POTASSIUM-TRANSPORT BY AMINO-ACID PERMEASES IN SACCHAROMYCES-CEREVISIAE

Deletion of the potassium transporter genes TRK1 and TRK2 impairs pota ssium uptake in Saccharomyces cerevisiae, resulting in a greatly incre ased requirement for the ion and the inability to grow on low pH mediu m. Selection for mutations that restored growth of trk1 Delta trk2 Del ta cells on low pH (3.0) medium led to the isolation of a dominant sup pressor that also partially suppressed the increased K+ requirement of these cells. Molecular analysis revealed the suppressor to be an alle le of BAP2 that encodes a permease for branched chain amino acids. The suppressor mutation (BAP2-1) converts a phenylalanine codon, highly c onserved among the amino acid permease genes, to a serine codon in a r egion predicted to lie within the sixth membrane-spanning domain. Gene ration of the analogous mutation in the histidine permease produced an allele, HIP1-293, that similarly suppressed the low pH sensitivity of trk1 Delta trk2 Delta cells. Suppression of trk1 Delta trk2 Delta phe notypes by BAP2-1 or HIP1-293 was correlated with increased Rb+ uptake . The presence of the substrate amino acids enhanced but was not essen tial for suppression of trk1 Delta trk2 Delta phenotypes and increased Rb+ uptake. The conserved site altered by the suppressor mutations ap pears to be important; his4 HIP1-293 cells show an increased requireme nt for histidine compared with his4 HIP1 cells.