manner. Such systems are often characterized by the requisite formation of a heterodimer where only one of the pair is subject to feedback. This ASymmetric Self-UpREgulation (ASSURE) motif is central to many biological systems, including cholesterol homeostasis (LXRα/RXRα), adipocyte differentiation (PPARγ/RXRα), development and differentiation (RAR/RXR), myogenesis (MyoD/E12) and cellular antiviral defense (IRF3/IRF7). To understand why this motif is so prevalent, we examined its properties in an evolutionarily conserved transcriptional regulatory network in yeast (Oaf1p/Pip2p). We demonstrate that the asymmetry in positive feedback confers a competitive advantage and allows the system to robustly increase its responsiveness while precisely tuning the response to a consistent level in the presence of varying stimuli. This study reveals evolutionary advantages for the ASSURE motif, and mechanisms for control, that are relevant to pharmacologic intervention and synthetic biology applications.