gyrA: Among the genes in Orientia that may be associated with potential drug resistance is the gyrA gene (coding for the protein DNA gyrase).
Ciprofloxacin, a member of the fluoroquinolones, is known to inhibit bacteria due to interaction with bacterial DNA gyrase. Mutations in gyrA gene in Bartonella was shown to be related to failure of treatment by ciprofloxacin. In Orientia, Tantibhedhyangkul et al. (2010) showed that the gyrA gene in the Kato isolate of O. tsutsugamushi contained an amino acid homolgous with that found in the gyrA gene of Bartonella (a Ser83Leu mutation in the QRDR domain of gyrA). They also showed that this amino acid configuration occurred in isolates from Laos.
gyrA in the genome of Orientia isolates
We have examined the gyrA gene in genome sequences of 38 isolates of O. tsutsugamushi and in O. chuto, as well as a representative sample of genomes from other members of the Rickettsiaceae.
As a standard, the gyrA gene is located at positions 244244 – 246961 within the genome of the Boryong isolate (accession # AM494475). The gyrA locus was found to be present in all isolates of O. tsutsugamushi and in the genome of O. chuto. The gene shows variation in both nucleic acid and amino acid sequences between isolates. However, in the genomes of all isolates of O. tsutsugamushi and of O. chuto, Leu was the amino acid found at position 83 of the QRDR domain of gyrA. As mentioned, Leu at this site has been shown experimentally to confer antibiotic resistance in Bartonella. In contrast, no other member of the Rickettsiaceae was found to possess Leucine at this position. All members of the genus Rickettsia that we examined had Ser at this position. Neorickettsia also had Ser, while Anaplasma isolates had threonine. Ehrlichia isolates had alanine. Wolbachia isolates had methionine. Finally, more distantly related intracellular rickettsiales generally had serine.
The occurrence of this resistance to ciprofloxacin seems to be uniquely a characteristic of Orientia, probably due to a mutational change Leu at poisition 83, and must have occurred very early after the divergence of Orientia from the common ancestor it shared with Rickettsia.