Extensive interplasmidic duplications change the virulence phenotype of the relapsing fever agent Borrelia turicatae

Abstract

SummaryThe relapsing fever agent Borrelia turicatae has two antigenically distinct serotypes, A and B, which differ in their variable small proteins (Vsps) and in their degree of virulence and neurotropism in mice. Each Vsp gene (vspA or vspB ) had an expression-linked copy that was unique to the serotype expressing it. This was located on one linear plasmid, which was defined by the upstream sequence. The archived copies of vspA and vspB were each located on different linear plasmids that were the same in both serotypes. In this feature, the mechanism of antigenic variation is similar to that of another relapsing fever agent, B. hermsii. However, in other features, the mechanisms of the two organisms differ. The expressed and archived loci for vspA and vspB of B. turicatae were near the centre of linear plasmids instead of near the telomeres. The vspA and vspB expression loci were duplicate copies of their respective silent loci: from the vsp itself to at least 13-14 kb downstream. Despite the extensive interplasmidic duplications and the internal position of the expression locus, the only detectable difference between serotypes A and B was in whether they expressed VspA or VspB.

Structural Conservation of Neurotropism-associated VspA within the Variable Borrelia Vsp-OspC Lipoprotein Family

“…CD spectroscopy demonstrated conserved, highly ␣-helical secondary structures that are predicted to fold into a four-helix bundle. Supporting this model were the findings that postswitch mutations in expressed vsp alleles are mainly conservative (30,52), and that non-conservative amino acid changes, insertions, and deletions cluster in regions outside the predicted helices (Fig. 1).…”

“…Vlp and Vsp proteins diverge even more, with 40 to 80% amino acid identities among them (17,28). In contrast to the single plasmid-encoded ospC of B. burgdorferi (29), several archival copies of B. hermsii vsp and vlp genes are maintained on linear plasmids and sequentially expressed from a promoter site after gene conversions or DNA rearrangements (21,30,31). The resulting multiphasic antigenic variation of Vsps and Vlps allows the spirochete to repeatedly evade the host's immune response, which leads to recurrent spirochetemia and the characteristic febrile episodes (16).…”

Typing African Relapsing Fever Spirochetes

“…Furthermore, the complexity of coexistent mechanisms for antigenic variation with associated potential for major genomic reorganization presents a problem for many microbiologic typing approaches. Isolates that are homogeneous by gene sequence–based approaches may present vastly different pulsed-field electrophoretic profiles ( 16 , 17 , 26 ), possibly through extensive genetic duplication associated with antigenic variation of major outer membrane proteins, whose genes are carried on large linear plasmids ( 7 , 11 ). …”

“…Many widely used techniques for molecular typing have been applied to Borrelia with variable success ( 5 , 6 ). Of concern when these methods are used for characterizing these spirochetes is the organisms' ability to undergo multiphasic antigenic variation, which can be associated with large inter- or intraplasmidic recombinations/duplications ( 7 , 8 ). Indeed, this ability is a likely cause of the variability in genomic organization of these spirochetes, which possess segmented genomes ( 9 – 11 ) composed of giant linear and circular plasmids.…”

IL-10 Prevents Apoptosis of Brain Endothelium during Bacteremia

“…Isogenic serotypes 1 (B. turicatae serotype 1 [Bt1]) and 2 (B. turicatae serotype 2 [Bt2]) of the RF spirochete B. turicatae have been characterized previously in our laboratory (9,10,15,18,26,27). Spirochetes were cultured in Barbour-Stoenner-Kelly H medium (Sigma-Aldrich) with 12% rabbit serum and grown at 37˚C; all bacterial strains were used at passages ,6 for all experiments.…”