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.