Genomic and structural analysis of major antigenic proteins' variability in the human relapsing fever spirochete Borrelia persica.

Antigenic variation enables immune evasion of pathogens, yet the mechanisms for the rapid change in their antigens without loss of structural and functional features are not fully known. Tick-borne relapsing fever (TBRF) spirochetes of the genus Borrelia can be used as a model because their variable major proteins (Vmps) drive antigenic variation followed by recurrent waves of bacteremia. In this study we generated the genome of Borrelia persica LMU-C01 and evaluated Vmp structure across relapsing fever Borrelia species. Borrelia persica was found to have a genome of 1.7Mb with one linear chromosome, 12 linear and one circular plasmids. Telomeric hairpin motifs characteristic of Borrelia linear replicons were detected in the chromosome and in the linear plasmids. Pulsed-field gel electrophoresis indicated that plasmid profiles remained identical after a year of continuous in-vitro cultivation. Genome annotation revealed 48 complete variable large proteins (Vlps) and one variable small protein (Vsp), all encoded in plasmids. Comparative structure modeling of B. persica Vlps together with 420 Vlps from twelve TBRF species revealed a conserved, surface-exposed variable region that occupies a similar external predicted position across structures, even as primary sequences diverge. This indicated that antigenic variation preserves topology while tolerating sequence plasticity, which may be shared with other immune evasive bacteria.