MBE Advance Access originally published online on July 14, 2009
Molecular Biology and Evolution 2009 26(11):2455-2462; doi:10.1093/molbev/msp151
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Research Articles |
Temporal Constraints on the Incorporation of Regulatory Mutants in Evolutionary Pathways
Department of Organismic and Evolutionary Biology, Harvard University, Cambridge, MA
E-mail: kylemichaelbrown{at}gmail.com.
Accepted for publication July 10, 2009.
Understanding the molecular details of the sequence of events in multistep evolutionary pathways can reveal the extent to which natural selection exploits regulatory mutations affecting expression, amino acid replacements affecting the active site, amino acid replacements affecting protein folding or stability, or variations affecting gene copy number. In experimentally exploring the adaptive landscape of the evolution of resistance to β-lactam antibiotics in enteric bacteria, we noted that a regulatory mutation that increases β-lactamase expression by about 2-fold has a very strong tendency to be fixed at or near the end of the evolutionary pathway. This pattern contrasts with previous experiments selecting for the utilization of novel substrates, in which regulatory mutations that increase expression are often fixed early in the process. To understand the basis of the difference, we carried out experiments in which the expression of β-lactamase was under the control of a tunable arabinose promoter. We find that the fitness effect of an increase in gene expression is highly dependent on the catalytic activity of the coding sequence. An increase in expression of an inefficient enzyme has a negligible effect on drug resistance; however, the effect of an increase in expression of an efficient enzyme is very large. The contrast in the temporal incorporation of regulatory mutants between antibiotic resistance and the utilization of novel substrates is related to the nature of the function that relates enzyme activity to fitness. A mathematical model of β-lactam resistance is examined in detail and shown to be consistent with the observed results.
Key Words: adaptive evolution • regulatory evolution • enzyme evolution • antibiotic resistance • beta-lactamase • evolutionary pathways
1 Present address: Broad Institute, 7 Cambridge Center, Cambridge, MA
2 Present address: Department of Ecology and Evolutionary Biology and Center for Computational Molecular Biology, Brown University Box G-W, Providence, RI