Preventing broken Borrelia telomeres: ResT couples dual hairpin telomere formation with product release

Spirochetes of the genus Borrelia include the tick-transmitted causative agents of Lyme disease and relapsing fever. They possess unusual genomes composed mainly of linear replicons terminated by closed DNA hairpins. Hairpin telomeres are formed from inverted repeat replicated telomere junctions (rTels) by the telomere resolvase ResT. ResT uses a reaction mechanism similar to that of the type IB topoisomerases and tyrosine recombinases. ResT can catalyze three distinct reactions: telomere resolution, telomere fusion, and Holliday junction (HJ) formation. HJ formation is known to occur only in the context of a synapsed pair of rTels. To test whether telomere resolution was synapsis-dependent, we performed experiments with rTel substrates immobilized on streptavidin-coated beads. We report that telomere resolution by ResT is synapsis-independent, indicating that alternative complexes are formed for telomere resolution and HJ formation. We also present evidence that dual hairpin telomere formation precedes product release. This mechanism of telomere resolution prevents the appearance of broken telomeres. We compare and contrast this mechanism with that proposed for TelK, the telomere resolvase of ϕKO2.