Characterization and genomic analysis of the Lyme disease spirochete bacteriophage [PHI]BB-1

Lyme disease is a tick-borne infection caused by the spirochete Borrelia (Borreliella) burgdorferi. Borrelia species have highly fragmented genomes composed of a linear chromosome and a constellation of linear and circular plasmids some of which are required throughout the enzootic cycle. Included in this plasmid repertoire by almost all Lyme disease spirochetes are the 32-kb circular plasmid cp32 prophages that are capable of lytic replication to produce infectious virions called [PHI]BB-1. While the B. burgdorferi genome contains evidence of horizontal transfer, the mechanisms of gene transfer between strains remain unclear. While we know that [PHI]BB-1 transduces cp32 and shuttle vector DNA during in vitro cultivation, the extent of [PHI]BB-1 DNA transfer is not clear. Herein, we use proteomics and long-read sequencing to further characterize [PHI]BB-1 virions. Our studies identified the cp32 pac region and revealed that [PHI]BB-1 packages linear cp32s via a headful mechanism with preferential packaging of plasmids containing the cp32 pac region. Additionally, we find [PHI]BB-1 packages fragments of the linear chromosome and full-length plasmids including lp54, cp26, and others. Furthermore, sequencing of [PHI]BB-1 packaged DNA allowed us to resolve the covalently closed hairpin telomeres for the linear B. burgdorferi chromosome and most linear plasmids in strain CA-11.2A. Collectively, our results shed light on the biology of the ubiquitous [PHI]BB-1 phage and further implicates [PHI]BB-1 in the generalized transduction of diverse genes and the maintenance of genetic diversity in Lyme disease spirochetes.