PHYLOGENETIC EVIDENCE FOR HORIZONTAL TRANSFER OF AN INTERVENING SEQUENCE BETWEEN SPECIES IN A SPIROCHETE GENUS

The 23S rRNA genes (rrl genes) of some strains of certain species of the spirochete genus Leptospira carry an intervening sequence (NS) of 485 to 759 bases flanked by terminal inverted repeat and encoding an open reading frame for a putative protein of over 120 amino acids. The structure and the sporadic distribution of the IVS suggest that it might be a mobile element that can be horizontally transferred within or between species. Phylogenetic hypotheses based on the sequences for six IVS open reading frames from various species were compared with hypotheses constructed by using DNA sequences from the 16S rRNA gene (rrs), which is not closely linked to rrl in this genus. The predicted phylogenies for the IVS and rrs differed in a major respect: one strain that claded with L. weillii in the tree based on the rrs data claded with L. noguchi in the tree based on the IVS data. Neither set of data supported a tree in which this strain was constrained to he in the same clade as was supported by the other set of data. This result indicates a probable horizontal transfer of the IVS from a recent ancestor of L. noguchi to a recent ancestor of one of the L. Weillii strains. This observation is the first indication of horizontal transfer of elements encoded on the chromosomes of spirochetes.