Natural insertion of the bro-1 β-lactamase gene into the gatCAB operon affects Moraxella catarrhalis aspartyl-tRNA(Asn) amidotransferase activity.

Abstract

Only about half of bacterial species use an asparaginyl-tRNA synthetase (AsnRS) to attach Asn to its cognate tRNA(Asn). Other bacteria, including the human pathogen Moraxella catarrhalis, a causative agent of otitis media, lack a gene encoding AsnRS, and form Asn-tRNA(Asn) by an indirect pathway catalysed by two enzymes: first, a non-discriminating aspartyl-tRNA synthetase (ND-AspRS) catalyses the formation of aspartyl-tRNA(Asn) (Asp-tRNA(Asn)); then, a tRNA-dependent amidotransferase (GatCAB) transamidates this 'incorrect' product into Asn-tRNA(Asn). As M. catarrhalis has a Gln-tRNA synthetase, its GatCAB functions as an Asp-tRNA(Asn) 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 GatA(wt) (residues 479-491) and of GatA(BRO-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 GatA(wt) of residues 479-492 of GatA(BRO-1) causes increased specificity for glutamine, and decreased specificity for Asp-tRNA(Asn) in the transamidation reaction. We conclude that the bro gene insertion has altered the kinetic parameters of Asp-tRNA(Asn) amidotransferase, and we propose a model for gatA evolution after the insertion of bro-1 at the carboxyl end of gatA.

• Authors: Pierre-Marie Akochy, Jacques Lapointe, Paul H Roy
• Journal: Microbiology (Reading, England) (Microbiology (Reading)) Vol. 158 Issue Pt 9 Pg. 2363-2371 (Sep 2012) ISSN: 1465-2080 [Electronic] England
• PMID: 22745266 (Publication Type: Journal Article, Research Support, Non-U.S. Gov't)

Chemical References

• Asp-tRNA(Asn) amidotransferase
• Nitrogenous Group Transferases
• BRO beta-lactamase
• beta-Lactamases

Topics

• Gene Transfer, Horizontal
• Humans
• Kinetics
• Moraxella catarrhalis (enzymology, genetics)
• Mutagenesis, Insertional
• Nitrogenous Group Transferases (genetics, metabolism)
• Operon
• beta-Lactam Resistance
• beta-Lactamases (genetics)