Part I.
A technique for inducing very high levels of tyrosinase activity with various antimetabol ites is described. A modification of this technique was used for studying tyrosinase induction in Neurospora overperiods of a few hours. An amount of the antimetabolite inducer sufficient to induce the enzyme is rapidly taken up into the mycelium. Following uptake, however, there is a lag period of about two hours before tyrosi nase is synthesized. During the lag period, some active, energy requiring process prepares the mycelium for tyrosinase synthesis. Rapid synthesis of tyrosinase then ensues. High concentrations ofcycloheximide, an inhibitor of protein synthesis, inhibit the development of any further enzyme activity when added to inducing cultures, indicating that the synthesis of tyrosinase is de novo.
Low concentrations of cycloheximide which had been previously shown to induce tyrosinase, partially inhibit general protein synthesis.Ethionine and parafluorophenyl alanine appear to induce the enzyme by being incorporated into proteins in place of methionine and phenylalanine, thus lowering the functional activity of newly synthesizedproteins. The partial inhibition of the synthesis of functional proteins, then, is sufficient, in some way, to induce tyrosinase.
Part II.
Kinetic studies have revealed the existence of two transport systems for amino acids in Neurospora. Transport system I corresponds to a system previously studied by Wiley and Matchett (24). Its activity is specifically missing in mtr mutant cultures previously described by Lester (26) and Stadler (25). It is capable of transporting mostneutral 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 sidechains, 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 showsfor many amino acids, suggests that amino acid transport system II serves a scavenger function, removing from the medium traces of exogenous amino acids.
