Genetic Control of Amino Acid Permeability in Neurospora crassa

Abstract

Neurospora crassa resistant to 4-methyltryptophan (4-MT) were isolated from populations of conidia exposed to ultraviolet light. In genetic crosses, 4-MT resistance behaved as a single-gene difference. Resistance to 4-MT could not be attributed to a relaxation of control of the formation or the activity of the enzymes of tryptophan biosynthesis. Growth studies involving tryptophan auxotrophs carrying the aberrant mt gene and uptake studies with normal and 4-MT-resistant strains showed that 4-MT resistance could be attributed to an inability of 4-MT-resistant strains to take up tryptophan and its methyl analogues. The mt gene is not specific for tryptophan; strains resistant to 4-MT are also resistant to ethionine, and they have a markedly reduced ability to take up serine, leucine, and a-aminoisobutyric acid. No difference was observed between strains carrying either mt allele in their ability to take up glucose; also, the uptake of anthranilic acid or of indole was not sufficiently impaired by the aberrant mt gene to prevent these tryptophan precursors from satisfying the nutritional requirement of certain tryptophan auxotrophs. The role of the mt gene in determining the permeability of N. crassa to amino acids is discussed.

End-Product Regulation of the Tryptophan-Nicotinic Acid Pathway in Neurospora crassa

“…The formamidase activities of these cultures was the same in every case and, thus, it appears that nicotinamide does not influence the formation of formamidase. Suspensions of germinated conidia which were grown on various levels of nicotinamide also showed no significant differences in their utilization of tryptophan; the values obtained are similar to those described elsewhere (18). These results are consistent with those of Table 4 and indicate that nicotinamide does not significantly affect the uptake of tryptophan nor its metabolism to compounds other than kynurenin (22).…”

PcMtr, an aromatic and neutral aliphatic amino acid permease of Penicillium chrysogenum

“…This shows that PcMtr has a low-affinity for citrulline, but is capable of transporting citrulline with high capacity into the cell, which explains why these cells can grow on citrulline as a sole nitrogen source. The substrate specificity of PcMtr corresponds with that reported for Mtr of N. crassa [20,19], with phenylalanine and tryptophan as preferred substrates (Fig. 2).…”

“…Interestingly, the mtr gene was found after selection of mutants resistant to 4-methyltryptophan, a toxic tryptophan analogue. These mutants are defective in the uptake of neutral aliphatic and aromatic amino acids [19][20][21][22][23]. Mtr does not belong to the AAP family nor does it show significant homology with any characterized permease (less than 22% amino acid sequence identity with first hit after Blast search).…”

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

“…The pmn; pmb; pmg strain of N. crassa is defective in constitutive amino acid transport by virtue of three single-gene mutations. The pmn mutation eliminates activity of the N system, the pmb mutation eliminates B system permease activity, and the pmg mutation eliminates the activity of the G system permease (15,28,29). The triple mutant was constructed in this laboratory and deposited with the Fungal Genetics Stock Center (Humboldt State University, Arcata, Calif.) as strain FGSC 2606.…”