Nuclear and extranuclear inheritance of oligomycin resistance in Aspergillus nidulans

Summary A number of spontaneously-occurring, stable oligomycin-resistant mutants have been isolated in Aspergillus nidulans. Genetic characterisation showed that while most of the mutants examined were nuclear, one mutant was extranuclear as judged by several criteria. While the nuclear mutants showed no abnormalities on drug-free medium, the extranuclear mutant exhibited impaired growth ability. This character never segregated from the oligomycin-resistance character in any of the genetic experiments carried out, and appeared to be a secondary effect of the same mutation. The extranuclear genetic element coding for the oligomycin-resistance character was unable to co-exist in a stable fashion within the same mycelium as the wild type element, and they tended to segregate into sectors consisting almost wholly of one type or the other. The nuclear mutants showed incomplete dominance in heterozygous diploids, segregating fully resistant homozygous areas. All nuclear mutants mapped on linkage group VII.     

Recombination between the extranuclear genes conferring oligomycin resistance and cold sensitivity in Aspergillus nidulans

Summary Recombination has been demonstrated between the extranuclear loci (oliA1) and (cs67) of Aspergillus nidulans. The stability of the double mutant recombinant and the fact that it formed smaller colonies than either parent at the non-permissive temperature are strong evidence that physical recombination of the extranuclear DNA has occurred rather than simple mixing. A method has been developed for quantifying the extranuclear recombination frequency, thus providing a means of mapping the A. nidulans mitochondrial genome. The data obtained suggests that the two loci are not closely linked.     

Supersuppressors in Aspergillus nidulans.

Summary Simultaneous reversion of mutations in two different Aspergillus nidulans loci adA and metG was found to be due monogenic suppressor mutations. Prelimirary evidence for the existance of supersuppressors in A. nidulans is presented.     

Molybdate metabolism in Aspergillus nidulans. I. Mutations affecting nitrate reductase and-or xanthine dehydrogenase

Summary Further evidence supports the hypothesis that nitrate reductase and xanthine dehydrogenase are molybdo-enzymes inAspergillus nidulans, probably sharing a molybdenum-containing cofactor. This evidence includes (1) five-fold greater toxicity of tungstate on nitrate and hypoxanthine than on other nitrogen sources, (2) locus-specific molybdate reparability of both nitrate reductase and xanthine dehydrogenase at one (cnxE) of five (cnx) loci where mutation can result in pleiotropic loss of both enzyme activities, and (3) an additional class of mutants (molB) which are both molybdate resistant and partially defective in utilization of nitrate and hypoxanthine as nitrogen sources. Moreover, the phenotypes on molybdate-containing media of various mutants altered in the regulation of nitrate reductase synthesis and the ability of nitrate to protect against molybdate toxicity suggest that incorporation of molybdenum into nitrate reductase or into something having the same control properties as nitrate reductase can detoxify molybdate. However, mutations affecting regulation of xanthine dehydrogenase synthesis do not affect growth responses to molybdate. The properties of another class of molybdate resistance mutations (molA) suggest that there is another nitrate-inducible intracellular molybdate detoxification mechanism in addition to the one having identical control properties to nitrate reductase.     

Pleiotropic mutants affecting the control of nitrogen metabolism in Aspergillus nidulans

Summary Mutants of Aspergillus nidulans with lesions in gene amdT are pleiotropically affected in their ability to utilize a wide variety of nitrogen sources in the presence of glucose. Ability to utilize a number of these compounds as sole sources of carbon and nitrogen is not altered. One of these mutants, amdT102, has properties consistent with it being derepressed for glucose repression of the utilization of most (but not all) nitrogen sources. The amdT102 mutant can grow strongly on histidine, lysine and cystine as sole nitrogen sources while the wild type strain grows extremely poorly on these amino acids. Similar but less extreme effects apply to many other nitrogen sources. The amdT19 mutant is unable to utilize most nitrogen sources in the presence of glucose, suggesting that it is subject to greatly increased repression of nitrogen source utilization. The amdT mutants are not affected in their ability to use many compounds as sole carbon sources. Carbon sources other than glucose also affect utilization of nitrogen sources in the amdT mutants.     

Identification of an intramitochondrially synthesized proteolipid associated with the mitochondrial ATPase complex as the product of a mitochondrial gene determining oligomycin resistance in Aspergillus nidulans.

The purification of an RNA-dependent DNA polymerase from the allantoic fluid of uninfected, embryonated chicken eggs is described in detail. Comparison to the polymerase of avian myeloblastosis virus shows that the two enzymes are different with respect to ion concentrations for optimal reaction, response to increasing concentrations of substrate, thermal stability and protection from thermal inactivation by viral RNA. It is concluded that the enzymes compared to each other are different proteins, which must have been coded by different genes. The RNA-dependent DNA polymerase in the allantoic fluid, therefore, does not derive from the partial or complete expression of the endogenous virus genome of the normal chicken cell or from infection by exogenous viruses.     

L-histidine utilization in Aspergillus nidulans.

Histidase activity rather than uptake of L-histidine is the limiting factor for the utilization of histidine as the sole nitrogen source for Aspergillus nidulans. Histidine cannot act as the sole carbon source, and evidence is presented indicating that this is attributable to an inability to convert histidine to L-glutamate in vivo. It has been shown that this fungus lacks an active urocanase enzyme and that histidine is quantitatively converted to urocanate, which accumulates in the extracellular medium. The use of histidine as a nitrogen source is regulated by nitrogen metabolite repression control of histidase synthesis. In addition, evidence for a requirement for a carbon source for histidase synthesis and for a minor form of control by nitrate is presented. The activity of the histidase enzyme is inhibited by micromolar concentrations of the product urocanate and by physiological levels of L-glutamate and L-glutamine.     

Codon usage in Aspergillus nidulans.

Summary Synonymous codon usage in genes from the ascomycete (filamentous) fungus Aspergillus nidulans has been investigated. A total of 45 gene sequences has been analysed. Multivariate statistical analysis has been used to identify a single major trend among genes. At one end of this trend are lowly expressed genes, whereas at the other extreme lie genes known or expected to be highly expressed. The major trend is from nearly random codon usage (in the lowly expressed genes) to codon usage that is highly biased towards a set of 19–20 optimal codons. The G+C content of the A. nidulans genome is close to 50%, indicating little overall mutational bias, and so the codon usage of lowly expressed genes is as expected in the absence of selection pressure at silent sites. Most of the optimal codons are C- or G-ending, making highly expressed genes more G+C-rich at silent sites.     

Protoplasts from Aspergillus nidulans.

A very effective lytic enzyme system for massive micro/macro-scale production of protoplasts from the filamentous fungus Aspergillus nidulans is described. A striking coincidence was observed between maximal lytic activity towards Aspergillus mycelium and the presece of both chitinase and alpha-(1 leads to 3)-glucanase activities. The release of protoplasts was greatly enhanced by preincubating the mycelium with 2-deoxy-D-glucose. Furthermore, protoplast formation was influenced by fungal age, culture conditions, pH of incubation and the osmotic stabilizer used. From 40 mg of fresh mycelium, grown for 14--16 h on 1% glucose in a low phosphate-citrate medium, preincubated with 2-deoxy-D-glucose for 45 min, and then incubated with the lytic enzyme mixture at pH 6.5 in the presence of 0.3--0.4 M (NH4) SO4, 2.5 x 10(8) stable protoplasts were produced within 3 h of incubation at 30 degrees C. Comparable results were obtained with 40--50 g of mycelium. At low osmotic stabilizer concentrations a peculiar type of regeneration was observed in the presence of the lytic enzyme system; within 12 h of incubation aberrant hyphal structure emerged from the large vacuolated protoplasts.     

Identification of genes involved in terbinafine resistance in Aspergillus nidulans

AIMS: To determine the pattern and the genetic basis of resistance to terbinafine, a drug extensively used for the treatment of fungal infections in humans. METHODS AND RESULTS: Four resistant mutants from Aspergillus nidulans isolated after irradiation with ultraviolet light were crossed with the master strain F (MSF). Genetic analysis revealed that a single gene, located on chromosome IV, is responsible for resistance to terbinafine and that the alleles responsible for this resistance in these mutants are of a codominant or dominant nature at high terbinafine concentrations. Furthermore, the interaction of this mutation with another one identified on chromosome II causes the double mutant to be highly resistant. CONCLUSIONS: Periodic surveillance of antimycotic susceptibility would be an important measure in detecting the emergence and spread of resistance. Mutation in a single gene could be responsible for resistance to terbinafine and a genic interaction may be responsible for a higher level of antimycotic resistance. SIGNIFICANCE AND IMPACT OF THE STUDY: The understanding of the mechanisms that lead to changes in the sensitivity of a fungus to a given antifungal agent is important both in order to define strategies for the use of such agent and to guide the development of new antifungal agents.     

Tagging of genes involved in multidrug resistance in Aspergillus nidulans

We have used a plasmid containing the Neurospora crassa pyr4 gene to transform an Aspergillus nidulans pyrG89 mutant strain in the presence of BamHI, and isolated multidrug-sensitive mutants among the transformants. Using this approach, we hoped to identify genes whose products are important for drug resistance by analyzing gene disruptions that alter the drug sensitivity of the cell. About 1300 transformants isolated following transformation were screened for sensitivity to drugs or various stress agents with different and/or the same mechanism of action. Seventy-seven of these transformants showed sensitivity to at least one drug, while fourteen transformants showed a complex phenotype of sensitivity to different drugs. The pyr4 marker was shown to be tightly linked to the mutant phenotype in only 36% of the pleiotropic mutants analyzed in sexual crosses. Genetic crosses between our multidrug-sensitive transformants and cycloheximide-sensitive and imazalil-resistant mutants of A nidulans were performed to determine whether mutations were present at the same loci. We have shown that the gene imaD that confers resistance to imazalil may also be involved in cycloheximide and hygromycin sensitivity, since this mutation is allelic to scyB (mutant scy290). In addition, the cross between the transformant R223 and the imazalil-resistant mutant ima535 showed that both mutations are in the same complementation group, suggesting that the gene imaG could also be involved in cycloheximide and itraconazole sensitivity. Received: 30 August 1999 / Accepted: 22 February 2000