Specificity of transport processes for sulfur, selenium, and molybdenum anions by filamentous fungi

Abstract

1. 1. Inorganic SO 4 2−, S 2O 3 2−, SeO 4 2− and MoO 4 2− enter mycelia of Penicillium and Aspergillus species by a common energy-, temperature-, pH-, and concentration-dependent permease. Evidence for a single permease is as follows: (a) All four anions exhibit reciprocal competitive inhibition. (b) Mycelia grown on sulfur sources that repress the sulfate permease ( e.g.), l-methionine) show similar low S 2O 3 2−, SeO 4 2−, and MoO 4 2− transport rates. Mycelia grown on sulfur sources that derepress the sulfate permease ( e.g.), l-djenkolic and l-cysteic acids) transport all four anions rapidly. (c) Sulfur starvation results in coincident derepression of SO 4 2−, S 2O 3 2−, SeO 4 2, and MoO 4 2 transport to at least 40 times the base level. Throughout the derepression period the SeO 4 2 /SO 4 2, MoO 4 2− and SeO 4 2−/MoO 4 2 transport rate ratios remain constant. (d) A mutant defective in SO 4 2− transport was equally defective in SeO 4 2 and MoO 4 2− transport. 2. 2. In addition to the sulfate (thiosulfate, selenate, molybdate) permease the mycelia possess distinct permeases for SO 3 2− and S 4O 6 2−. The sulfite and tetrathionate permeases are under metabolic control by some intracellular sulfur-containing metabolite. 3. 3. Inorganic S 2O 3 2− enters the mycelium of a sulfate (thiosulfate) permease-negative mutant at 1–5% of the wild-type rate at low extracellular S 2O 2 2 concentrations. S 2O 3 2− transport by the mutant is not inhibited by SO 4 2−. The v max for S 2O 3 2− transport by the mutant is almost the same as that of the sulfate permease-positive parent (about 2–3 μmoles/g per min). The K m value, however, is about 30-fold higher (2 mM vs. 59 μM). The tetrathionate permease may be responsible for S 2O 3 2-transport in the mutant under standard assay conditions. However, S 2O 3 2 is unstable at the pH values of most fungal cultures. Consequently, most of the sulfur incorporated from S 2O 3--containing media in long-term growth studies is probably in the form of breakdown products (SO 3 2-, SO 4 2-, S 2-). 4. 4. S 2- uptake is 2,4-dinitriphenol- and azide-sensitive, but shows a low Q 10 (1.15 vs. 2.1 for the sulfate permease), is non-saturable, and does not depend on the degree of sulfur sufficiency of the mycelium. 5. 5. No SCN - uptake could be detected.