5-Ethynyl-2'-deoxycytidine and 5-ethynyl-2'-deoxyuridine are differentially incorporated in cells infected with HSV-1, HCMV, and KSHV viruses.

Nucleoside analogs are a valuable experimental tool. Incorporation of these molecules into newly synthesized DNA (i.e. pulse-labeling) is used to monitor cell proliferation or to isolate nascent DNA. Some of the most common nucleoside analogs used for pulse-labeling of DNA in cells are the deoxypyrimidine analogs 5-ethynyl-2'-deoxyuridine (EdU) and 5-ethynyl-2'-deoxycytidine (EdC). Click chemistry enables conjugation of an azide molecule tagged with a fluorescent dye or biotin to the alkyne of the analog, which can then be used to detect incorporation of EdU and EdC into DNA. The use of EdC is often recommended because of the potential cytotoxicity associated with EdU during longer incubations. Here, by comparing the relative incorporation efficiencies of EdU and EdC during short 30-min pulses, we demonstrate significantly lower incorporation of EdC than of EdU in non-infected human fibroblast cells or in cells infected with either human cytomegalovirus (HCMV) or Kaposi's sarcoma-associated herpesvirus (KSHV). Interestingly, cells infected with herpes simplex virus type-1 (HSV-1) incorporated EdC and EdU at similar levels during short pulses. Of note, exogenous expression of HSV-1 thymidine kinase (TK) increased the incorporation efficiency of EdC. These results highlight the limitations when using substituted pyrimidine analogs in pulse-labeling and suggest that EdU is the preferable nucleoside analog for short pulse-labeling experiments, resulting in increased recovery and sensitivity for downstream applications. This is an important discovery that may help to better characterize the biochemical properties of different nucleoside analogs with a given kinase, ultimately leading to significant differences in labeling efficiency of nascent DNA.