Translation of genetic information into proteins involves a def-
inite set of nucleic acids and proteins. A cell free translation system
has been reconstituted with 46 tRNA species and 32 protein com-
ponents isolated from
Escherichia coli
(initiation, elongation and re-
lease factors, aminoacyl-tRNA synthetases, ribosome and
formylase)
[1]
. An essentially similar array of molecules is required
for ribosomal protein synthesis in eubacteria, archaea and eukary-
otes. The substrates of the protein-making machine, the ribosome,
are aminoacylated tRNAs that should be delivered in the form of a
ternary complex with elongation factor EF1A and GTP. Accordingly,
the EF1A
GTP
aa-tRNA should be non-limiting and represents the
major form of EF1A and tRNA in growing cells. After transfer of
the amino acid to the growing polypeptide chain on the ribosome,
deacylated tRNA replenishes the pool of EF1A
GTP
aa-tRNA follow-
ing aminoacylation by a family of 20 enzymes, one per amino acid,
the aminoacyl-tRNA synthetases. Several lines of evidence suggest
that during the elongation step of translation in the cytoplasm of
eukaryote cells tRNAs flow in a closed circuit referred to as the
tRNA cycle (
Fig. 1