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Abstract
Only about half of bacterial species use an asparaginyl-tRNA synthetase (AsnRS) to attach Asn to its cognate tRNAAsn. Other bacteria, including the human pathogen Moraxella catarrhalis, a causative agent of otitis media, lack a gene encoding AsnRS, and form Asn-tRNAAsn by an indirect pathway catalysed by two enzymes: first, a non-discriminating aspartyl-tRNA synthetase (ND-AspRS) catalyses the formation of aspartyl-tRNAAsn (Asp-tRNAAsn); then, a tRNA-dependent amidotransferase (GatCAB) transamidates this ‘incorrect’ product into Asn-tRNAAsn. As M. catarrhalis has a Gln-tRNA synthetase, its GatCAB functions as an Asp-tRNAAsn amidotransferase. This pathogen rapidly evolved to about 90 % ampicillin resistance worldwide by insertion of a bro-1 β-lactamase gene within the gatCAB operon. Comparison of the GatCAB subunits from bro-1 β-lactamase-positive and bro-negative strains showed that the laterally transferred bro-1 gene, inserted into the gatCAB operon, affected the C-terminal sequence of GatA. The identity between the C-terminal sequences of GatAwt (residues 479–491) and of GatABRO-1 (residues 479–492) was about 36 %, whereas the rest of the GatA sequence was relatively conserved. The characterization of these two distinct GatCABs as well as the hybrid GatCAB containing GatA(1–478)wt(479–492)BRO-1 and truncated GatCAB enzymes of M. catarrhalis showed that the substitution in GatAwt of residues 479–492 of GatABRO-1 causes increased specificity for glutamine, and decreased specificity for Asp-tRNAAsn in the transamidation reaction. We conclude that the bro gene insertion has altered the kinetic parameters of Asp-tRNAAsn amidotransferase, and we propose a model for gatA evolution after the insertion of bro-1 at the carboxyl end of gatA.
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