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Regulation of Sulfate Transport in Filamentous Fungi ¹

^a Department of Biochemistry and Biophysics, University of California, Davis, California 95616

Inorganic sulfate enters the mycelia of Aspergillus nidulans, Penicillium chrysogenum, and Penicillium notatum by a temperature-, energy-, pH-, ionic strength-, and concentration-dependent transport system ("permease"). Transport is unidirectional. In the presence of excess external sulfate, ATP sulfurylase-negative mutants will accumulate inorganic sulfate intracellularly to a level of about 0.04 M. The intracellular sulfate can be retained against a concentration gradient. Retention is not energy-dependent, nor is there any exchange between intracellular (accumulated) and extracellular sulfate. The sulfate permease is under metabolic control. Sulfur starvation of high methionine-grown mycelia results in about a 1000-fold increase in the specific sulfate transport activity at low external sulfate concentrations. L-Methionine is a metabolic repressor of the sulfate permease, while intracellular sulfate and possibly L-cysteine (or a derivative of L-cysteine) are feedback inhibitors. Sulfate transport follows hyperbolic saturation kinetics with a Michaelis constant (Km) value of 6 x 10^–5 to 10^–4M and a V_max (for maximally sulfurstarved mycelia) of about 5 micromoles per gram per minute. Refeeding sulfur-starved mycelia with sulfate or cysteine results in about a 10-fold decrease in the V_max value with no marked change in the Km. Azide and dinitrophenol also reduce the V_max.

¹ This research was supported by United States Public Health Service Research Grant GM-12292 and National Science Foundation Research Grants GB-5376 and GB-7736 and was conducted over the past 5 years by undergraduate students (first six authors) as part of their independent study or special summer research programs. The first six authors were undergraduate research students in the Departments of Biochemistry and Biophysics, Bacteriology, Family and Consumer Sciences, Biological Sciences, Biochemistry and Biophysics and Chemistry, respectively.

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