A single point mutation in CTP synthetase of Chlamydia trachomatis confers resistance to cyclopentenyl cytosine.

A Chlamydia trachomatis strain (L2/CPEC) resistant to the cytotoxic effects of cyclopentenyl cytosine (CPEC) was isolated by a stepwise selection procedure. This strain showed an approximate 350-fold increase in resistance to CPEC. Sequencing of the gene encoding CTP synthetase from this resistant strain revealed a single point mutation, resulting in a change of amino acid 149 from Asp to Glu. This appeared to be the only mutation in L2/CPEC, because no changes in CTP transport, CTP synthetase expression, or incorporation of CPEC into DNA or RNA could be detected. The mutation in the chlamydial CTP synthetase resulted in a loss of CTP feedback inhibition. This was demonstrated both in vivo using Escherichia coli cells carrying the cloned gene, and an in vitro assay using partially purified preparations of CTP synthetase. As a result of the loss of feedback inhibition, E. coli cells carrying the CPECR CTP synthetase showed a 22-fold increase in their CTP pools. However, examination of the CTP pools of L2/CPEC revealed no change in CTP levels when compared with wild type C. trachomatis.