Methionine transport by mycelia of Fusarium oxysporum f. sp. lycopersici.

Mycelia of Fusarium oxysporum f. sp. lycopersici accumulated L-methionine by an energy-dependent process, and the energy required for uptake may be derived from either respiration or glycolysis. The pH optimum for transport was 4 and the temperature optimum was 35 degrees C. The apparent Km for methionine uptake was 2.5-3.3 microM and the Vmax 0.24-0.30 mmol . min-1 . mg dry weight-1. S-adenosylhomocysteine (Ado-Hcys) was the major metabolic product of methionine although S-adenosylmethionine (Ado-Met), homocysteine (Hcys), and an unidentified metabolite (compound X) were also detected. The failure to demonstrate efflux of accumulated methionine in the presence of the uncoupler 2,4-dinitrophenol or excess unlabeled methionine was probably due to the fact that methionine was rapidly metabolized within the cells. Acidic and basic amino acids, and those amino acids having less than a four-carbon chain, did not inhibit methionine uptake. The rate of uptake of methionine, which was greatest in log phase mycelia, decreased substantially as the cells entered the stationary phase.