Off topic:How can MMLV enzyme add cytosines to the 3' of cDNA

Question

[SDin]

Hi there, I am working on the mechanism of SMART , a kind of reverse transcription strategy.

However, I am confused about the terminal transferase activity of MMLV enzyme. It have been reported since 1998, but I cannot find any resource to figure out HOW can it add Cs on the 3' of cDNA. Can anyone provide any message? I will be very appreciated. Cheers.