Multiple drug resistance in the fission yeast Schizosaccharomyces pombe: Correlation between drug and amino acid uptake and membrane ATPase activities

Thialysine-resistant mutants and uptake of lysine in Schizosaccharomyces pombe

Mutants defective in lysine transport were isolated and characterized. After UV-mutagenesis colonies resistant to thialysine, a toxic analogue of lysine, were isolated and L-lysine uptake into the mutant strains was analyzed. Among the thialysine-resistant strains a group of mutants was found, where the half-saturation constant, KT, of the high-affinity transport system for lysine was higher than in the wild-type, the high-affinity transport system for basic amino acids being specifically affected. This was confirmed by a complementation test in which all the thialysine-resistant strains with a higher KT for lysine uptake belonged to one complementation group. Kinetic and genetic analysis showed that our mutants were identical with can1-1 mutants, showing that a single high-affinity system for the transport of basic amino acids exists in S. pombe.

Transport of L-lysine in the fission yeast Schizosaccharomyces pombe

Systems of L-lysine transport in Schizosaccharomyces pombe are not constitutive, as at no phase of growth in a rich medium is lysine taken up. Transport activity appears only after preincubation of harvested cells with glucose or another suitable source of energy. If cycloheximide is added during this preincubation no transport systems are synthesized. After removal of glucose, the activity of the transport system decays with a half-time of 13 min. The transport of L-lysine into S. pombe cells from the stationary phase of growth preincubated for 60 min with 1% D-glucose is mediated by at least two systems, the high-affinity one with a Kt of 26 mumol/l and Jmax of 4.95 nmol/min per mg dry wt., the low-affinity one with a KT of 1.1 mmol/l and Jmax of 11.8 nmol/min per mg dry wt. The transport of lysine mediated by these two systems proceeds uphill. The high-affinity system has a pH optimum at 4.0-4.2, the accumulation ratio is highest at a cell density 2-5 mg dry wt. per ml and decreases with increasing lysine concentrations. Lysine accumulated by this system does not exit from cells. The only potent competitive inhibitors are L-arginine, L-histidine and D-lysine. The other amino acids tested do not behave as competitive inhibitors. Of the various metabolic inhibitors tested, the most potent were proton conductors and antimycin A.

Some general characteristics of the transport of nutrients and toxic compounds in yeasts and fungi

This essay deals in turn with the six main mechanisms that are believed to be involved in the passage of small molecules across the cell membrane of yeasts and fungi. Such solutes include amino acids, carbohydrates, purines and pyrimidines and other nutrients, among which are the ions K+,,or. The subject has been reviewed extensively (see, for example, Eddy, 1982; Cooper, 1982; Serrano, 1984; Harold, 1986). Transport systems relate to chemotherapy in two ways. First, they may themselves be potential targets for drug action. Second, they may deliver a drug to its intracellular target, the drug being an illicit passenger in a transport system with a different primary role. Most of our knowledge about yeast and fungal transport is currently based on studies with relatively few species selected because they are known to be amenable to genetic and biochemical analysis. The organisms include representative strains ofNeurospora crassaor ofSaccharomyces cerevisiaeand selected species ofRhodotorulaandSchizosaccharomyces. Fungal organisms pathogenic to man, such asCandida albicans, those causing either systemic fungal infections (see Kerridge, 1986; Shepherd, Poulter & Sullivan, 1985), or the numerous fungal diseases of plants have been little used as models of nutrient transport, despite their clinical or economic importance. Whether all these ecologically diverse organisms, which in evolutionary terms are only distantly related, employ only the transport mechanisms so far discovered inSaccharomyces cerevisiaeis an open question.

A new mutation for multiple drug resistance and modified plasma membrane ATPase activity in Schizosaccharomyces pombe

The mutant JV66 was selected from the wild type strain of S. pombe 972h- ade7-413 by its ability to grow on solid rich medium containing 200 micrograms Dio-9/ml. The single nuclear mutation, designated pma1 gives resistance towards diguanidines and several other positively charged compounds. The pma1 mutation also decreases plasma membrane ATPase activity and confers resistance of ATPase to vanadate. The pma1 locus is localized on chromosome I at 5.3 map units from cyh1-C7 and at about 20.7 map units from the centromere. This new mutation is genetically and phenotypically different from the mutation cyh3 and cyh4 previously described (Johnston and Coddington 1983).