S-adenosyl-L-methionine-dependent methyltransferase that specifically methylates the C5 position of cytosine 72 in several tRNAs. This modification appears to slightly promote the thermal stability of P.horikoshii tRNAs, but does not affect their amino acid accepting activity. Four elements in the acceptor stems of tRNAs are essential for substrate recognition by this enzyme: the target site C72, the 3'-CCA terminus, U73 or G73, and the second base pair C2:G71. cytidine(72) in tRNA + S-adenosyl-L-methionine = 5-methylcytidine(72) in tRNA + H(+) + S-adenosyl-L-homocysteine cytidine(72) in tRNA(Thr) + S-adenosyl-L-methionine = 5-methylcytidine(72) in tRNA(Thr) + H(+) + S-adenosyl-L-homocysteine cytidine(72) in tRNA(Cys) + S-adenosyl-L-methionine = 5-methylcytidine(72) in tRNA(Cys) + H(+) + S-adenosyl-L-homocysteine kcat is 3.04 min(-1) with tRNA(Cys)(GCA) as substrate. kcat is 4.30 min(-1) with tRNA(Thr)(CGU) as substrate. kcat is 4.34 min(-1) with tRNA(Thr)(GGU) as substrate. kcat is 4.43 min(-1) with tRNA(Thr)(GGU) as substrate. kcat is 2.23 min(-1) with tRNA(Ser)(UGA) as substrate. kcat is 1.50 min(-1) with tRNA(Ser)(CGA) as substrate. kcat is 1.19 min(-1) with tRNA(Ser)(GGA) as substrate. kcat is 0.37 min(-1) with tRNA(Ser)(GCU) as substrate. kcat is 1.76 min(-1) with tRNA(Asn)(GUU) as substrate. kcat is 3.71 min(-1) with tRNA(Asp)(GUC) as substrate. kcat is 0.67 min(-1) with tRNA(Arg)(GCG) as substrate. Methyltransferase activity is much more higher at 65 degrees Celsius than at 55 and 37 degrees Celsius. Belongs to the class I-like SAM-binding methyltransferase superfamily. RsmB/NOP family.