Biosynthesis of tRNA

Nuclear DNA transcribes precursor tRNA through RNA polymerase. In prokaryotes precursor tRNA is a single molecule with a ‘leader’ sequences of nucleotides at the 5’ end and ‘trailor’ sequences at the 3’ end. Multimeric precursor tRNA arises from a cluster of genes. It may contain many copies of a single tRNA species, or single copies of two or more tRNAs. 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:
 
5’ P – leader – (tRNA – spacer) tRNA – trailer – OH3’

Precursor tRNA is processed to form mature tRNA. Alterations during processing are of three types: nucloelytic reactions, nucleoside modifications, terminal additions of nucleotides.

(1) Nucleolytic reactions include cleavage and trimming of precursor tRNA molecules. At least two enzymes are involved. RNase P removes the leader sequences from monomeric tRNA precursors and is apparently also involved in separating tRNA sequences from multimeric precursors.

(2) Nucleoside modifications result in the alteration of some of the usual bases (A, U, G and C) of precursor tRNA to unusual bases. Modification of nucleosides can occur either in precursor tRNA or in the cleaved products.

(3) Terminal addition of nucleotides. All tRNA molecules have a –CCA-OH3’ terminus. IN many tRNA species the –CCA sequence is part of the precursor tRNA. It becomes the terminus after cleavage by RNase Q.
In eukaryotes there do not appear to be any large dimeric and multitermeric 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, consists only of a few nucleotides.