Characterization of L-asparagine transport systems in Stemphylium botryosum

Abstract

L-Asparagine uptake by Stemphylium botryosum is mediated by two distinct energy- and temperature-dependent transport systems. One permease is relatively specific for L-asparagine and L-glutamine and is present in nutrient-sufficient mycelium. The specific permease shows an optimum pH at 5.2, saturation kinetics (Km = 4.4 x 10(-4) M, Vmax = 1.1 mumol/g per min), competitive gradient of L-asparagine, and higher affinity towards the L-isomer of asparagine. Amide derivatives of L-asparagine (5-diazo-4-oxo-L-norvaline or L-aspartyl hydroxamate) are the most effective competitors, alpha-amino derivative (N-acetyl asparagine) is a moderate competitor, and alpha-carboxyl derivative (L-asparagine-t-butylester) shows only slight inhibition of the specific permease. Derivatives of L-glutamine are significantly less effective competitors than those of L-asparatine. The level of the specific permease is affected by nitrogen sources and increases approximately threefold upon starvation. The nonspecific permease possesses an optimum pH at 6.8, saturation kinetics (Km = 7 x 10(-5) M, Vmax = 5 mumol/g per min, Kt = 7.4 x 10(-5) M for L-leucine), and high affinity towards various types of amino acids.