Stability in numbers

Two subunits are better than one when the goal is a bistable or oscillating circuit. The findings, based on a mathematical model by Nicolas Buchler (The Rockefeller University, New York, NY) and colleagues, suggest another reason why homodimers are common in biology. “When dimers are more stable than monomers,” says Buchler, “it can have a big impact on genetic circuits.”

Experimental evidence has shown that dimerization can hide a proteolytic tag or stabilize unfolded monomers. For such proteins, the degradation rate will decrease as the concentration of the protein (and thus of its dimer) increases. Buchler et al. modeled the effect of this cooperative stability in a simple, bistable bacterial genetic circuit in which a transcription factor dimer activates its own gene expression.

This bistable circuit can settle in either...