COMPLETE NUCLEOTIDE-SEQUENCE OF A CIRCULAR PLASMID FROM THE LYME-DISEASE SPIROCHETE, BORRELIA-BURGDORFERI

We have determined the complete nucleotide sequence of a small circula r plasmid from the spirochete Borrelia burgdorferi Ip21, the agent of Lyme disease. The plasmid (cp8.3/Ip21) is 8,303 bp long, has a 76.6% A +T content, and is unstable upon passage of cells in vitro. An analysi s of the sequence revealed the presence of two nearly perfect copies o f a 184-bp inverted repeat sequence separated by 2,675 bp containing t hree closely spaced, but nonoverlapping, open reading frames (ORFs). E ach inverted repeat ends in sequences that may function as signals for the initiation of transcription and translation of flanking plasmid s equences. A unique oligonucleotide probe based on the repeated sequenc e shelved that the DNA between the repeats is present predominantly in a single orientation. Additional copies of the repeat were not detect ed elsewhere in the Ip21 genome. An analysis for potential ORFs indica tes that the plasmid has nine highly probable protein-coding ORFs and one that is less probable; together, they occupy almost 71% of the nuc leotide sequence. Analysis of the deduced amino acid sequences of the ORFs revealed one (ORF-9) with features in common with Borrelia lipopr oteins and another (ORF-2) having limited homology with a replication protein, RepC, from a gram-positive plasmid that replicates by a rolli ng circle (RC) mechanism. Known collectively as RC plasmids, such plas mids require a double-stranded origin at which the Rep protein nicks t he DNA to generate a single-stranded replication intermediate. cp8.3/I p21 has three copies of the heptameric motif characteristically found at a nick site of most RC plasmids. These observations suggest that cp 8.3/Ip21 may replicate by an RC mechanism.