DIFFERENCES IN CHROMOSOME-NUMBER AND GENOME REARRANGEMENTS IN THE GENUS BRUCELLA

We have studied the genomic structure and constructed the Spel, Pad an d I-Ceul restriction maps of the four biovars of the pathogenic bacter ium Brucella suis. B. suis biovar 1 has two chromosomes of 2.1 Mb and 1.15 Mb, similar to those of the other Brucella species: B. melitensis , B. abortus, B. ovis and B. neotomae. Two chromosomes were also obser ved in the genome of B. suis biovars 2 and 4, but with sizes of 1.85 M b and 1.35 Mb, whereas only one chromosome with a size of 3.1 Mb was f ound in B. suis biovar 3. We show that the differences in chromosome s ize and number can be explained by rearrangements at chromosomal regio ns containing the three rrn genes. The location and orientation of the se genes confirmed that these rearrangements are due to homologous rec ombination at the rrn loci. This observation allows us to propose a sc heme for the evolution of the genus Brucella in which the two chromoso me-containing strains can emerge from an hypothetical ancestor with a single chromosome, which is probably similar to that of B. suis biovar 3. As the genus Brucella is certainly monospecific, this is the first time that differences in chromosome number have been observed in stra ins of the same bacterial species.