Nucleotide modification in vitro of the precursor of transfer RNA of Escherichia coli.

Certain nucleotides in precursor RNA of tRNA(Tyr) of Escherichia coli were modified in vitro with a preparation of partially purified E. coli enzyme containing ribothymidine- and pseudouridine-forming activity. The only nucleotides modified in vitro are the same as those found modified in mature tRNA. The best substrate for these modifying enzymes is the RNase P cleavage product of the precursor RNA, which contains the mature tRNA sequence. Of the two pseudouridines found in mature tRNA, one (in the TPsiC sequence) can be formed in intact precursor RNA. The other (in the anticodon stem) can only be formed in the cleaved precursor RNA. The presence of modified nucleotides in the precursor RNA does not enhance its rate of cleavage by RNase P.