Task: Class Definition Understanding (ontology: Sciences/Biology)

Which of the following definitions best describes 'protein-L-histidine N-pros-methyltransferase activity'?

A. Catalysis of the reaction: L-histidyl-[protein] + S-adenosyl-L-methionine = N(pros)-methyl-L-histidyl-[protein] + S-adenosyl-L-homocysteine.
B. Catalysis of the reaction: S-adenosyl-L-methionine + guanosine in tRNA = S-adenosyl-L-homocysteine + tRNA containing methylguanine.
C. Catalysis of the reaction: 3',4',5,7-tetrahydroxy-3-methoxyflavone + S-adenosyl-L-methionine(1+) = 3',4',5-trihydroxy-3,7-dimethoxyflavone + S-adenosyl-L-homocysteine.
D. Catalysis of the reaction: L-histidine + 3 S-adenosyl-L-methionine <=> 3 H(+) + hercynine + 3 S-adenosyl-L-homocysteine. This reaction is the addition of three methyl groups to L-histidine to form N-alpha,N-alpha,N-alpha-trimethyl-L-histidine (also known as hercynine).

Answer: Catalysis of the reaction: L-histidyl-[protein] + S-adenosyl-L-methionine = N(pros)-methyl-L-histidyl-[protein] + S-adenosyl-L-homocysteine.