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Molecular and Cellular Biology, April 2008, p. 2567-2578, Vol. 28, No. 8
0270-7306/08/$08.00+0     doi:10.1128/MCB.01785-07
Copyright © 2008, American Society for Microbiology. All Rights Reserved.

Evolution of New Function through a Single Amino Acid Change in the Yeast Repressor Sum1p{triangledown}

Alexias Safi, Kelley A. Wallace,{dagger} and Laura N. Rusche*

Institute for Genome Sciences and Policy and Biochemistry Department, Duke University, Durham, North Carolina 27710

Received 28 September 2007/ Returned for modification 21 October 2007/ Accepted 2 February 2008

The SUM1-1 mutation is an example of a single amino acid change that results in new function. Wild-type Sum1p in Saccharomyces cerevisiae is a DNA-binding repressor that acts locally, whereas mutant Sum1-1p forms an extended repressive chromatin structure. By characterizing a panel of mutations in which various amino acids replaced the critical residue, threonine 988, we found that threonine was required for wild-type function and that in the absence of threonine the association of Sum1p with DNA was reduced. Isoleucine, the amino acid in mutant Sum1-1p, was required for the novel spreading property. Thus, the SUM1-1 mutation results in both a loss and a gain of function. The presence of isoleucine caused Sum1-1p to self-associate, a property that may promote spreading. In addition, isoleucine enabled Sum1-1p to associate with the origin recognition complex (ORC) and accumulate near ORC binding sites. Thus, both threonine and isoleucine at position 988 enable Sum1p to form intermolecular interactions. We propose that interaction domains may be hotspots for gain-of-function mutations because alterations in such domains have the potential to redirect a protein to new sets of binding partners. In addition, self-association of chromatin proteins may promote the formation of extended chromatin structures.

* Corresponding author. Mailing address: Institute for Genome Sciences and Policy, Box 3382, Duke University, Durham, NC 27710. Phone: (919) 684-0354. Fax: (919) 668-0795. E-mail: lrusche{at}biochem.duke.edu

{triangledown} Published ahead of print on 11 February 2008.

{dagger} Present address: College of Osteopathic Medicine of the Pacific, Western University of Health Sciences, Pomona, CA 91766.

Molecular and Cellular Biology, April 2008, p. 2567-2578, Vol. 28, No. 8
0270-7306/08/$08.00+0     doi:10.1128/MCB.01785-07
Copyright © 2008, American Society for Microbiology. All Rights Reserved.





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