Structure, function, and evolution of linear replicons in Borrelia

Spirochetes of the genus Borrelia include important human pathogens that cause Lyme borreliosis and relapsing fever. The genomes of Borrelia species can be composed of up to 24 DNA molecules, most of which are linear. The plasmid content and linear replicon sequence arrangement vary widely between isolates. The linear replicons are terminated by covalently closed DNA hairpins or hairpin telomeres. Replication of these elements involves a unique reaction, called telomere resolution, to produce hairpin telomeres from replicative intermediates. The telomere resolvase, ResT, is thought to contribute to the genetic flux of the linear molecules by promoting stabilized telomere fusions. Telomere resolvases are related to the tyrosine recombinases and ResT can generate the crucial reaction intermediate of this class of enzyme, the Holliday junction. This observation has led to the proposal that telomere resolvases evolved from tyrosine recombinases inducing DNA linearization in the genomes that acquired them.