UCK2-dependent conversion of cytidine to CTP is required for CTP uptake by Chlamydia trachomatis

<i>Chlamydia trachomatis</i> , an obligate intracellular bacterium, develops into a vacuolar compartment called the inclusion. The bacteria import nucleoside triphosphates (NTPs) present in the inclusion lumen. It remains unclear whether nucleosides enter the inclusion lumen in their native form or as phosphorylated nucleotides. Using click chemistry coupled with fluorescence microscopy we provide evidence that cytidine requires phosphorylation by host uridine-cytidine kinase 2 (UCK2) prior to its incorporation into bacterial nucleic acids. These findings support the hypothesis that nucleosides are converted into nucleotides in the host cytoplasm prior to translocation into the inclusion lumen. Future work should therefore focus on the identification of nucleotide transporter(s) at the inclusion membrane.