Index >>Ribose Nucleic Acids >>Biosynthesis of  Tranfer RNA - tRNA

These are separated by spacer segments and have leader and trailer sequences at either end. The general structure of precursor molecules can be represented thus: Precursor tRNA is processed to form mature tRNA. Alterations during processing are of three types: nucleolytic reactions, nucleoside modification, and terminal addition of nucleotides.

(1) Nudeolytic reactions include cleavage and trimming of precursor tRNA molecules. At least two enzymes are involved. RNase P removes the leader sequence from monomeric tRNA precursors and is apparently also involved in separating tRNA sequences from multimeric precursors. In the latter case a second enzyme, RNase P2 (probably the same as RNase 0), is also involved. RNase Q or RNase PIII removes the trailer sequence from the 3' end of precursor tRNA.

In eukaryotes there do riot appear to be any large dimeric and multimeric precursor tRNAs which are found in prokaryotes. This is because spacer segments separating tRNA genes are about 10 times the lengths of the genes, and it would not be economical to transcribe multimeric precursors. In prokaryotes the spacers separating the tRNA genes, consist only of a few nucleotides. In mammals the precursor tRNA are transcribed by polymerase III, and most or all terminate in U-OH.

Precursor tRNAs from many eukaryote cells have been found to be 4.8S and mature tRNAs are 3.8S. The average difference in length between precursor and mature tRNAs is about 20 nucleotides (range, 15-35 nucleotides). Processing includes trimming. Modification reactions, e.g. methylation and conversion of uridine to pseudouridine apparently occur after the pre tRNA is trimmed to the final size.