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Science 21 October 2005:
Vol. 310. no. 5747, p. 401
DOI: 10.1126/science.310.5747.401k
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Epistatic mutations, which have a nonadditive effect on phenotype, may be important in evolution because they could generate rugged adaptive landscapes. Alternatively, epistasis may be relatively unimportant in natural selection. Lunzer et al. (p. 499; see the Perspective by Ellington and Bull) construct a biochemical adaptive landscape for cofactor use by the Escherchia coli enzyme isopropylmalate dehydrogenase (IMDH). The enzyme normally uses nicotinamide adenine dinucleotide (NAD) as a coenzyme, but can be engineered through five amino acid changes to use nicotinamide adenine dinucleotide phosphate (NADP). More than 150 single and double intermediate mutants were assayed for performance and coenzyme preference, and mutant bacteria were assayed for fitness. Each amino acid change contributes additively to enzyme function, whereas they show epistatic contributions to fitness. All natural IMDHs use NAD, which suggests that an ancient adaptive landscape has been conserved.




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