Part I. Tyrosinase Induction by Antimetabolites in Neurospora. Part II. Amino Acid Transport in Neurospora
<p>Part I.</p> <p>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 antimetabo...
| Summary: | <p>Part I.</p>
<p>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.</p>
<p>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.</p>
<p>Part II.</p>
<p>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.</p>
<p>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.</p>
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