Movement of sodium and cell volume changes in a sodium-rich yeast

DISCRIMINATION BETWEEN ALKALI METAL CATIONS BY YEAST. I. EFFECT OF PH ON UPTAKE

Extracellular pH markedly influences the ability of yeast cells to discriminate between K⁺ and Na⁺, with K⁺ favored to a greater degree at low pH. Studies of the kinetics of uptake of individual alkali metal cations by fermenting yeast indicate three zones relative to pH. Between pH 6 and 8, H⁺ has no effect. Below pH 4, H⁺ competitively inhibits the transport of each cation. Between pH 4 and 6, H⁺ acts kinetically as a predominantly non-competitive inhibitor. Both effects can be reversed by increasing the concentrations of cations. However, the concentrations required to reverse the competitive effect are considerably lower than those required to reverse the apparently non-competitive effect. It is suggested that H⁺ and the alkali metal cations can combine with two sites, a transport or carrier site, and a second, non-transporting site that influences the maximal rate of transport. Because the non-competitive inhibitory effect of H⁺ is considerably greater on the other cations than on K⁺, the discrimination in favor of K⁺ is increased severalfold at low pH, beyond that predicted on the basis of the relative affinities for the transport site.

Regulation of Sugar Transport Systems in Fusarium oxysporum var. lini

Fusarium oxysporum var. lini (ATCC 10960) formed a facilitated diffusion system for glucose ( K s , about 10 mM) when grown under repressed conditions. Under conditions of derepression, the same system was present together with a high-affinity ( K s , about 40 μM) active system. The maximum velocity of the latter was about 5% of that of the facilitated diffusion system. The high-affinity system was under the control of glucose repression and glucose inactivation. When lactose was the only carbon source in the medium, a facilitated diffusion system for lactose was found ( K s , about 30 mM).

Cation exchanges of yeast in the absence of magnesium

Environmental Mg2+ was found to influence the K+/Na+ exchange rate of metabolizing yeast. Addition of EDTA increased the exchange rate and Mg2+ reversed the effect of EDTA. Yeast starved in the absence of Mg2+ exchanged cellular K+ or Na+ for external H+ when maintained at acidic pH. The exchange rate depended on cellular pH and showed the same kinetics for both K+ and Na+. At acidic pH, the presence of external cations neither inhibited H+ absorption nor changed the cation/H+ 1 : 1 stoichiometry. At neutral pH, external cations inhibited H+ influx but did not change the cation efflux. The K+/Na+ exchange is discussed as electrically coupled and the K+/H+ and Na+/H+ exchanges as electroneutral antiports.

Relationship between solute permeability and osmotic remediability in a galactose-negative strain of Saccharomyces cerevisiae

An osmotic remedial allele, gal 7-1, in the galactose pathway of Saccharomyces cerevisiae responds to either penetrating (ethylene glycol and diethylene glycol) or nonpenetrating (KCl, NaCl, and sorbitol) solutes in the growth medium. Extracts from cells grown under restrictive conditions gave no increase in enzyme activity (gal-1-phosphate, uridylyl transferase) when exposed to the penetrating solutes; thus protein synthesis or possibly polymer assembly is proposed as the critical step remedied by the addition of the solutes.