LeuRS can leucylate type I and type II tRNA(Leu)s in Streptomyces coelicolor

Transfer RNAs (tRNAs) are divided into two types, type I with a short variable loop and type II with a long variable loop. Aminoacylation of type I or type II tRNA(Leu) is catalyzed by their cognate leucyl-tRNA synthetases (LeuRSs). However, in Streptomyces coelicolor, there are two types of tRNA(Leu) and only one LeuRS (ScoLeuRS). We found that the enzyme could leucylate both types of ScotRNA(Leu), and had a higher catalytic efficiency for type II ScotRNA(Leu)(UAA) than for type I ScotRNA(Leu)(CAA). The results from tRNA and enzyme mutagenesis showed that ScoLeuRS did not interact with the canonical discriminator A73. The number of nucleotides, rather than the type of base of the variable loop in the two types of ScotRNA(Leu)s, was determined as important for aminoacylation. In vitro and in vivo assays showed that the tertiary structure formed by the D-loop and TψC-loop is more important for ScotRNA(Leu)(UAA)(.) We showed that the leucine-specific domain (LSD) of ScoLeuRS could help LeuRS, which originally only leucylates type II tRNA(Leu), to aminoacylate type I ScotRNA(Leu)(CAA) and identified the crucial amino acid residues at the C-terminus of the LSD to recognize type I ScotRNA(Leu)(CAA). Overall, our findings identified a rare recognition mechanism of LeuRS to tRNA(Leu).