PLASMID LOCATION OF BORRELIA PURINE BIOSYNTHESIS GENE HOMOLOGS

The Lyme disease spirochete Borrelia burgdorferi must survive in both its tick vector and its mammalian host to be maintained in nature. We have identified the B. burgdorferi guaA gene encoding GMP synthetase, an enzyme involved in de novo purine biosynthesis that is important fo r the survival of bacteria in mammalian blood. This gene encodes a fun ctional product that will complement an Escherichia coli GMP synthetas e mutant. The gene is located on a 26-kb circular plasmid, adjacent to and divergent from the gene encoding the outer surface protein C (Osp C). The guaB gene homolog encoding IMP dehydrogenase, another enzyme i n the purine biosynthetic pathway, is adjacent to guaA. In Borrelia he rmsii, a tick-borne relapsing fever spirochete, the guaA and guaB gene s are located on a linear plasmid. These are the first genes encoding proteins of known function to be mapped to a borrelial plasmid and the only example of genes encoding enzymes involved in the de novo purine biosynthesis pathway to be mapped to a plasmid in any organism. The u nique plasmid location of these and perhaps other housekeeping genes m ay be a consequence of the segmented genomes in borreliae and reflect the need to adapt to both the arthropod and mammalian environments.