Evolution of antibiotic resistance genes: the DNA sequence of a kanamycin resistance gene from Staphylococcus aureus

The kanamycin resistance gene from Staphylococcus aureus has been sequencedand its structure compared with similar genes isolated from Streptomycesfradiae and from two transposons, Tn5 and Tn903, originally isolated fromKlebsiella pneumoniae and Salmonella typhimurium, respectively. The genesare all homologous but, since their common ancestor, have undergoneextensive divergence, with more than 43% divergence between the closestpair. The phylogeny of the genes cannot be made congruent to the phylogenyof the taxa from which they were isolated without requiring ratherimprobable differences in rates. One is therefore led to conclude thatthere have been multiple occurrences of gene transfer between thesespecies. Thus, although they are homologous, they are neither orthologousnor paralogous. It is suggested that homologous genes of this type becalled xenologous.