Effects of modification of 4-thiouridine in E. coli tRNAfMet on methylation reaction with a thermostable Gm-methylase.

Gm-methylase isolated from an extreme thermophile, Thermus thermophilus HB 27 methylates the 2'-OH of the ribose of G18 in the consensus GG sequence in the D loop, by recognizing the D loop-and-stem structure as a minimal substrate. Modification of s4U8 of E. coli tRNAfMet with S-benzylthioisothiourea resulted in a considerable decrease in methylation activity of Gm-methylase. The effect was cancelled by reduction with beta-mercaptoethanol. However, aminoacylation activity and methylation activity of m1A-methylase were scarcely influenced by the modification. These results suggest the involvement of s4U8 residue of tRNA in the recognition of Gm-methylase.