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Journal of Bacteriology, February 2006, p. 909-918, Vol. 188, No. 3
0021-9193/06/$08.00+0     doi:10.1128/JB.188.3.909-918.2006
Copyright © 2006, American Society for Microbiology. All Rights Reserved.

Function and Evolution of Plasmid-Borne Genes for Pyrimidine Biosynthesis in Borrelia spp.

Jianmin Zhong,1 Stephane Skouloubris,2 Qiyuan Dai,1 Hannu Myllykallio,2 and Alan G. Barbour1*

Departments of Microbiology and of Molecular Genetics and Medicine, University of California Irvine, Irvine, California,1 Université Paris-Sud, Institut de Génétique et Microbiologie, INSERM Avenir Group, CNRS UMR8621, Orsay, France2

Received 23 August 2005/ Accepted 8 November 2005

The thyX gene for thymidylate synthase of the Lyme borreliosis (LB) agent Borrelia burgdorferi is located in a 54-kb linear plasmid. In the present study, we identified an orthologous thymidylate synthase gene in the relapsing fever (RF) agent Borrelia hermsii, located it in a 180-kb linear plasmid, and demonstrated its expression. The functions of the B. hermsii and B. burgdorferi thyX gene products were evaluated both in vivo, by complementation of a thymidylate synthase-deficient Escherichia coli mutant, and in vitro, by testing their activities after purification. The B. hermsii thyX gene complemented the thyA mutation in E. coli, and purified B. hermsii ThyX protein catalyzed the conversion of dTMP from dUMP. In contrast, the B. burgdorferi ThyX protein had only weakly detectable activity in vitro, and the B. burgdorferi thyX gene did not provide complementation in vivo. The lack of activity of B. burgdorferi's ThyX protein was associated with the substitution of a cysteine for a highly conserved arginine at position 91. The B. hermsii thyX locus was further distinguished by the downstream presence in the plasmid of orthologues of nrdI, nrdE, and nrdF, which encode the subunits of ribonucleoside diphosphate reductase and which are not present in the LB agents B. burgdorferi and Borrelia garinii. Phylogenetic analysis suggested that the nrdIEF cluster of B. hermsii was acquired by horizontal gene transfer. These findings indicate that Borrelia spp. causing RF have a greater capability for de novo pyrimidine synthesis than those causing LB, thus providing a basis for some of the biological differences between the two groups of pathogens.

* Corresponding author. Mailing address: Pacific-Southwest Center, 3046 Hewitt Hall, University of California Irvine, Irvine, CA 92697-4028. Phone: (949) 824-5626. Fax: (949) 824-6452. E-mail: abarbour{at}uci.edu.

Journal of Bacteriology, February 2006, p. 909-918, Vol. 188, No. 3
0021-9193/06/$08.00+0     doi:10.1128/JB.188.3.909-918.2006
Copyright © 2006, American Society for Microbiology. All Rights Reserved.



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