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Journal of Bacteriology, April 2008, p. 2266-2274, Vol. 190, No. 7
0021-9193/08/$08.00+0     doi:10.1128/JB.01841-07
Copyright © 2008, American Society for Microbiology. All Rights Reserved.

Elucidation of an Alternate Isoleucine Biosynthesis Pathway in Geobacter sulfurreducens{triangledown}

Carla Risso,1* Stephen J. Van Dien,2 Amber Orloff,1 Derek R. Lovley,1 and Maddalena V. Coppi1

Department of Microbiology, 203N Morrill Science Center IVN, University of Massachusetts Amherst, Amherst, Massachusetts 01003,1 Genomatica, Inc., 5405 Morehouse Drive, Suite 210, San Diego, California 921212

Received 21 November 2007/ Accepted 18 January 2008

The central metabolic model for Geobacter sulfurreducens included a single pathway for the biosynthesis of isoleucine that was analogous to that of Escherichia coli, in which the isoleucine precursor 2-oxobutanoate is generated from threonine. 13C labeling studies performed in G. sulfurreducens indicated that this pathway accounted for a minor fraction of isoleucine biosynthesis and that the majority of isoleucine was instead derived from acetyl-coenzyme A and pyruvate, possibly via the citramalate pathway. Genes encoding citramalate synthase (GSU1798), which catalyzes the first dedicated step in the citramalate pathway, and threonine ammonia-lyase (GSU0486), which catalyzes the conversion of threonine to 2-oxobutanoate, were identified and knocked out. Mutants lacking both of these enzymes were auxotrophs for isoleucine, whereas single mutants were capable of growth in the absence of isoleucine. Biochemical characterization of the single mutants revealed deficiencies in citramalate synthase and threonine ammonia-lyase activity. Thus, in G. sulfurreducens, 2-oxobutanoate can be synthesized either from citramalate or threonine, with the former being the main pathway for isoleucine biosynthesis. The citramalate synthase of G. sulfurreducens constitutes the first characterized member of a phylogenetically distinct clade of citramalate synthases, which contains representatives from a wide variety of microorganisms.

* Corresponding author. Mailing address: Department of Microbiology, 203N Morrill Science Center IVN, University of Massachusetts Amherst, Amherst, MA 01003. Phone: (413) 577-2439. Fax: (413) 577-4660. E-mail: crisso{at}microbio.umass.edu

{triangledown} Published ahead of print on 1 February 2008.

Journal of Bacteriology, April 2008, p. 2266-2274, Vol. 190, No. 7
0021-9193/08/$08.00+0     doi:10.1128/JB.01841-07
Copyright © 2008, American Society for Microbiology. All Rights Reserved.



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