The telomere resolvase of the Lyme disease spirochete, Borrelia burgdorferi, promotes DNA single-strand annealing and strand exchange

Abstract

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 hairpin telomeres. Hairpin telomeres present an uninterrupted DNA chain to the replication machinery overcoming the ‘end-replication problem’ for the linear replicons. Hairpin telomeres are formed from inverted repeat replicated telomere junctions by the telomere resolvase, ResT. ResT uses a reaction mechanism similar to that of the type IB topoisomerases and tyrosine recombinases. We report here that ResT also possesses single-strand annealing activity and a limited ability to promote DNA strand exchange reactions on partial duplex substrates. This combination of activities suggests ResT is a nexus between the seemingly distinct processes of telomere resolution and homologous recombination. Implications for hairpin telomere replication and linear plasmid recombination, including antigenic variation, are discussed.

Hairpin Telomere Resolvases

“…Interestingly, the ResT N-terminal domain, ResT 1-163 was found to promote SSA and strand exchange on its own (23). The properties of these reactions promoted by ResT are consistent with a mode of action similar to single-strand annealing proteins (SSAPs) typified by phage λ Beta of the λ Red recombination system and of Rad52, the eukaryotic SSAP that participates in RecA/Rad51-dependent and -independent homologous recombination reactions (64).…”

“…At this writing telomere resolvases have been purified from three phage and seven bacterial species: E. coli phage N15 (3), Klebsiella oxytoca phage φKO2, Yersinia enterocolitica phage PY54 (5), Agrobacterium tumefaciens (8), the Lyme spirochete Borrelia burgdorferi (6), the relapsing fever borreliae B. hermsii, B. parkeri, B. recurrentis, B. turicatae, and the avian spirochete B. anserina (7). The B. burgdorferi enzyme, ResT (Resolvase of Telomeres) has been the most extensively studied at the biochemical level (6,7,10,11,12,13,14,15,16,17,18,19,20,21,22,23) and is the primary focus of this review, with properties of the other enzymes noted (3,4,5,8,24). Structural studies of the Klebsiella phage φKO2 (25) and the Agrobacterium (26) resolvases have been reported and have shed additional light on reaction mechanisms and on differences between the resolvases from different organisms.…”

“…Recently, an unexpected activity of ResT was uncovered: a vigorous single-strand DNA annealing activity (SSA) and the ability to promote DNA strand exchange between single-stranded donor DNAs and homologous partial duplex target DNAs (23). Interestingly, the ResT N-terminal domain, ResT 1-163 was found to promote SSA and strand exchange on its own (23).…”

DNA Recombination Strategies During Antigenic Variation in the African Trypanosome

“…Gene conversion during antigenic variation that is independent of the HR recombinase is not unique to T. brucei: Borrelia burgdorferi is a tick-borne spirochete where recombination of silent vls gene segments into the single vlsE expression site is unaffected by mutation of RecA (169), though it does require RuvAB, a helicase that processes Holliday junction HR intermediates (170,171). Like T. brucei, how homology-directed strand transfer initiates in B. burgdorferi without RecA/RAD51 is unknown, though the potential involvement of a novel, telomere-directed resolvase would be very interesting (172).…”

The Unusual Linear Plasmid Generating Systems of Prokaryotes

“…It remains to be determined whether the bacterial protelomerases have their origin in phage as both, to some extent, share a common substrate recognition and a DNA cleavage/rejoining mechanism. In addition, some protelomerases have been identified as having roles in vivo unrelated to telomere resolution such as single strand annealing [8] and ATP-dependent helicase activities [9]. Further research into this important class of enzymes should help elucidate the significance of single strand annealing and DNA unwinding activities during closed linear chromosome replication.…”