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Abstract :

Several intra-erythrocytic parasites of the genus Babesia are responsible of ovine babesiosis in China. They belong to two phylogenetic groups. One includes only Babesia sp. Xinjiang ; the other, close to B. motasi, gathers several different parasites whose status as genus or species remains to be clarified. To develop a vaccination strategy aiming at blocking the parasite multiplication within its host, the characterization of the multigene family rap-1 (rhoptry-associated-protein-1) has been undertaken, as these proteins are known to be involved in the process of the erythrocyte invasion by the parasite.
In the B. motasi-like group, the rap-1 locus is complex in the 4 parasites studied (Babesia sp. Tianzhu, Babesia sp. BQ1 Lintan and BQ1 Ningxian, Babesia sp. Hebei) with 3 types of genes : rap-1a (6 polymorphic copies), rap-1b (5 identical copies intercalated with rap-1a genes) and rap-1c (1 copy at the 3' end of the locus). The sequences are similar for 3 of these parasites (99.9% over the whole locus), but Babesia sp. Hebei rap-1 sequences are more divergent (78%). The 3 types of genes are transcribed in vitro by Babesia sp. BQ1 Lintan. In vitro, only some copies of rap-1a are translated. In vivo, all genes seem to be translated with different kinetics.
Seven rap-1a genes have been characterized in Babesia sp. Xinjiang, belonging to two types, α and β, according to their 3' end sequences. These tandemly arranged genes are identical over 936 nt in 5', and differ in 3' by a variable number of more or less conserved 36 nt repeats. At least 5 of these genes are transcribed in in vitro culture.
These results are discussed in terms of taxonomy, evolution and vaccination.

Key words :

Babesia, ovine babesiosis, rhoptry-associatedprotein, protein expression, red blood cell invasion, taxonomy, vaccine development, protein repeats