Involved in the biosynthesis of theacrine, a caffeine-like xanthine alkaloid with diverse beneficial biological activities including anti-depressive, sedative, and hypnotic activities, improving learning and memory, increasing exercise activity, and preventing nonalcoholic fatty liver disease (PubMed:32193380). Mediates the conversion of 1,3,7-trimethyluric acid to theacrine (PubMed:32193380). 1,3,7-trimethylurate + S-adenosyl-L-methionine = H(+) + S-adenosyl-L-homocysteine + theacrine Binds 1 Mg(2+) ion per subunit. kcat is 0.01142 sec(-1) with 1,3,7-trimethyluric acid as substrate. Alkaloid biosynthesis. Expressed in leaves of cv. Kucha, but not in cv. Puer. Caffeine is catabolized to produce theacrine in Camellia sinensis var. assamica cv. Kucha, but not in cv. Puer. Belongs to the methyltransferase superfamily. Type-7 methyltransferase family.