Cloning of two genes encoding potassium transporters in Neurospora crassa and expression of the corresponding cDNAs in Saccharomyces cerevisiae

Two Neurospora crassa genes, trk-1 and hak-1, encode K+ transporters that s how sequence similarities to the TRK transporters described in Saccharomyce s cerevisiae and Schizosaccharomyces pombe, and to the HAK transporters des cribed in Schwanniomyces occidentalis and barley. The N. crassa TRK1 and HA K1 transporters expressed by the corresponding cDNAs in a trk1 Delta trk2 D elta mutant of S. cerevisiae exhibited a high affinity for Rb+ and K+. Nort hern blot analysis and comparison of the kinetic characteristics of the two transporters in the trk1 Delta trk2 Delta mutant with the kinetic characte ristics of Kf uptake in N. crassa cells allowed TRK1 to be identified as th e dominant K+ transporter and HAK1 as a transporter that is only expressed when the cells are K+ starved. The HAK1 transporter showed a high concentra tive capacity and is identified as the K+-H+ symporter described in N. cras sa, whereas TRK1 might be a K+ uniporter. Although the co-existence of K+ t ransporters of the TRK and HAK types in the same species had not been repor ted formerly, we discuss whether this co-existence may be the normal situat ion in soil fungi.