Entry of methotrexate () into the folate prototrophic bacterium was poorly inhibited by folate or its natural derivative folinic acid, suggesting that if is transported via a folate transporter, the affinity of that transporter for is higher than for folate. In the range of concentrations tested, uptake was non-concentrative and decreased in -depleted bacteria. When the external concentration of was increased from 1 x 10 M to 1 x 10 M, uptake became saturated and was insensitive to ionophores. However when external concentrations were increased to 1 x 10 M, uptake increased linearly, and was inhibited by the ionophores carbonyl cyanide -chlorophenylhydrazone (CCCP) and valinomycin, suggesting that the process was energized by the protonmotive force (δ) at this concentration. A model for entry in S. is proposed with respect to these results. The high level of resistance to of the nonsense mutant cannot be entirely explained by a decrease in uptake.


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