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Molecular characterization and analysis of a gene encoding the acidic repeat protein (Arp) of Treponema
pallidum.
J Med Microbiol. 2007; 56:715-721.
Liu H, Rodes B, George R, Steiner B.
Abstract
The acidic repeat protein (arp) genes from three subspecies of the treponeme
Treponema pallidum (T. pallidum subsp. pallidum, Nichols strain; T. pallidum
subsp. pertenue, CDC-1 and CDC-2 strains; and T. pallidum subsp. 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 of T. pallidum
subsp. pallidum contained a set of 14 nearly identical repeats of a 60 bp
sequence, which occupied approximately 51 % of the length of the gene. Analyses
of arp from 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. The arp gene 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 against T. 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 of T. pallidum
infection.