Horizontal transfer of drug‐resistant aminoacyl‐transfer‐RNA synthetases of anthrax and Gram‐positive pathogens

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Abstract
The screening of new antibiotics against several bacterial strains often reveals unexpected occurrences of natural drug resistance. Two examples of this involve specific inhibitors of Staphylococcus aureus isoleucyl‐transfer‐RNA synthetase 1 (IleRS1) and, more recently, Streptococcus pneumoniae methionyl‐tRNA synthetase 1 (MetRS1). In both cases, resistance is due to the presence of a second gene that encodes another synthetase (IleRS2 or MetRS2). Here, we show that both S. pneumoniae MetRS2 and S. aureus IleRS2 have closely related homologues in the Gram‐positive bacterium Bacillus anthracis , the causative agent of anthrax. Furthermore, similar to drug‐resistant pathogens, strains of B. anthracis and its closest relative, B. cereus , also have wild‐type ileS1 and metS1 genes. Clostridium perfringens , the causative agent of gangrene, also has two metS genes, whereas Oceanobacillus iheyensis isolated from deep‐sea sediments has a single ileS2 ‐type gene. This study shows the importance of understanding complex evolutionary networks of ancient horizontal gene transfer for the development of novel antibiotics.