These are only two simple examples of mechanisms that regulate gene activity, and they are functionally equivalent. "The question was raised long ago as to whether nature's choice in favor of one of the two mechanisms is only random, or whether there are specific criteria," reports Gerland. "Studies have shown that the demand for a gene product is a decisive factor: it is mostly genes whose proteins are required most of the time that are directly activated. Proteins such as lactase, on the other hand, which are only used some of the time, tend to have genetic codes that are only released from their repressor when needed." The "use-it-or-lose-it" principle was proposed to explain this, which demands the frequent use of regulation factors because they would otherwise be subject to damaging effects.

Using computer simulations and theoretical calculations, Gerland and Hwa have now demonstrated that a second - even opposing - principle also comes into play: "wear-and-tear". Its name reflects the fact that constant use of regulators can also lead to detrimental consequences. The two researchers therefore investigated whether other factors that can affect the evolution of microorganisms play a role. "Our results clearly show that both principles are valid, even though they are actually contradictory," Gerland says. "In this conflict between maximal and minimal use of the regulators, other criteria do in fact come into play: the population size and the periods over which the environmental changes stretch."

Genetic regulation in small populations existing in an environment that only slowly changes is governed by the "use-it-or-lose-it" principle, with maximal use of the regulation proteins. In the opposite case, however, it is more "wear-and-tear" that comes into play with minimal use of the proteins. "The evolution of regulatory systems is still barely understood," reports Gerland. "So far, appropriate theoretical methods have also been largely lacking. But the time-dependent selection shown in our example may now prove to be an important factor in regulatory development. Many questions are still open, and our results will hopefully prompt further investigations."

Full bibliographic information: "Evolutionary selection between alternative modes of gene regulation", Ulrich Gerland and Terence Hwa, PNAS Early Edition, 22 May 2009

en.uni-muenchen.de/