Genome organization of Pseudomonas stutzeri and resulting taxonomic and evolutionary considerations

In order to determine the genome variability within Pseudomonas stutzeri, 20 strains representing the seven described genomovars and strain JM300 were analyzed by using various resolution levels of rare cutting enzymes. XbaI and SpeI 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 P. stutzeri strains. The I-CeuI, PacI, and SwaI low-resolution map of P. stutzeri type strain CCUG 11256 shows the locations of 12 genes, including rrn operons and the origin of replication. I-CeuI digests of the 20 strains studied plus the positions of six genes allowed a comparison of the rm backbone organization within genomovars; the four rrn 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-CeuI cleavage maps of the genomovar reference strains revealed a diverse genome organization in the genomovars relative to rrn operons and gene locations. In most genomovars, rrn 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 P. slutzeri diversification and niche colonization. An ancestral chromosome is suggested, and some plausible pathways for the generation of the various genome structures are proposed.