AMINO ACID TRANSPORT IN NEUROSPORA CRASSA .I. PROPERTIES OF 2 AMINO ACID TRANSPORT SYSTEMS

Kinetic studies have revealed the existence of two transport systems for amino acids in Neurospora crassa. Transport system I corresponds to a system previously studied by Wiley and Matchett. Its activity is specifically missing in mtr mutant cultures previously described by Lester and Stadler. It is capable of transporting most neutral l-amino acids. Amino acid transport system II has not been described previously. It has an affinity for a wide variety of amino acids. It transports amino acids with hydrophobic and hydrophilic side chains, both basic and neutral amino acids, and d- as well as l-amino acids. Transport system II has an affinity for both β- and α-amino acids. Transport system I has high activity in young, rapidly growing cultures. Transport system II has little or no activity in young cultures. In older, carbon-starved cultures, however, it is more active than transport system I. This, together with the high affinities it shows for many amino acids, suggests that amino acid transport system II serves a scavenger function, removing from the medium traces of exogenous amino acids. © 1969.