Mechanism of mupirocin transport into sensitive and resistant bacteria

Pseudomonic acid A (mupirocin) blocks protein synthesis in bacteria by inhibition of bacterial isoleucyl-tRNA synthetase. [16, 17-3H]mupirocin, isolated from a methionine auxotroph of Pseudomonas fluorescens, was used to study transport of this antibiotic into sensitive and resistant strains of Bacillus subtilis, Staphylococcus aureus, and Escherichia coli. The transport of mupirocin into sensitive bacteria was energy independent and temperature dependent (decreased uptake at lower temperatures), indicating non-carrier-mediated passive diffusion. Uptake was also saturable with time or increasing antibiotic concentration. The saturable intracellular binding site, most likely the target isoleucyl-tRNA synthetase as determined by the amount of bound mupirocin (2,700 to 3,100 molecules per cell), caused concentration of the antibiotic within the cell. E. coli transformed with a plasmid containing ileS overproduced the target enzyme and demonstrated greater accumulation of mupirocin than a strain containing a control plasmid. The concentrations needed to half saturate (Kd) these binding sites in B. subtilis and S. aureus were 35 and 7 nM, respectively. In gram-positive organisms trained for mupirocin resistance, uptake was not saturable with increasing antibiotic concentration, and intra- and extracellular concentrations of drug equilibrated with time. Kinetic analysis of crude isoleucyl-tRNA synthetase from trained and untrained B. subtilis strains revealed differences in apparent Ki for mupirocin (resistant strain SB23T, Ki = 71.1 nM; sensitive strain SB23, Ki = 33.5 nM), while the Km for isoleucine remained unchanged (2.7 to 2.9 microM). A Km of 0.4 micromolar isoleucine and Ki of 24 nM mupirocin was demonstrated for isoleucyl-tRNA synthetase from sensitive S. aureus 730a, while no isoleucyl-tRNA synthetase activity was detected in extracts of resistance-trained S. aureus 3000 even at 40 micromolar isoleucine, suggesting instability of the enzyme. Free isoleucine pools differed between sensitive (0.26 micromolar) and resistance-trained (1.06 micromolar) S. aureus. Our results demonstrate that (i) mupirocin enters cells by passive diffusion, (ii) mupirocin concentrates in sensitive bacteria due to binding to isoleucyl-tRNA synthetase, and (iii) resistance to mupirocin involves restricted access to the binding site of isoleucyl-tRNA synthetase.