The specificity and recognition of tRNA-modifying enzymes may be accounted for in part by nucleotide sequences which are localized next to the modifiable nucleoside. In order to determine the sequence specificity of tRNA-modifying enzymes, we have surveyed 55 published tRNA sequences from Escherichia coli, Salmonella typhimurium and T4 phage. For each modified nucleoside, the nucleotide sequence surrounding the modification site was determined for all tRNAs known to contain the modified nucleoside. Subsequently all tRNAs not containing the modified nucleoside were examined for the absence of the putative recognition site. We present the detailed analysis of 12 modified nucleosides for which we found a strong correlation between the modified nucleoside and the local nucleotide sequence. This suggests that these sequences may be recognition sites for tRNA-modifying enzymes. For each of the 12 modified nucleosides we have identified a recognition sequence present in the tRNA set containing the modification and not in the set without it. All 203 other published tRNA sequences were then examined to see if the sequence specificity rules apply to other organisms, including both prokaryotes and eukaryotes. In several cases a good adherence was found, indicating conservation of the putative recognition sequences.
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