CT406 encodes a chlamydial ortholog of NrdR, a repressor of ribonucleotide reductase.

Chlamydia trachomatis is an obligate intracellular bacterium that is dependent on its host cell for nucleotides. Chlamydia imports ribonucleotide triphosphates (NTPs) but not deoxyribonucleotide triphosphates (dNTPs) and instead uses ribonucleotide reductase to convert imported ribonucleotides into deoxyribonucleotides for DNA synthesis. The genes encoding ribonucleotide reductase have been recently shown to be negatively controlled by a conserved regulator called NrdR. In this study, we provide direct evidence that Escherichia coli NrdR is a transcriptional repressor and that C. trachomatis CT406 encodes its chlamydial ortholog. We showed that CT406 binds specifically to two NrdR boxes upstream of the nrdAB operon in C. trachomatis. Using an in vitro transcription assay, we confirmed that these NrdR boxes function as an operator since they were necessary and sufficient for CT406-mediated repression. We validated our in vitro findings with reporter studies in E. coli showing that both E. coli NrdR and CT406 repressed transcription from the E. coli nrdH and C. trachomatis nrdAB promoters in vivo. This in vivo repression was reversed by hydroxyurea treatment. Since hydroxyurea inhibits ribonucleotide reductase and reduces intracellular deoxyribonucleotide levels, these results suggest that NrdR activity is modulated by a deoxyribonucleotide corepressor.