Genetic variation in the dimorphic regions of RAP-1 genes and rap-1 loci of Babesia bigemina

Abstract

The rhoptry-associated protein-1 (RAP-1) of Babesia bigemina induces protective immune responses in cattle. RAP-1 has two regions of sequence dimorphism at the carboxy and amino terminal ends, respectively. Neutralization-sensitive, surface-exposed B-cell epitopes are present in the amino terminal variant type 1 (NT-1), and CD4+ T-cell epitopes in the carboxy terminal variant type 1 (CT-1). Importantly, antibodies recognizing NT-1 epitopes do not cross react with NT-2 and CD4+ T-cells recognizing epitopes in CT-1 do not cross react with CT-2, suggesting that variation in dimorphic regions of RAP-1 is immunologically significant. We evaluated rap-1 locus structure and the extent of sequence variation in the dimorphic regions of rap-1 genes from geographically diverse strains of B. bigemina. All strains contained NT-1 and NT-2 the encoding sequences were highly conserved, with at least 99%, nucleotide identity among strains. However, the Puerto Rico strain encoded a hybrid NT-1/NT-2 sequence which appears to have originated by a gene conversion event. The 3' ends of rap-1 genes, which include the carboxy terminal variants, are conserved among strains. A new and conserved CT variant (CT-3), with a region of sequence identity to CT-2 and a sequence not related to either CT-1 or CT-2, was identified in all strains of B. bigemina. All but one strain encode both NTs and the three CT variants. The S1A strain, an attenuated strain from Argentina, does not encode CT-2. While NT-1 is associated only with CT-1, NT-2 can be associated with all three CT variants in RAP-1. Within the genome, rap-1 genes are arranged in tandem repeats but with different gene copy number and arrangements among strains. Collectively, the data suggest that gene conversion and unequal recombination events contribute to overall rap-1 sequence conservation among gene variants and strains but may also generate new rap-1 variants.