Genome structure and phylogeny in the genus Brucella.

PacI and SpeI restriction maps were obtained for the two chromosomes of each of the six species of the genus Brucella: B. melitensis, B. abortus, B. suis, B. canis, B. ovis, and B. neotomae. Three complementary techniques were used: hybridization with the two replicons as probes, cross-hybridization of restriction fragments, and a new mapping method. For each type strain, a unique I-SceI site was introduced in each of the two replicons, and the location of SpeI sites was determined by linearization at the unique site, partial digestion, and end labeling of the fragments. The restriction and genetic maps of the six species were highly conserved. However, numerous small insertions or deletions, ranging from 1 to 34 kb, were observed by comparison with the map of the reference strain of the genus, B. melitensis 16M. A 21-kb Spel fragment specific to B. ovis was found in the small chromosome of this species. A 640-kb inversion was demonstrated in the B. abortus small chromosome. All of these data allowed the construction of a phylogenetic tree, which reflects the traditional phenetic classification of the genus.