Abstract:
Babesia bovis, a tick-transmitted apicomplexan protozoon, infects cattle in tropical and
subtropical regions around the world. In the apicomplexans Toxoplasma gondii and Plasmodium
falciparum, rhomboid serine protease 4 (ROM4) fulfills an essential role in host cell invasion. We thus
investigated B. bovis ROM4 coding genes; their genomic organization; their expression in in vitro
cultured asexual (AS) and sexual stages (SS); and strain polymorphisms. B. bovis contains five rom4
paralogous genes in chromosome 2, which we have named rom4.1, 4.2, 4.3, 4.4 and 4.5. There are
moderate degrees of sequence identity between them, except for rom4.3 and 4.4, which are almost
identical. RT-qPCR analysis showed that rom4.1 and rom4.3/4.4, respectively, display 18-fold and
218-fold significantly higher (p < 0.01) levels of transcription in SS than in AS, suggesting a role in
gametogenesis-related processes. In contrast, transcription of rom4.4 and 4.5 differed non-significantly
between the stages. ROM4 polymorphisms among geographic isolates were essentially restricted to
the number of tandem repeats of a 29-amino acid sequence in ROM4.5. This sequence repeat is highly
conserved and predicted as antigenic. B. bovis ROMs likely participate in relevant host–pathogen
interactions and are possibly useful targets for the development of new control strategies against
this pathogen.
