Linear chromosomes of Lyme disease agent spirochetes: genetic diversity and conservation of gene order

Abstract

We have constructed physical and genetic maps of the chromosomes of 21 Lyme disease agent spirochetes from geographically diverse locations. All have linear chromosomes whose lengths range from 935 to 955 kbp, and all contain multiple linear plasmids in the 16-to 175-kbp size range. The locations of 11 gene clusters on the chromosomes of these different isolates are indistinguishable at the resolution achieved in this study, indicating that the members of this related group of species have highly conserved chromosomal gene orders. However, chromosomal restriction endonuclease cleavage site maps are unique for nearly all isolates. The 22 chromosomal maps currently available define eight classes of Lyme disease agents. Four of these correspond to the previously proposed species Borrelia burgdorferi, Borrelia garinii, Borrelia afzelii, and Borrelia japonica. In addition, the North American isolates 21038, DN127 cl9-2, 25015, and CA55 typify four additional chromosomal types that are as phylogenetically distinct as the species listed above. These findings support the idea that comparison of restriction maps is currently the most robust and definitive method for determining overall chromosomal relationships among closely related bacteria. In the course of this work, we located on the chromosome the previously unmapped outer surface protein-encoding LA7 gene and genes homologous to the Escherichia coli priA, plsC, parE, and parC genes, and we have substantially refined the locations of the recA, fla, p22A, and flgE genes.

Telomeres of the linear chromosomes of Lyme disease spirochaetes: nucleotide sequence and possible exchange with linear plasmid telomeres

“…Most restriction sites in the region appear to be conserved among the strains analysed. Double digests with NcoI (or SacI) and SgrAI (or BssHII) were able to position the local maps with respect to these highly conserved rightmost SgrAI and BssHII sites (Casjens et al, 1995). Isolate CA-11.2A DNA was found to have restriction sites tested in this region that are indistinguishable from those in B31 DNA, and HB19 DNA was found to be indistinguishable from WI91-23 DNA.…”

“…The genomes of these two extremely closely related species are very similar; they appear to have identical lifestyles and both cause Lyme disease, their chromosomes are extremely similar in size and gene order (Casjens et al, 1995), and they carry related plasmid-encoded surface protein genes on similar plasmids (e.g. Jonsson et al, 1992;Dykhuizen et al, 1993;Theisen et al, 1993;Caporale and Kocher, 1994).…”

“…Ten isolates have a right-end SgrAI fragment 18 kbp long (similar to B. afzelii R-IP3, which corresponds to having only a few hundred bp distal to ORF473; see above), 14 isolates have 24 kbp right-end fragments, one has a 30 kbp rightend fragment, and five have 32 kbp right-end fragments. To begin to analyse the nature of these variable lengths, we characterized the location of the extra sequences in a Length of the rightmost chromosomal SgrAI fragment (see Casjens et al (1995) for complete chromosomal SgrAI maps of several B. burgdorferi isolates). b The sizes of linear plasmids containing chromosomal right-end sequences are given in kbp; a dash (¹) indicates that no linear plasmids hybridized (in Southern analyses) with the probes and conditions used in this study.…”

“…1). Finally, B. burgdorferi isolates B31, Sh-2-82, JD1, CA-11.2A, HB19 and WI91-23 have each been probed with more than 12 non-telomeric probes (Casjens et al, 1995), and none hybridized to linear plasmids in these isolates. These findings are consistent with the historical exchange of genetic information specifically between the right chromosomal telomere and linear plasmids.…”

“…The isolates analysed were as follows: B. afzelii R-IP3, B. garinii 20047 and G2, B. andersonii 19857, B. japonica HO14 and IKA2, as well as isolates DN127, CA55, 25015 (see Casjens et al, 1995 for references for these nine isolates); PotiB1 and VS116 (Postic et al, 1994);and M63 (Nohlmans et al, 1995). These isolates typify the four named and five unnamed non-burgdorferi (sensu stricto) Lyme ᮊ 1997 Blackwell Science Ltd, Molecular Microbiology, 26, 581-596 Fig.…”