Evolution of new tRNA-synthetase classes

Four primordial modes of tRNA-synthetase recognition, determined by the (G,C) operational code

In distinction to single-stranded anticodons built of G, C, A, and U bases, their presumable double-stranded precursors at the first three positions of the acceptor stem are composed almost invariably of G-C and C-G base pairs. Thus, the "second" operational RNA code responsible for correct aminoacylation seems to be a (G,C) code preceding the classic genetic code. Although historically rooted, the two codes were destined to diverge quite early. However, closer inspection revealed that two complementary catalytic domains of class I and class II aminoacyl-tRNA synthetases (aaRSs) multiplied by two, also complementary, G2-C71 and C2-G71 targets in tRNA acceptors, yield four (2 x 2) different modes of recognition. It appears therefore that the core four-column organization of the genetic code, associated with the most conservative central base of anticodons and codons, was in essence predetermined by these four recognition modes of the (G,C) operational code. The general conclusion follows that the genetic code per se looks like a "frozen accident" but only beyond the "2 x 2 = 4" scope. The four primordial modes of tRNA-aaRS recognition are amenable to direct experimental verification.

Asparaginyl-tRNA synthetase from Escherichia coli has significant sequence homologies with yeast aspartyl-tRNA synthetase

The Escherichia coli asnS gene codes for asparaginyl-tRNA synthetase (NRSEC). We have sequenced the asnS region, including 382 bp of the 5'-untranslated region, 1398 bp of the coding region and 280 bp of the 3'-untranslated region. The DNA-derived NRSEC amino acid (aa) sequence was confirmed by direct aa sequencing of the N-terminal parts of the native protein and of a 28-kDa internal fragment generated by trypsin digestion. The asnS gene product has been purified to homogeneity using three chromatographic steps. Sequence comparison of the deduced NRSEC sequence with all aminoacyl-tRNA synthetase sequences showed significant homologies with the yeast aspartyl-tRNA synthetase and weaker relationships with other aminoacyl-tRNA synthetases for aa with an XAX codon.