Crystallization and preliminary X-ray crystallographic analysis of a bacterial Asn-transamidosome

LC-MS assay targeting the mycobacterial indirect aminoacylation pathway uncovers glutaminase activities of the nondiscriminating aspartyl-synthetase

The synthesis of asparagine (Asn)-tRNAAsn in most prokaryotes uses an indirect aminoacylation pathway involving a nondiscriminating aspartyl synthetase (ND-AspRS) and a glutamine amidotransferase (GatCAB). This was recently implicated as an adaptive mistranslation mechanism for antimicrobial resistance in Mycobacterium tuberculosis, but it remains poorly understood. We report an accessible liquid chromatography-mass spectrometry method with unparalleled chemical specificity, sensitivity, and quantification over the current assays to enable the direct analysis and drug screening campaigns of this pathway. Through this method, we show that the mycobacterial ND-AspRS stimulates the glutaminase activity of GatCAB. We further uncover novel glutaminase activity of the synthetase. These biological insights help better understand the indirect aminoacylation biology and allude to new roles beyond protein translation.

Crystal structures reveal an elusive functional domain of pyrrolysyl-tRNA synthetase

Pyrrolysyl-tRNA synthetase (PylRS) is a major tool in genetic code expansion with non-canonical amino acids, yet understanding of its structure and activity is incomplete. Here we describe the crystal structure of the previously uncharacterized essential N-terminal domain of this unique enzyme in complex with tRNA^Pyl. This structure explains why PylRS remains orthogonal in a broad range of organisms, from bacteria to humans. The structure also illustrates why tRNA^Pyl recognition by PylRS is anticodon-independent; the anticodon does not contact the enzyme. Using standard microbiological culture equipment, we then established a new method for laboratory evolution – a non-continuous counterpart of the previously developed phage-assisted continuous evolution. With this method, we evolved novel PylRS variants with enhanced activity and amino acid specificity. We finally employed an evolved PylRS variant to determine its N-terminal domain structure and show how its mutations improve PylRS activity in the genetic encoding of a non-canonical amino acid.