Why does cordycepin end transcription?

An adenosine analogue, cordycepin is easily phosphorylated to its mono, di, and triphosphate forms within cells. Triphosphate Cordycepin can be integrated into RNA and, because it lacks a hydroxyl moiety at the 3' position, inhibits RNA synthesis and transcription elongation.

Similar to how adenosine and cordycepin are so similar that some enzymes are unable to distinguish between the two, cordycepin can take part in specific biochemical reactions. For instance, it can be integrated into an RNA molecule, resulting in the premature termination of its synthesis. At high doses, cordycepin reduces protein synthesis to very low levels indirectly by shutting down the mTOR pathway, a signal transduction pathway that regulates protein synthesis.

In vitro, cordycepin has been shown to be cytotoxic to certain leukaemia cell lines. Several biological activities of cordycepin have also been documented, such as the inhibition of platelet aggregation, the induction of apoptosis, the inhibition of cell proliferation, the inhibition of cell migration and invasiveness, and the inhibition of inflammation.

Cordycepin terminates transcription by acting as an analog of adenosine, which is incorporated into the growing RNA chain during transcription. Once cordycepin is incorporated, it lacks the necessary 3'-OH group required for further RNA chain elongation, leading to premature termination of transcription.