Mitochondrial biogenesis during fungal spore germination: effects of the antilipogenic antibiotic cerulenin upon Botryodiplodia spores

Cytochrome c oxidase in Neurospora crassa contains myristic acid covalently linked to subunit 1

Radiolabel from [3H]myristic acid was incorporated by Neurospora crassa into the core catalytic subunit 1 of cytochrome c oxidase (EC 1.9.3.1), as indicated by immunoprecipitation. This modification of the subunit, which was specific for myristic acid, represents an uncommon type of myristoylation through an amide linkage at an internal lysine, rather than an N-terminal glycine. The [3H]myristate, which was chemically recovered from the radiolabeled subunit peptide, modified an invariant Lys-324, based upon analyses of proteolysis products. This myristoylated lysine is found within one of the predicted transmembrane helices of subunit 1 and could contribute to the environment of the active site of the enzyme. The myristate was identified by mass spectrometry as a component of mature subunit 1 of a catalytically active, purified enzyme. To our knowledge, fatty acylation of a mitochondrially synthesized inner-membrane protein has not been reported previously.

Effect of antifungal agents on lipid biosynthesis and membrane integrity in Candida albicans

Eight antifungal agents were examined for effects on lipid biosynthesis and membrane integrity in Candida albicans. Lipids were labeled in vivo or in vitro with [14C]acetate and analyzed by thin-layer and gas chromatography. Membrane integrity was measured by a recently developed [14C]aminoisobutyric acid radiolabel release assay. The imidazole antifungal agents miconazole, econazole, clotrimazole, and ketoconazole, at concentrations inhibiting ergosterol biosynthesis (0.1 microM), decreased the ratio of unsaturated to saturated fatty acids in vivo but not in vitro. Similarly, naftifine, tolnaftate, and the azasterol A25822B, at concentrations inhibiting ergosterol biosynthesis (10, 100, and 1 microM, respectively), decreased the ratio of unsaturated to saturated fatty acids in vivo only. This suggests that the effect on fatty acids observed with ergosterol biosynthesis inhibitors may be secondary to the effect on ergosterol. With imidazoles, oleic acid antagonized inhibition of cell growth but not inhibition of ergosterol. This suggests that, with the C-14 demethylase inhibitors, decreased unsaturated fatty acids, rather than decreased ergosterol, are responsible for growth inhibition. Cerulenin, previously reported to be a potent inhibitor of both fatty acid and ergosterol biosynthesis, was found in the present study to inhibit the former (at 5 microM) but not the latter (up to 100 microM). Of the antifungal agents tested, econazole and miconazole (at 100 microM) produced complete release of [14C]aminoisobutyric acid, which is consistent with membrane damage.

Ribosomal RNA contents of maize genotypes with different ribosomal RNA gene numbers

Ribosomal RNA (rRNA) contents were determined in 16 maize genotypes whose individual rRNA gene numbers varied from 5000 to 23,000 per 2C nucleus. Analytical polyacrylamide gel electrophoresis of total RNA showed that no obvious relation existed between rRNA gene number and rRNA content. Only two of nine common inbred lines contained more rRNA than W-23, the inbred with the lowest rRNA gene number. Two of four lines with altered protein content (due to long-term experimental selection) had rRNA contents significantly reduced from those of W-23. A line with an apparent duplication of the nucleolus organizer region of chromosome 6 (called 2-NOR) was expected to possess an elevated quantity of rRNA because it possesses a larger nucleolus; however, we produced a 2-NOR isogenic version and found no difference in rRNA content. The rRNA genes in maize are distributed throughout the NOR-heterochromatin and the NOR-secondary constriction portions of the NOR. The absence of an obvious correlation between rRNA gene number and cellular rRNA content may reflect the presence of a large number of rRNA genes in an inactive state, at least during the stage of growth examined in these experiments.

Inhibitory effect of cerulenin and sodium butyrate on germination of Candida albicans

Candida albicans germination in liquid medium was inhibition by the antilipogenic agent cerulenin and the fatty acid sodium butyrate. Although these inhibitors prevented germ tube emergence at concentrations of 1 microgram/ml and 20 mM, respectively, neither significantly affected cell viability as judged by trypan blue staining or the rate of protein biosynthesis throughout the time course of the experiments. Cerulenin treatment resulted in inhibition of lipid biosynthesis, but lipid biosynthetic capabilities remained unaltered in sodium butyrate-supplemented cultures. Because each inhibitor blocks germination by different mechanisms, their utility in distinguishing events directly correlated to germination was examined. In this context, chitin synthase activity was inhibited by both compounds, confirming the importance of chitin biosynthesis in C. albicans germination.

Lipid synthesis during morphogenesis of Mucor racemosus

Lipid synthesis increases coordinately with protein and RNA synthesis during morphogenesis of Mucor racemosus. The lipid synthesis inhibitor cerulenin can completely block morphogenesis under conditions in which cell growth continues. An increase in phospholipid turnover may be an important correlate to morphogenesis of Mucor spp., especially the turnover of phosphotidyl inositol and phosphatidyl ethanolamine. The increase in ornithine decarboxylase, which occurs during morphogenesis, is inhibited by the addition of cerulenin.

The use of antibiotics for studies of morphogenesis and differentiation in microorganisms

Numerous antibodies with a known mechanism of action are utilized as possible means for studying morphogenesis and differentiation. Inhibitors of biosynthesis of nucleic acids and proteins, compounds intervening with the synthesis and/or function of cell walls and membranes or compounds influencing the energy metabolism are particularly useful. The use of antibiotics for studies of the life cycle of viruses, bacteria, fungi, myxomycetes, protozoa and algae is analyzed in the present communication. Certain aspects of morphogenesis and functions of mitochondria and plastids were clarified with the aid of antibiotics. Relationships between production of antibiotics and differentiation of their producers are discussed in the final part of the paper.

Mitochondrial biogenesis during fungal spore germination. Purification, properties and biosynthesis of cytochrome c oxidase from Botryodiplodia theobromae

1. Cytochrome c oxidase (ferrocytochrome c:oxygen oxidoreductase, EC 1.9.3.1) was purified from the mycelial fungus Botryodiplodia theobromae by sodium cholate and ammonium sulfate solubilization, ammonium sulfate fractionation, and DEAE-cellulose chromatography. The purified enzyme contained 6--7 nmol heme a/mg of protein. The specific activity of the purified enzyme was 2.1--2.3 . 10(3) k (min-1) per mg of protein with 15 mumol ferrocytochrome c and at pH 5.9 and optimal phosphate and Tween 80 concentrations (65 mM and 0.1%, respectively). The Km for ferrocytochrome c was determined to be 1.2--1.3 . 10(-5) M, while at infinite substrate concentration the enzyme catalyzed the oxidation of about 60 mumole of ferrocytochrome c/min per mg of protein. Sedimentation behavior and kinetic evidence suggest that the purified enzyme exists as aggregates of the single molecule. The purified B. theobromae cytochrome c oxidase with its 428 nm Soret absorption maximum may be similar if not identical to oxygenated forms of the enzyme from other fungal species. 2. The purified enzyme was shown by sodium dodecyl sulfate polyacrylamide gel electrophoresis to consist of seven polypeptides with the following respective molecular weights: I, 41 000; II, 28 000; III, 19 000; IV, 14 800; V, 12 800; VI, 11 500; and VII, 9300. Biosynthesis studies showed that the three highest molecular weight polypeptides of the enzyme were synthesized on mitochondrial ribosomes, and the four smaller polypeptides were products of cytoplasmic ribosomes.

Nuclear mutants of Neurospora crassa temperature-sensitive for the synthesis of cytochrome aa3. Mitochondrial protein synthesis and analysis of the polypeptide composition of cytochrome c oxidase

Three previously isolated mutants of Neurospora crassa, temperature-sensitive for the production of cytochrome aa3, have been further analyzed. These mutants have a slightly reduced capacity for mitochondrial protein synthesis when grown at 41 degrees C, although this relative deficiency appeared to be no greater than the deficiency in other cytochrome-aa3-deficient mutants. Thermolability studies revealed that the cytochrome c oxidase purified from each of the mutants grown at 23 degrees C is no more sensitive to heat inactivation than the enzyme isolated from wild-type cells. Sodium dodecylsulfate gel electrophoresis of immunoprecipitates obtained from the mitochondria of each of the mutants grown at 23 degrees C, using antiserum directed against holocytochrome c oxidase, indicated that all the subunits of cytochrome c oxidase were present in relative amounts similar to those found in mitochondria from wild-type cultures. However, when the mitochondria from mutant cultures grown at 41 degrees C were examined in the above fashion, only subunits 5 and 6 of the oxidase were detected. Nonetheless, the mitochondrially synthesized subunit 1, 2 and 3 polypeptides could be immunoprecipitated from mitochondria isolated from mutant cells grown at 41 degrees C and labelled with [3H]leucine in medium containing cycloheximide. Although subunits 4 and 7 could not be detected, because a suitable antibody was not available, the fact that five of the seven subunits were present, but not associated with each other, suggested that the genetic defects in these mutants may affect the process of cytochrome c oxidase assembly.