Acidic and basic amino acid transport systems of Penicillium chrysogenum

Penicillium chrysogenum possesses a relatively specific constitutive transport system for the basic amino acids l -lysine and l -arginine (Km = 6 × 10−6 m , Vmax = 1 μmole/g-min). This system has a low affinity for 2,4-diaminobutryate, l -ornithine, and l -histidine. The basic amino acid transport system can be assayed readily in nutrient-sufficient mycelia where a nonspecific amino acid transport system (Benko, P. V., Wood, T. C., and Segel, I. H., Arch. Biochem. Biophys.129, 498, 1969), which also transports basic amino acids, is extremely low in activity. Nutrient-sufficient mycelia also contain a low-level lysine-insensitive arginine transport system and a low-level arginine-insensitive lysine transport system. The basic amino acid transport system can be assayed in nitrogen-starved and in carbon-starved mycelia if a large excess of, e.g., l -leucine is added to the assay mixture in order to suppress transport of the labeled substrate by the nonspecific amino acid transport system. The acidic amino acids, l -glutamate and l -aspartate, are also substrates of the general amino acid transport system. However, the preferred ionic form is the uncharged species. l -Glutamate and l -aspartate are also transported by a distinct stereospecific acidic amino acid transport system that develops along with the nonspecific system during nitrogen starvation or carbon starvation. The acidic amino acid system can be assayed at pH 6.0 in the presence of a large excess of, e.g., l -leucine. These conditions minimize the transport of glutamate and aspartate by the nonspecific system. A distinct proline transport system has also been detected in nitrogen-starved and in carbon-starved mycelia. No distinct or quantitatively significant specific systems for aromatic amino acids or histidine were detected. The general and acidic amino acid transport systems are subject to feedback inhibition (“transinhibition”) by their intracellular substrates. The transinhibition is most evident under conditions in which the amino acid is transported much faster than it is metabolized. In this respect, l -α-aminoadipate is a better transinhibitor of the acidic system than the preferred substrates, aspartate and glutamate. The feedback inhibitor of the nonspecific system has been shown to be the accumulated substrate rather than NH4+.