In order to determine the genome variability within , 20 strains representing the seven described genomovars and strain JM300 were analyzed by using various resolution levels of rare cutting enzymes. I and I fingerprints revealed a high degree of heterogeneity of restriction patterns that did not correlate with the division into genomovars. However, a fragment pattern comparison led to the establishment of several groups of clonal variants within genomovars. One circular chromosome ranging in size from 3.75 to 4.64 Mb constitutes the genome of strains. The I-I, I, and I low-resolution map of type strain CCUG 11256 shows the locations of 12 genes, including operons and the origin of replication. I-I digests of the 20 strains studied plus the positions of six genes allowed a comparison of the backbone organization within genomovars; the four operons seemed to be at similar locations with respect to the origin of replication, as did the rest of the genes. However, a comparison of I-I cleavage maps of the genomovar reference strains revealed a diverse genome organization in the genomovars relative to operons and gene locations. In most genomovars, operons are not arranged around the origin of replication but are equally distributed on the chromosome. Strain JM300 does not belong to any described genomovar, as determined from the organization of its genome. Large chromosomal rearrangements seem to be responsible for the differences in superordinate genome structure and must have played an important role in diversification and niche colonization. An ancestral chromosome is suggested, and some plausible pathways for the generation of the various genome structures are proposed.


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