Cysteinyl‐tRNA synthetase is a direct descendant of the first aminoacyl‐tRNA synthetase

Abstract

The gene encoding the cysteinyl‐tRNA synthetase of E. coli was cloned from an E. coli genomic library made in λ2761, a lambda vector which can integrate and which carries a chloramphenicol resistance gene. A thermosensitive cysS mutant of E. coli was lysogenised and chloramphenicol‐resistant colonies able to grow at 42°C were selected to isolate phages containing the wild‐type cysS gene. The sequence of the gene was determined. It codes for a 461 amino‐acid protein and includes the sequences HIGH and KMSK known to be involved in the ATP and TRNA binding respectively of class I synthetases. The cysteinyl enzyme has segments in common with the cytoplasmic leucyl‐tRNA synthetase of Neurospora crassa, the tryptophanyl‐tRNA synthetase of Bacillus stearothermophilus, and the phenylalanyl‐tRNA synthetase of Saccharomyces cerevisiae. Sequence comparisons show that the amino end of the cysteinyl‐tRNA synthetase has similarities with prokaryotic elongation factors Tu; this region is close to the equivalent acceptor binding domain of the glutaminyl‐tRNa synthetase of E. coli. There is a further similarity with the seryl enzyme (a class II enzyme) which has led us to propose that both classes had a common origin and that this was the ancestor of the cysteinyl‐tRNA synthetase.