A genomic island of the pathogen Leptospira interrogans serovar Lai can excise from its chromosome

An examination of the two Leptospira interrogans genomes sequenced so far reveals few genetic differences, including an extra DNA region, 54 kb in length, in L. interrogans serovar Lai. This locus contains 103 predicted coding sequences that are absent from the genome of L. interrogans serovar Copenhageni, of which only 20% had significant BLASTP hits in GenBank. By analyzing the L. interrogans serovar Lai genome by pulsed-field gel electrophoresis, we also found that this 54-kb DNA fragment exists as a circular plasmid. This was confirmed by amplification of a DNA fragment corresponding to that of the predicted fragment if this region excised from the chromosome and its left and right ends joined together. In addition, cloning of the putative rep gene of this DNA region was responsible for autonomous replication in Leptospira spp., therefore generating a new Escherichia coli-Leptospira sp. shuttle vector. Taken together, our results show that this genomic island can excise from the chromosome and form a replicative plasmid. Analysis of the distribution of this genomic island revealed that highly related sequences exist in other L. interrogans virulent strains. This genomic island, containing a high proportion of novel genes, may have an important role in spreading genes, including virulence factors, among bacterial populations.