Molecular characterization and analysis of a gene encoding the acidic repeat protein (Arp) of Treponema pallidum

Abstract

The acidic repeat protein (arp) genes from three subspecies of the treponemeTreponema pallidum(T. pallidumsubsp.pallidum, Nichols strain;T. pallidumsubsp.pertenue, CDC-1 and CDC-2 strains; andT. pallidumsubsp.endemicum, Bosnia A strain) were cloned and sequenced. The predicted protein sequence contained a high percentage of glutamic acid, hence the name acidic repeat protein, or Arp. The protein had a potential membrane-spanning domain and a signal peptidase I site. The gene from the Nichols strain ofT. pallidumsubsp.pallidumcontained a set of 14 nearly identical repeats of a 60 bp sequence, which occupied ∼51 % of the length of the gene. Analyses ofarpfrom laboratory strains showed that the 5′ and 3′ ends of the genes were conserved, but there was considerable heterogeneity in the number of repeats of this 60 bp sequence. Based on amino acid variations, the 14 sequence repeats could be classified into three types, which were named type I, type II and type III repeats. The type II repeat was the most common in the strains examined. Thearpgene of the Nichols strain was subsequently cloned into the expression vector pBAD/TOPO ThioFusion. The expressed protein was detected in a Western blot assay using rabbit immune sera produced againstT. pallidum, or synthetic peptides derived from the repeat sequences. Using an ELISA, rapid plasma reagin (RPR) test-positive sera reacted with synthetic peptides derived from the repeat region but not with peptides derived from N and C termini of the Arp protein. These results show that the Arp protein is immunogenic and could prove to be a useful target for serological diagnosis ofT. palliduminfection.