Gel mobility shift assay of the non-covalent complexes between MbtRNASer and various mMbSerRS mutants. MbtRNASer was incubated with different mMbSerRS variants (final concentration of tRNA and enzymes was 1.1 and 0.6 μm, respectively) and subjected to PAGE under native conditions: WT, lane b; R38A, lane c; R76A, lane d; R78A, lane e; K79A, lane f; K87A, lane g; K88A, lane h; Y89A, lane i; K90A, lane j; R94A, lane k; K141A, lane l; N142A, lane m; and R143A, lane n. Non-complexed mMbSerRS (8.3 pmol) and MbtRNASer (14.8 pmol) were loaded on the gel as electrophoretic mobility markers (lanes a and o, respectively). Non-covalent complexes and non-complexed tRNA are marked with black and white arrows, respectively.