Expression and strain variation of the novel "small open reading frame" (smorf) multigene family in Babesia bovis

Abstract

Small open reading frame (smorf) genes comprise the second largest Babesia bovis multigene family. All known 44 variant smorf genes are located in close chromosomal proximity to ves1 genes, which encode proteins that mediate cytoadhesion and contribute to immune evasion. In this study, we characterised the general topology of smorf genes and investigated the gene repertoire, transcriptional profile and SMORF expression in two distinct strains, T2Bo and Mo7. Sequence analysis using degenerate primers identified additional smorf genes in each strain and demonstrated that the smorf gene repertoire varies between strains, with conserved and unique genes in both. Smorf genes have multiple semi-conserved and variable blocks, and a large hypervariable insertion in 20 of the 44 genes defines two major branches of the family, termed smorf A and smorf B. A total of 32 smorf genes are simultaneously transcribed in T2Bo strain B. bovis merozoites obtained from deep brain tissue of an acutely infected animal. SMORF peptide-specific antiserum bound in immunoblots to multiple proteins with a range of sizes predicted by smorf genes, confirming translation of smorf gene products from these transcripts. These results indicate that the smorf multigene family is larger than previously described and demonstrate that smorf genes are expressed and are undergoing variation, both within strains and in a lineage-specific pattern independent of strain specificity. The function of these novel proteins is unknown.