Analysis of a genomic island housing genes for DNA S‐modification system inStreptomyces lividans66 and its counterparts in other distantly related bacteria

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Summary: The complete sequence (92 770 bp) of a genomic island (GI) named SLG fromStreptomyces lividans66, encoding a novel DNA S‐modification system (dnd), was determined. Its overall G+C content was 67.8%, lower than those of three sequencedStreptomycesgenomes. Among 85 predicted open reading frames (ORFs) in SLG, 22 ORFs showed little homology with previously known proteins. SLG displays a mosaic structure composed of four modules, indicative of multiple recombination events in its formation. Spontaneous excision and circularization of SLG was observed, and the excision rate appeared to be induced at least fivefold by MNNG exposure. Using constructed mini‐islands of SLG, we demonstrated that Slg01, a P4‐like integrase, was sufficient to promote SLG integration, excision and circularization. Eleven counterpartdndclusters, which also mapped to GIs in 10 chromosomes and a plasmid, were found in taxonomically unrelated bacterial species from various geographic niches. Additionally,c.10% of actinomycetes were found to possess adndcluster in a survey involving 74 strains. Comparison ofdndclusters in the 12 bacteria strongly suggests that thesednd‐bearing elements might have evolved from a common ancestor similar to plasmid‐originated chromosome II ofPseudoalteromonas haloplanktisTAC125.

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