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Applied and Environmental Microbiology, June 2006, p. 3993-4000, Vol. 72, No. 6
0099-2240/06/$08.00+0     doi:10.1128/AEM.00684-06
Copyright © 2006, American Society for Microbiology. All Rights Reserved.

Cysteine Metabolism in Legionella pneumophila: Characterization of an L-Cystine-Utilizing Mutant

Fanny Ewann1,2 and Paul S. Hoffman1,2*

Division of Infectious Diseases and International Health, Department of Internal Medicine,1 Department of Microbiology, School of Medicine, University of Virginia, Charlottesville, Virginia2

Received 24 March 2006/ Accepted 27 March 2006

Growth of Legionella pneumophila on buffered charcoal-yeast extract (BCYE) medium is dependent on L-cysteine (but not L-cystine), which is added in excess over what is required for nutrition. We investigated the biochemical and genetic bases for this unusual requirement and determined that much of the L-cysteine in BCYE medium is rapidly oxidized to L-cystine and is unavailable to the bacteria. Analysis of cysteine consumption during bacterial growth indicated that of the 11% consumed, 3.85% (~0.1 mM) was incorporated into biomass. The activities of two key cysteine biosynthetic enzymes (serine acetyltransferase and cysteine synthase) were not detected in cell extracts of L. pneumophila, and the respective genes were not present in the genome sequences, confirming cysteine auxotrophy. Kinetic studies identified two energy-dependent cysteine transporters, one with high affinity (apparent Km, 3.29 µM) and the other with low affinity (apparent Km, 93 µM), each of which was inhibited by the uncoupling agent carbonyl cyanide m-chlorophenylhydrazone. Cystine was not transported by L. pneumophila; however, a mutant strain capable of growth on L-cystine (CYS1 mutant) transported L-cystine with similar kinetics (Km, 4.4 µM and 90 µM). Based on the bipartite kinetics, requirement for proton motive force, and inhibitor studies, we suggest that a high-affinity periplasmic binding protein and a major facilitator/symporter (low affinity) mediate uptake. The latter most likely is functional at high cysteine concentrations and most likely displays altered substrate specificity in the CYS-1 mutant. Our studies provide biochemical evidence to support a general view that L. pneumophila is restricted to an intracellular lifestyle in natural environments by an inability to utilize cystine, which most likely ensures that the dormant cyst-like transmissible forms do not germinate outside suitable protozoan hosts.

* Corresponding author. Mailing address: Division of Infectious Diseases, University of Virginia Health Systems, MR-4 Building, Room 2146, 409 Lane Road, Charlottesville, VA 22908. Phone: (434) 924-2893. Fax: (434) 924-0075. E-mail: psh2n{at}virginia.edu.

Applied and Environmental Microbiology, June 2006, p. 3993-4000, Vol. 72, No. 6
0099-2240/06/$08.00+0     doi:10.1128/AEM.00684-06
Copyright © 2006, American Society for Microbiology. All Rights Reserved.



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