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Infection and Immunity, July 2002, p. 3566-3575, Vol. 70, No. 7
0019-9567/02/$04.00+0     DOI: 10.1128/IAI.70.7.3566-3575.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.

The Babesia bovis Merozoite Surface Antigen 2 Locus Contains Four Tandemly Arranged and Expressed Genes Encoding Immunologically Distinct Proteins

Monica Florin-Christensen,^1, Carlos E. Suarez,^1,2 Stephen A. Hines,¹ Guy H. Palmer,¹ Wendy C. Brown,¹ and Terry F. McElwain^1*

Program in Vector-Borne Diseases, Department of Veterinary Microbiology and Pathology, Washington State University, Pullman, Washington 99164-7040,¹ Animal Disease Research Unit, USDA Agricultural Research Service, Pullman, Washington 99164-6630²

Received 26 December 2001/ Returned for modification 25 February 2002/ Accepted 17 April 2002

Members of the variable merozoite surface antigen (vmsa) gene family of Babesia bovis encode membrane proteins involved in erythrocyte invasion. In this study, we have identified and sequenced the complete 8.3-kb genomic locus containing msa-2, a member of the vmsa family, in the biologically cloned Mexico Mo7 strain. Four tandemly arranged copies of msa-2-related genes were found in the locus. The four genes, designated msa-2a₁ (which corresponds to the originally described msa-2 gene), msa-2a₂, msa-2b, and msa-2c, were shown to be transcribed and expressed and encode proteins with open reading frames ranging in size from 266 (MSA-2c) to 317 (MSA-2a₁) amino acids. MSA-2a₁ and -2a₂ are the most closely related of the four proteins (90% identity), differing by (i) the number of 24-amino-acid repeats that comprise a surface-exposed B-cell epitope and (ii) the presence of a 32-amino-acid area of recombination between MSA-2a₂ and -2b. In contrast, msa-2c is most closely related to the previously described babr 0.8 gene in Australia strains of B. bovis. Comparison of MSA-2 proteins in the Argentina R1A strain of B. bovis with the Mexico Mo7 clone revealed a relatively high degree of conservation (83.6, 69.4, 79.1, and 88.7% amino acid identity for MSA-2a₁, -2a₂, -2b, and -2c, respectively), in contrast to the extensive MSA-1 sequence variation (52% identity) between the same two strains. Postinfection bovine immune serum contains antibodies that bound to each of the recombinant MSA-2 proteins. Blocking assays demonstrated the presence of unique B-cell epitopes in MSA-2a₁, -2b, and -2c. The results support the evolution of the msa-2 locus through at least two gene duplications, with selection for multiple related but antigenically distinct merozoite surface proteins.

Editor: W. A. Petri, Jr.

Present address: Institute of Virology, CICVyA, INTA-Castelar, 1712 Castelar, Argentina.

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