engleski

IleRS2 is responsible for mupirocin resistance of Bacillus megaterium

Isoleucyl-tRNA synthetase (IleRS) contributes to protein biosynthesis by coupling cognate Ile- tRNAIle pair. Based on phylogenetic analysis, bacterial IleRSs can be grouped in two distinct clades: bIleRS1 (bacteria-like) and bIleRS2 (eukaryote-like). Generally, either bIleRS1 or bIleRS2 is present in the cell. However, some bacteria have both IleRSs. These two enzymes, despite catalysing the same reaction, differ substantially in mupirocin susceptibility. Mupirocin is an antibiotic that inhibits formation of Ile-tRNAIle by binding to the enzyme active site. bIleRS2 is generally less susceptible to mupirocin inhibition than bIleRS1. Genome analysis revealed that B. megaterium carries two ileS genes: BmileS1 and BmileS2. B. megaterium shares biological niche with mupirocin-producing Pseudomonas fluorescens, indicating that resistance to mupirocin could be an evolutionary driving force for acquiring bIleRS2. Indeed, B. megaterium showed no growth inhibition even at high micromolar mupirocin concentrations. To test if BmIleRS2 confers resistance to mupirocin, both ileS genes were cloned and overexpressed in Escherichia coli. Kinetic analysis revealed that mupirocin present at nanomolar concentration inhibits BmIleRS1. Opposing that, BmIleRS2 showed significantly higher inhibitory constant, with the Ki value in the micromolar range. Our data indicate that BmIleRS2 provides B. megaterium with a high mupirocin resistance, while still catalysing Ile-tRNAIle synthesis efficiently. Why then BmIleRS2 did not replace BmIleRS1? A trade- off between catalytic activity and antibiotic resistance is not uncommon. Thus, BmIleRS2 might have sacrificed some features of catalytic performance and/or aminoacylation accuracy for higher antibiotic resistance. Further kinetic and in vivo experiments are on-going to answer this question.

isoleucyl-tRNA synthetase, Bacillus megaterium, mupirocin resistance

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