The enzyme from Saccharomyces cerevisiae is responsible for complete 5-methylcytosine methylations of yeast tRNA. The incidence of modification depends on the cytosine position in tRNA. At positions 34 and 40, 5-methylcytosine is found only in two yeast tRNAs (tRNALeu(CUA) and tRNAPhe(GAA), respectively), whereas most other elongator yeast tRNAs bear either 5-methylcytosine48 or 5-methylcytosine49, but never both in the same tRNA molecule [1]. The formation of 5-methylcytosine34 and 5-methylcytosine40 is a strictly intron-dependent process, whereas the formation of 5-methylcytosine48 and 5-methylcytosine49 is an intron-independent process [2,3].
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(1) S-adenosyl-L-methionine + cytosine(34) in tRNA precursor = S-adenosyl-L-homocysteine + 5-methylcytosine(34) in tRNA precursor. (2) S-adenosyl-L-methionine + cytosine(40) in tRNA precursor = S-adenosyl-L-homocysteine + 5-methylcytosine(40) in tRNA precursor. (3) S-adenosyl-L-methionine + cytosine(48) in tRNA = S-adenosyl-L- homocysteine + 5-methylcytosine(48) in tRNA. (4) S-adenosyl-L-methionine + cytosine(49) in tRNA = S-adenosyl-L- homocysteine + 5-methylcytosine(49) in tRNA.
