tRNA Processing Enzymes from Escherichia coli

The primary transcript of a tRNA gene undergoes several enzyme catalyzed transformations before it becomes a functioning tRNA molecule. Depending on its chromosomal environment, a tRNA gene is transcribed as either a monomeric or multimeric precursor molecule that is rapidly processed by both endo- and exonucleases. Further processing steps involve enzymatic modification of nucleotides and the addition of terminal CCA residues to processed precursor molecules. This chapter discusses the endo- and exonucleases enzymes that alter the size of tRNA gene transcripts. It describes the enzymes identified in extracts of E. coli; some of these enzymes are very well characterized. RNase P is an endoribonuclease that generates then 5´ termini of all mature tRNAs in E. coli. An essential role for this enzyme in the biosynthesis of tRNA in E. coli has been demonstrated through the use of various mutants that affect the enzymatic activity. Escherichia coli RNase III plays an important role in the processing of rRNA transcripts. It also plays an indirect role in the maturation of tRNAs.