Gene structure of the extracellular glutathione S-transferase from Onchocerca volvulus and its overexpression and promoter analysis in transgenic Caenorhabditis elegans

Two highly similar genes encoding unique extracellular, glycosylated glutathione S-transferases (GSTs) of the human-pathogenic nematode, Onchocerca volvulus (Ov-GST1a and Ov-GST1b), have been isolated and characterised. The genes are ≈ 3 kb in length and consist of seven exons interrupted by introns of ≈ 100 bp in length, with the exception of intron II, which is ≈ 1.6 kb in length. Interestingly, exon I and II encode a signal peptide and an N-terminal extension before sequence homology to other GSTs begins. The 5′ flanking region was sequenced and analysed for transcription factor binding sites. Consistent with the lack of a TATA box, analysis of the mRNAs by primer extension showed multiple transcription start sites spread over a 60 bp region. To examine the activity and specificity of the Ov-GST1a gene promoter, we have exploited Caenorhabditis elegans as a heterologous transformation system. To analyse whether transgenic C. elegans are able to carry out processing and post-transcriptional modifications of the Ov-GST1a correctly, the protein was ectopically overexpressed in C. elegans. The parasite-derived Ov-GST1a gene product was correctly processed in transgenic C. elegans and posttranslational modifications, such as signal peptide cleavage and N-glycosylation, were performed successfully. This further demonstrates the potential of C. elegans as a host for expression of candidate vaccine antigens from O. volvulus and affirms the role of C. elegans as a model for parasitic nematodes.