TRK2 is required for low affinity K⁺ transport in Saccharomyces cerevisiae

TRK1, the gene encoding the high affinity K⁺ transporter in Saccharomyces cerevisiae, is nonessential due to the existence of a functionally independent low affinity transporter. To identify the gene(s) encoding the low affinity K⁺ transporter, we screened trk1 Δ cells for mutants (Kla^-) that require higher concentrations of K⁺ in the medium to support growth. trkI Δ trk2 mutants require up to tenfold higher concentrations of K⁺ to exhibit normal growth compared to trk1Δ TRK2 cells. K⁺ and ⁸⁶Rb⁺ transport assays demonstrate that the mutant phenotype is due to effective K⁺ transport (uptake). Each of 38 independent mutants contains a mutation in the same gene, TRK2. Cells deficient for both high and low affinity K⁺ transport (trk1Δ trk2) exhibit hypersensitivity to low extracellular pH that can be suppressed by high concentrations of K⁺ but not Na⁺. TRK1 completely suppresses both the K⁺ transport defect and low pH hypersensitivity of trk2 cells, suggesting that TRK1 and TRK2 are functionally independent.