Summary auto-generated
This study describes an unusual illegitimate recombination event in the ospA gene of Borrelia afzelii strain R9, isolated from a patient with chronic Lyme disease meningitis. The mutant strain produced a 28 kDa outer-surface protein A (OspA) instead of the typical 32 kDa variant. DNA sequencing revealed a 96 base-pair deletion in the ospA structural gene. The deletion mechanism involved strand slippage stimulated by an 18-nucleotide palindromic sequence and 5-nucleotide direct repeats flanking the deleted region. The deleted DNA sequence could form a complex hairpin structure that likely caused DNA polymerase pausing during replication. The hairpin formation facilitated slippage of the nascent DNA strand across the replication fork. Notably, strain R9 was isolated exclusively from a human patient with chronic infection and was never found in environmental samples, suggesting the OspA variation may have been selected during human infection as an immune evasion mechanism. The findings demonstrate that Borrelia species can undergo antigenic variation through illegitimate recombination, potentially allowing the pathogen to evade host immune responses and establish persistent infections.
Key findings
- A 96 bp deletion in B. afzelii ospA gene caused OspA protein size reduction from 32 to 28 kDa through strand slippage-mediated illegitimate recombination
- The deletion was facilitated by an 18-nucleotide palindromic sequence and 5-nucleotide direct repeats that promoted DNA polymerase pausing and hairpin formation
- Strain R9 with the ospA deletion was isolated exclusively from a chronic Lyme disease patient, suggesting immune-driven selection during human infection
- The low GC content of Borrelia DNA may facilitate formation of unusual secondary structures promoting recombination events
- OspA variation through illegitimate recombination represents a potential antigenic variation mechanism for immune evasion, though it is not universal among all clinical isolates
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Abstract
In this study, we describe an unusual illegitimate recombination in the linear-plasmid-encoded outer-surface protein A gene of Borrelia afzelii. A 96 bp DNA segment was deleted from the ospA structural gene of B. afzelii strain R9. The nature of the rearrangement suggested that it arose by a strand slippage mechanism, which was stimulated by a 18- mer palindromic sequence and 5-mer short direct repeats at both termini of the deleted DNA. The deleted sequence could form a complex hairpin structure suggesting that it may have played important roles in pausing of replication and slippaging of the nascent strand across the replication fork. In addition, the mutant strain was isolated from a chronic Lyme disease patient, implying that the variation mechanism may have been used by the borrelial strain to avoid host immune elimination.