Essentiality Assessment of Cysteinyl and Lysyl-tRNA Synthetases of <i>Mycobacterium smegmatis</i>

<div><p>Discovery of mupirocin, an antibiotic that targets isoleucyl-tRNA synthetase, established aminoacyl-tRNA synthetase as an attractive target for the discovery of novel antibacterial agents. Despite a high degree of similarity between the bacterial and human aminoacyl-tRNA synthetases, the selectivity observed with mupirocin triggered the possibility of targeting other aminoacyl-tRNA synthetases as potential drug targets. These enzymes catalyse the condensation of a specific amino acid to its cognate tRNA in an energy-dependent reaction. Therefore, each organism is expected to encode at least twenty aminoacyl-tRNA synthetases, one for each amino acid. However, a bioinformatics search for genes encoding aminoacyl-tRNA synthetases from <i>Mycobacterium smegmatis</i> returned multiple genes for glutamyl (GluRS), cysteinyl (CysRS), prolyl (ProRS) and lysyl (LysRS) tRNA synthetases. The pathogenic mycobacteria, namely, <i>Mycobacterium tuberculosis</i> and <i>Mycobacterium leprae</i>, were also found to possess two genes each for CysRS and LysRS. A similar search indicated the presence of additional genes for LysRS in gram negative bacteria as well. Herein, we describe sequence and structural analysis of the additional aminoacyl-tRNA synthetase genes found in <i>M</i>. <i>smegmatis</i>. Characterization of conditional expression strains of Cysteinyl and Lysyl-tRNA synthetases generated in <i>M</i>. <i>smegmatis</i> revealed that the canonical aminoacyl-tRNA synthetase are essential, while the additional ones are not essential for the growth of <i>M</i>. <i>smegmatis</i>.</p>