tRNAGln-discrimination in bacterial GluRS
Eco -GluRS efficiently glutamylates tRNAGlu but is strictly discriminating against tRNAGln. The current understanding of the molecular origin behind the observed tRNAGlx-specificity comes from experiments performed on tRNAGln. As shown in in Figure 1A, a feature of tRNA that was shown to play an important role in whether it is discriminated against or recognized by GluRS is whether its D stem was augmented (H-bonded 13:22 nucleotide pair) or non-augmented (non H-bonded 13:22 nucleotide pair). The augmented D stem was also shown to be correlated with the absence of nucleotide 47. For example, an augmented D-helix (and a deletion of nucleotide 47) in the tRNAGlx (tRNAGlu1, tRNAGlu2 and tRNAGln2 isoacceptors) in A. ferrooxidans was shown to be responsible for the efficient glutamylation of all three tRNAs by GluRS1 (11). On the other hand, the presence of a non-augmented D-helix in tRNAGln1 (the34UUG36 isoacceptor) was responsible for the inability of GluRS1 to glutamylate the tRNAGln1 isoacceptor (discrimination). Similarly, when unique identity elements on tRNAGlu (U34, U35, C36, A37, G1•C72, U2•A71, U11•A24, U13•G22••Α46, and Δ47) were transplanted into tRNAGln, the latter could be efficiently glutamylated by GluRS (10,25).
The clear signatures of tRNAGlx-discrimination by GluRS on tRNAGlx beg a larger question – are there also signatures on GluRS, that dictate tRNAGlx-discrimination by GluRS? If present, augmented/non-augmented tRNA discrimination signatures must lie in parts of GluRS that interacts with the tRNAGlx D-helix. An earlier report from our laboratory had shown that a C-terminal truncated version of Eco- GluRS could efficiently discriminate against tRNAGln (15), suggesting the presence of tRNAGln-discriminatory features in the catalytic domain of Eco -GluRS. However, their exact identitity features on the protein have not been explored.