Molecular transport. I. In vivo studies of transport mutants of Neurospora crassa with altered amino acid competition patterns

Six new loci controlling resistance top-fluorophenylalanine inAspergillus nidulans

Twelve FPA-resistant mutants were selected on medium containingp-fluorophenylalanine and ethionine. Dominance tests in heterozygous diploids showed that 8 out of 12 are dominant and 4 recessive to their wild-type alleles. One mutant,fpa60, showed a partial requirement for tyrosine and was found to be allelic to anfpaAmutant described previously. A tyrosine non-requirer,fpa65, was also assigned to this locus. The other 10 mutants did not show any growth requirement and were simultaneously resistant to ethionine and 3-amino-L-tyrosine. Of the 8 dominant mutants, 3 were allelic to the permease-mutants at the locusfpaD.Dominant mutants showed higher degrees of resistance than recessive ones. Six new loci, identified after preliminary genetic analysis, were located on 3 linkage groups: 3 on linkage group VI, and one each on linkage groups I, V, and VIII. The recombinantfpaD11;fpaK69 was found to be sensitive to FPA.

Changes in properties of glutamate transport in Trichoderma viride vegetative mycelia upon adaptation to glutamate as carbon source

The U-(14)C-labelled glutamate uptake was measured in both sucrose- and glutamate-grown mycelia of Trichoderma viride. The biomass yield was five-fold lower with glutamate as a sole carbon source. The rate of glutamate transport measured at a glutamate concentration of 1 mM remained unchanged in glutamate-grown mycelia whereas the properties of the glutamate transport were substantially changed compared to sucrose-grown mycelia. The glutamate uptake in both sucrose- and glutamate-grown mycelia was inhibited by an uncoupler (3,3',4',5-tetrachlorosalicylanilide) but the inhibitory efficiency was higher in the latter. The affinity of the permease to glutamate increased approximately five-fold in the glutamate-grown mycelia (about 76 microM compared to about 16 microM). The pH optimum for glutamate uptake was 4 in sucrose-grown mycelia but the glutamate-grown mycelia had two pH optima, one at pH 4 and the second between pH 6 and 7. The inhibition of glutamate uptake by other amino acids yielded different inhibitory patterns in the two mycelia under study. The glutamate uptake in mycelia of different ages also showed differences in both transport rate and temporal pattern. The results show that the growth of mycelia on glutamate led to the appearance of an additional permease with different properties and suggest that only this permease is operating in mycelia grown on glutamate.

A chimeric light-regulated amino acid transport system allows the isolation of blue light regulator (blr) mutants of Neurospora crassa

We have developed a system for the isolation of Neurospora crassa mutants that shows altered responses to blue light. To this end we have used the light-regulated promoter of the albino-3 gene fused to the neutral amino acid permease gene mtr. The product of the mtr gene is required for the uptake of neutral aliphatic and aromatic amino acids, as well as toxic analogs such as p-flurophenylalanine or 4-methyltryptophan. mtr trp-2-carrying cells were transformed with the al-3 promoter-mtr wild-type gene (al-3p-mtr+) to obtain a strain with a light-regulated tryptophan uptake. This strain is sensitive to p-fluorophenylalanine when grown under illumination and resistant when grown in the dark. UV mutagenesis of the al-3p-mtr(+)-carrying strain allowed us to isolate two mutant strains, BLR-1 and BLR-2 (blue light regulator), that are light-resistant to p-fluorophenylalanine and have lost the ability to grow on tryptophan. These two strains have a pale-orange phenotype and show down-regulation of all the photoregulated genes tested (al-3, al-1, con-8, and con-10). Mutations in the BLR strains are not allelic with white collar 1 or white collar 2, regulatory genes that are also involved in the response to blue light.

Kinetic properties, nutrient-dependent regulation and energy coupling of amino-acid transport systems in Penicillium cyclopium

In submerged grown hyphae of Penicillium cyclopium the activities of seven transport systems could be distinguished which share in the uptake of L-arginine, L-glutamic acid, L-phenylalanine and L-leucine. They include the specific systems a (accepting L-arginine and L-lysine), b (L-phenylalanine, L-tyrosine), c (L-glutamic acid) and d (L-leucine), system I (a 'general amino-acid permease') and the low-affinity systems II and III, which accept acidic or basic amino acids, respectively, but also L-phenylalanine. In nutrient-sufficient cells, systems I, II and III remain repressed; uptake is dominated by the specific systems b, c, d and a, the latter reaching its maximum activity. Nitrogen starvation is the most powerful signal for the development of systems I, II and III, whereas, in carbon-starved cells, systems b, c and d reach maximum activities. The development of the general amino-acid permease in nitrogen-starved cells requires both translational and--with a few hours delay--transcriptional events as indicated by the influence of cycloheximide and 5-fluorouracil. The uptake of all amino acids is accompanied by a transient acidification of the cellular interior. Short-time preaccumulation of several anions, such as citrate, alpha-oxo-glutarate, glutamate (but not glutamine), increases the initial rate of amino-acid uptake at a pH above the optimum. Uncouplers inhibit the uptake not only under aerobic but also under anaerobic conditions, where the ATP content is not influenced by these compounds. These findings point to an H+/amino acid symport, which is tightly connected with the recycling of the incoming protons by the plasmalemma H+-ATPase.

Participation of an extracellular deaminase in amino acid utilization by Neurospora crassa

A strain of Neurospora crassa defective in amino acid transport can utilize a variety of amino acids for growth when readily metabolizable nitrogen is limiting. Growth is accompanied by the production of an extracellular deaminase that converts the amino acid to its respective keto acid plus equimolar quantities of utilizable nitrogen in the ammonium ion form. Production of the deaminase is subject to ammonium repression. The relationship between the ability of an amino acid to trigger deaminase production and the presence of particular amino acid permease deficiencies is complex. Four classes of amino acids have been defined with respect to this relationship. The existence of multiple extracellular deaminases is discussed.

Characterization of 2-aminoisobutyric acid transport in Neurospora crassa: a general amino acid permease-specific substrate

We report the characterization of an amino acid 2-aminoisobutyric acid was transported solely by the general amino acid permease and not by the neutral amino acid permease. Furthermore, this substrate was not metabolized after transport. The potential for a system-specific nonmetabolizable substrate as a tool in the analysis of amino acid transport and its regulation is discussed.

Physiological and regulatory properties of the general amino acid transport system of Neurospora crassa

The fundamental properties of the general amino acid transport system of Neurospora crassa were investigated in the conidial stage of the life cycle. The transport activity was found to be under genetic control, and an isogenic set of mutants deficient for the neutral, basic, or general amino acid transport systems and combinations thereof was constructed and used for analyzing the properties specific to the general permease. Amino acid transport by this system was found to be a carrier-mediated active process with broad specificity for the neutral and basic amino acids. Kinetic analysis revealed that a common binding site functioned to transport both neutral and basic amino acids and that the permease had a high affinity for its substrates. The kinetic parameters Km, Vmax, and Ki were defined for several substrates. Two modes of regulation were detected: substrate inhibition and ammonium repression. Activity of the general system was enhanced by the removal of ammonium ions from the incubation medium with a concomitant decline in either neutral or basic permease activity, suggesting that a common component exists between the neutral and the general systems and between the basic and the general systems.

Mutations in Neurospora crassa which affect multiple amino acid transport systems

Characterization of a double mutant, his-6: hgu-4, which is unable to utilize L-histidyl-glycine as a source of histidine has revealed a new locus on linkage group V. The hgu-4 genotype results in a generalized reduced transport activity for amino acids, with a concomitant increased resistance to amino acid analogs. Transport rates and analog resistance for amino acids by this mutant are compared to the previously reported transport deficient mutants fpr-1, nap and un-3. Transport of L-aspartate as a function of temperature is examined in a variety of transport deficient strains in an attempt to explain the mode of action of mutation which pleiotropically affect several genetically and biochemically distinct amino acid transport systems.

Isolation and characterization of a mutant of Neurospora crassa deficient in general amino acid permease activity

A mutant of Neurospora crassa (pm-nbg27) was isolated on the basis of its resistance of p-fluoro-phenylalanine on ammonium-deficient Vogel's medium. This mutant was found to be devoid of both conidial and post-conidial (after 180 min of preincubation) transport activity of all amino acids. Genetic analysis of pm-nbg27 by crossing it to wild-type (74A) resulted in the predicted segregants exhibiting transport characteristics of pm-n, pm-b, pm-g, pm-nb, pm-ng, pm-bg and parental types. The above observations confirm the postulated general amino acid permease system as well as a single genetic locus control of that activity.

Peptide utilization by amino acid auxotrophs of Neurospora crassa

The ability of auxotrophs of Neurospora crassa to grow on certain tripeptides, despite the presence of excess competing amino acids, suggests it has an oligopeptide transport system. In general, dipeptides did not support growth except in those instances where extracellular hydrolysis occurred, or where the dipeptide appeared to be accumulated by an uptake system which is sensitive to inhibition by free amino acids. Considerable intracellular peptidase activity toward a large number of peptides was demonstrated, including a number of peptides which could not be utilized for growth. The intracellular peptidase activity was shown to be selective for amino acid composition and sequence (N-terminal or C-terminal) within the peptide; glycine-containing peptides were particularly poor substrates for peptidase activity. Only a small amount of extracellular peptidase activity could be detected.

Histidine uptake in mutant strains of Neurospora crassa via the general transport system for amino acids

A transport double mutant of Neurospora crassa has been isolated that has only one of the three transport systems capable of l-histidine uptake. The substrate specificity of the remaining transport system, termed the general transport system, has been fully characterized with regard to the contributions to binding of the side chain, the alpha-amino group, and the carboxylate group. The positively charged alpha-amino group is necessary for binding; the negatively charged carboxylate group is of less importance, since its replacement by a neutral carbonyl functional group does not completely abolish binding. The greatest structural latitude for binding was found in the side chain; affinity for alpha-amino acids was uniformly high except for l-aspartic and l-glutamic acids, l-asparagine, and l-proline. Thus, this transport system is "general" with these restrictions.