Trans-editing of Cys-tRNAPro by Haemophilus influenzae YbaK protein

Prolyl-tRNA synthetases (ProRSs) from all three domains of life have been shown to misactivate cysteine and to mischarge cysteine onto tRNA(Pro). Although most bacterial ProRSs possess an amino acid editing domain that deacylates mischarged Ala-tRNA(Pro), editing of Cys-tRNA(Pro) has not been demonstrated and a double-sieve mechanism of editing does not appear to be sufficient to eliminate all misacylated tRNA(Pro) species from the cell. It was recently shown that a ProRS paralog, the YbaK protein from Haemophilus influenzae, which is homologous to the ProRS editing domain, is capable of weakly deacylating Ala-tRNA(Pro). This function appears to be redundant with that of its corresponding ProRS, which contains a canonical bacterial editing domain. In the present study, we test the specificity of editing by H. influenzae YbaK and show that it efficiently edits Cys-tRNA(Pro) and that a conserved Lys residue is essential for this activity. These findings represent the first example of an editing domain paralog possessing altered specificity and suggest that similar autonomous editing domains could act upon different mischarged tRNAs thus providing cells with enhanced proofreading potential. This work also suggests a novel mechanism of editing wherein a third sieve is used to clear CystRNA(Pro) in at least some organisms.