ABSTRACT
The genomes of the classical Brucella species and their biovars have two chromosomes with the exception of B. suis biovar 3 strain 686 which has a single chromosome. The larger chromosome has approximately 2.1 Mbp and has a bacterial origin of replication. The smaller chromosome has approximately 1.2 Mbp and has plasmid replication functions. There is a large inversion within the small chromosome of B. abortus biovars 1, 2, and 4. There is a single large unique genetic island among the genomic sequences of B. melitensis, B. suis, and B. abortus. This island resides in the small genome of B. suis and encodes homologs of transfer functions and phage related genes. Given the high similarity among the genomic sequences of brucellae, differences among them with regards to host preference, virulence and infectious cycle could be due to subtle variations in the conserved DNA and differential expression of conserved genes, rather than due to unique genomic DNA fragments. Detailed comparative sequence analysis identified common and unique regions and diverged regions within conserved genes, and suggests sequence targets to be used in a comparative approach to functional genomics experiments.
