Part I. Tyrosinase Induction by Antimetabolites in Neurospora. Part II. Amino Acid Transport in Neurospora

Citation

Pall, Martin Lawrence (1968) Part I. Tyrosinase Induction by Antimetabolites in Neurospora. Part II. Amino Acid Transport in Neurospora. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/3025-N696. https://resolver.caltech.edu/CaltechTHESIS:04202018-102319427

Abstract

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 over periods 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 of cycloheximide, 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 synthesized proteins. 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 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.

Thesis Files