Comparison of genetic maps for two Leptospira interrogans serovars provides evidence for two chromosomes and intraspecies heterogeneity

Abstract

Genetic maps were constructed for Leptospira interrogans serovars icterohaemorrhagiae and pomona. Previously we independently constructed physical maps of the genomes for these two serovars. The genomes of both serovars consist of a large replicon (4.4 to 4.6 Mb) and a small replicon (350 kb). Genes were localized on the physical maps by using Southern blot analysis with specific probes. Among the probes used were genes encoding a variety of essential enzymes and genes usually found near bacterial chromosomal replication origins. Most of the essential genes are on the larger replicon of each serovar. However, the smaller replicons of both serovars contain the asd gene. The asd gene encodes aspartate 0-semialdehyde dehydrogenase, an enzyme essential in amino acid and cell wall biosyntheses. The finding that both L. interrogans replicons contain essential genes suggests that both replicons are chromosomes. Comparison of the genetic maps of the larger replicons of the two serovars showed evidence of large rearrangements. These data show that there is considerable intraspecies heterogeneity in L. interrogans.Leptospirosis is an anthropozoonose, a disease common among most animal species, including humans. Leptospirosis is caused by pathogenic members of the genus Leptospira. Until recently, all pathogenic leptospires were classified within the same species, Leptospira interrogans (sensu lato). Recent DNA homology studies have resulted in reclassification of L. interrogans with the formation of seven and L. weilii (34, 44). Because of extensive antigenic heterogeneity, each species is further divided according to antigenic composition into serogroups and serovars. Some serovars occur in more than one of the newly formed species (e.g., serovar hardjo is found in L. interrogans [sensu stricto] and L.

Dynamic bacterial genome organization

“…In R. sphaeroides, two rRNA operons are located on the smaller replicon of 900 kb, while one rRNA operon is present on the 3 Mb replicon; both replicons are considered to be chromosomes (Suwanto and Kaplan, 1989). In B. cereus, the chromosome appears to vary in size between 2.4 and 6.3 Mb Leptospira interrogans is another bacterium in which housekeeping genes are present on two replicons: asd is localized on a 350 kb replicon, rather than on the chromosome of 4.4-4.6 Mb (Zuerner et al, 1993). Clearly, complex genomes in bacteria are more common than hitherto anticipated, and the presence of two or more chromosomes, each with housekeeping genes, may not be so rare in bacteria.…”

Physical map of the Clostridium beijerinckii (formerly Clostridium acetobutylicum) NCIMB 8052 chromosome

“…Some bacteria, such as Borrelia burgdorferi and Rhodococcus fascians, have linear chromosomes (4,20,24), whereas Streptomyces lividans and Streptomyces coelicolor have linear chromosomes that may become circular as a result of deletions spanning both telomeres (34,35). Still others, such as Brucella melitensis and Leptospira interrogans, have segmented genomes (40,63). The segmented genome of Agrobacterium tumefaciens has one circular and one linear component (1).…”

Molecular biology of the Borrelia, bacteria with linear replicons

“…Analysis of different strains of Clostriditrm perfringens belonging to different biotypes showed that the overall organization of the chromosome in that bacterium is wellconserved (Canard e t al., 1992). By contrast, the chromosomes of two strains of L. interrogans exhibit several major D N A rearrangements (Zuerner et al, 1993) …”