Six of the eight transfer RNAs coded by bacteriophage T4 are synthesized
via three dimeric precursor molecules. The sequences of two of these have been determined. Both of these precursors give rise to equimolar amounts of the cognate tRNA molecules
in vivo. In contrast, even in wild-type infections, tRNA
Ile is present in ≤ 30% the amount of tRNA
Thr, with which it is processed from a common dimeric precursor.We have now determined the sequence of this dimer. In addition to the nucleotides present in tRNA
Thr and tRNA
Ile, it contains nine precursor-specific residues, located at the 5′ and 3′ termini and at the interstitial junction of the two tRNA sequences. While the three dimers share the majority of structural features in common, pre-tRNA
Thr + Ile is the only case in which an encoded tRNA 3′ -C-C-A terminus is present in the interstitial region.The processing of this dimer in various biosynthetic mutants has been analyzed
in vivo and
in vitro and shown to be anomalous in several respects. These results suggest that the apparent underproduction of tRNA
Ile can be explained by a novel processing pathway that generates a metabolically unstable tRNA
Ile product. Data from DNA sequence analysis of the T4 tRNA gene cluster (Fukada & Abelson, 1980) support the conclusion that the asymmetric maturation of this precursor is a consequence of the unique disposition of the -C-C-A sequence. These results argue that gene expression can be modulated at the level of RNA processing. The biological significance of this phenomenon is discussed in relation to evidence that tRNA
Ile has a unique physiological role.
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