Sequence and organization of the rhoptry-associated-protein-1 (rap-1) locus for the sheep hemoprotozoan Babesia sp. BQ1 Lintan (B. motasi phylogenetic group)

Abstract Babesiosis is a frequent infection of animals worldwide by tick borne pathogen Babesia , and several species are responsible for ovine babesiosis. Recently, several Babesia motasi -like isolates were described in sheep in China. In this study, we sequenced the multigenic rap-1 gene locus of one of these isolates, Babesia sp. BQ1 Lintan. The RAP-1 proteins are involved in the process of red blood cells invasion and thus represent a potential target for vaccine development. A complex composition and organization of the rap-1 locus was discovered with: (1) the presence of 3 different types of rap-1 sequences ( rap-1a , rap-1b and rap-1c ); (2) the presence of multiple copies of rap-1a and rap-1b ; (3) polymorphism among the rap-1a copies, with two classes (named rap-1a61 and rap-1a67 ) having a similarity of 95.7%, each class represented by two close variants; (4) polymorphism between rap-1a61-1 and rap-1a61-2 limited to three nucleotide positions; (5) a difference of eight nucleotides between rap-1a67-1 and rap-1a67-2 from position 1270 to the putative stop site of rap-1a67-1 which might produce two putative proteins of slightly different sizes; (6) the ratio of rap-1a copies corresponding to one rap-1a67 , one rap-1a61-1 and one rap-1a61-2 ; (7) the presence of three different intergenic regions separating rap-1a , rap-1b and rap-1c ; (8) interspacing of the rap-1a copies with rap-1b copies; and (9) the terminal position of rap-1c in the locus. A 31 kb locus composed of 6 rap-1a sequences interspaced with 5 rap-1b sequences and with a terminal rap-1c copy was hypothesized. A strikingly similar sequence composition ( rap-1a , rap-1b and rap-1c ), as well as strong gene identities and similar locus organization with B. bigemina were found and highlight the conservation of synteny at this locus in this phylogenetic clade.