Haem ligands of the ferricytochrome c of Ustilago sphaerogena

The mammalian-type cytochrome c of the basidiomycete Ustilago sphaerogena contains in a single polypeptide chain of 107 residues, two histidine residues located at positions 18 and 33, and one methionine residue situated at position 80 (Bitar et al., 1972). The reaction of Ustilago ferricytochrome c with bromoacetate at neutral pH resulted in the modification of histidine-33, but not of histidine-18 or of the invariant methionine residue. The activities of Ustilago cytochrome c with mitochondrial cytochrome c oxidase and with NADH-cytochrome c reductase were unaltered by the modification. The equilibrium constants for the formation of low-spin complexes of the ferrihaem octapeptide of horse cytochrome c (residues 14-21, including the haem bound covalently to cysteines 14 and 17) with imidazole, N(2)-acetylhistidine and monocarboxymethyl derivatives of N(2)-acetylhistidine were determined spectrophotometrically. Alkylation of the imidazole side-chain group of N(2)-acetylhistidine resulted in a marked decrease in its ability to form low-spin ferrihaem complexes. These results indicate that in Ustilago ferricytochrome c in solution histidine-33 is not involved in the central co-ordination complex. Since side-chain groups of residues other than histidine and methionine do not appear to be involved in the central complexes of other mammalian-type cytochromes c (Hettinger & Harbury, 1964, 1965; Myer & Harbury, 1965) it is likely that in Ustilago ferricytochrome c in solution at neutral pH, the side-chain groups of histidine-18 and methionine-80 are involved in the central co-ordination complex. The latter is stable over the pH range 2.6-8.4.     

Ethoxyformylation of ribonuclease U2 form Ustilago sphaerogena.

RNase U2 was inactivated by incubation with ethoxyformic anhydride at pH 6.0 and pH 4.5. The absorbance of the RNase U2 increased at around 250 nm and decreased at around 280 nm. The inactivation occurred in parallel with the amount of modified histidine and plots of the relationship between the remaining activity and the modified histidine suggested that the modification of one of the two histidine residues totally inactivated the enzyme. The inactivated enzyme RNase U2 was reactivated by a low concentration of hydroxyamine, with removal of the ethoxyformyl group from the modified histidine residue. At pH 4.5, 2'-adenylate and 2'-guanylate protected RNase U2 from inactivation by ethoxyformic anhydride. The difference CD spectra showed that the ability of RNase U2 to form a complex with 2'-adenylate was lost on ethoxyformylation.     

The role of ferrichrome reductase in iron metabolism of Ustilago sphaerogena.

Ferrichrome, the ferric ionophore for Ustilago sphaerogena, can serve as a source of iron for the enzyme ferrochelatase (protoheme ferrolyase, EC 4.99.1.1) in this organism, but only after enzymatic removal of the iron from its carrier. U. sphaerogena contains a specific ferrichrome reductase (NADH:ferrichrome oxidoreductase) which catalyzes cellular dissociation of the complex by reduction of the metal to the ferrous state. A spectrophotometric assay was developed based on trapping of the ferrous ion produced by ferrozine. There is an apparent inhibition by oxygen which is thought to be due to re-oxidation of the metal under the assay conditions. The close structural analogue, ferrichrome A, is not a substrate, nor is the ester type siderochrome ferric hexahydro-N,N',N"-triacetylfusarinine C. Aluminum desferriferrichrome is inhibitory. The importance of this enzyme for the metabolism of iron in this organism is discussed.     

The primary structure of cytochrome c from the rust fungus Ustilago sphaerogena

1. The complete amino acid sequence of cytochrome c from the basidiomycete Ustilago sphaerogena was determined from the amino acid compositions and sequences of either tryptic or chymotryptic peptides, and in homology with at least thirty other established sequences of cytochrome c. 2. The primary structure of the molecule bears all of the characteristics of a mammalian-type cytochrome c, showing the typical clustered distribution of hydrophobic and basic residues with a single polypeptide chain of 107 residues. 3. Like all other fungal cytochromes c, it possesses a free N-terminus, and one less residue at the C-terminus than vertebrate cytochromes c. The region of residues 70-80 is strictly conserved, as is histidine at position 18. Position 26 is occupied by an asparagine residue, in contrast to histidine which occurs at this location in most of the known sequences of mammalian-type cytochromes c. 4. In contrast to some other fungal and plant cytochromes c of known primary structures, the Ustilago cytochrome c molecule does not contain trimethyl-lysine. 5. The sequence of Ustilago cytochrome c differs from the sequences of human, horse, chicken, tuna, wheat, and baker's yeast proteins at loci 47, 43, 44, 44 and 38 respectively.     

The disulfide bridges of ribonuclease U2 from Ustilago sphaerogena.

RNase U2 was partially hydrolyzed with chymotrypsin [EC 3.4.21.1] and sulfuric acid, and in each case the resulting peptides were separated by gel filtration, ion exchange column chromatography and paper electrophoresis. From the results of amino acid analysis of cystine-containing peptides and their oxidized components, the three disulfide bridges were located between the cystine residues at positions 1 and 53, 9 and 112, and 54 and 95.     

Iron uptake from ferrichrome A and iron citrate in Ustilago sphaerogena.

Double radioactive label transport assays with iron, chromium, and gallium chelates were used to investigate the mechanism of iron uptake by Ustilago sphaerogena. In iron-deficient cells, ferrichrome A iron was taken up without appreciable uptake of the ligand. Iron-sufficient cells partially accumulated the ligand with the metal. The chromium- and gallium-containing analogs of ferrichrome A were transported as intact chelates. Ferrichrome A iron uptake was inhibited by dipyridyl. The data suggest that the intact ferrichrome A chelate binds to a specific receptor, the iron is then separated from the ligand at the membrane by reduction, and the metal is released to the inside of the cell while the ligand is released to the exterior. The reduction step is not transport rate limiting. Iron chelated to citrate was taken up by an energy-dependent process. The citrate ligand was not taken up with the metal. Uptake was sensitive to dipyridyl and ferrozine. Chromic ion chelated to citrate was not transported, suggesting that the iron, rather than the chelate, is recognized by the receptor or that reduction of the metal is required for transport.     

Studies on extracellular ribonucleases of Ustilago sphaerogena. Purification and properties

1. Four ribonucleases were isolated from culture media of Ustilago sphaerogena. They were designated ribonucleases U(1), U(2), U(3) and U(4). 2. They were purified about 1600-, 3700-, 1100- and 16-fold respectively. 3. It was shown by gel filtration that ribonucleases U(1), U(2) and U(3) have molecular weights about 10000 like ribonuclease T(1), and that ribonuclease U(4) is much larger. 4. Ribonucleases U(1), U(2) and U(3) are thermostable, but ribonuclease U(4) is not. 5. The pH optimum of ribonucleases U(1) and U(4) is pH8.0-8.5, and that of ribonucleases U(2) and U(3) is pH4.5.     

The amino acid sequence of ribonuclease U2 from Ustilago sphaerogena.

1. RNAase (ribonuclease) U2, a purine-specific RNAase, was reduced, aminoethylated and hydrolysed with trypsin, chymotrypsin and thermolysin. On the basis of the analyses of the resulting peptides, the complete amino acid sequence of RNAase U2 was determined, 2. When the sequence was compared with the amino acid sequence of RNAase T1 (EC 3.1.4.8), the following regions were found to be similar in the two enzymes; Tyr-Pro-His-Gln-Tyr (38-42) in RNAase U2 and Tyr-Pro-His-Lys-Tyr (38-42) in RNAase T1, Glu-Phe-Pro-Leu-Val (61-65) in RNAase U2 and Glu-Trp-Pro-Ile-Leu (58-62) in RNAase T1, Asp-Arg-Val-Ile-Tyr-Gln (83-88) in RNAase U2 and Asp-Arg-Val-Phe-Asn (76-81) in RNAase T1 and Val-Thr-His-Thr-Gly-Ala (98-103) in RNAase U2 and Ile-Thr-His-Thr-Gly-Ala (90-95) in RNAase T1. All of the amino acid residues, histidine-40, glutamate-58, arginine-77 and histidine-92, which were found to play a crucial role in the biological activity of RNAase T1, were included in the regions cited here. 3. Detailed evidence for the amino acid sequence of the sequence of the proteins has been deposited as Supplementary Publication SUP 50041 (33 PAGES) AT THE British Library (Lending Division)(formerly the National Lending Library for Science and Technology), Boston Spa, Yorks. LS23 7BQ, U.K., from whom copies can be obtained on the terms indicated in Biochem. J. (1975), 145, 5.     

Role of two siderophores in Ustilago sphaerogena: Regulation of biosynthesis and uptake mechanisms

Under iron-deficient conditions the smut fungus Ustilago sphaerogena produces two kinds of siderophores, ferrichrome and ferrichrome A. Regulation of ligand biosyntheses and uptake mechanisms of the iron chelates were studied to determine the role of each chelate in U. sphaerogena. The biosynthesis of each ligand was differentially regulated. Ferrichrome A, the more effective chelate, was preferentially synthesized under more extreme conditions of iron stress, but completely repressed when the cell was supplied with sufficient iron. In contrast, biosynthesis of ferrichrome was strongly but not completely repressed by iron. The mechanism of repression was examined using a newly developed in vivo synthesis assay. Chromium and gallium-containing siderophore analogs had no effect on siderophore ligand biosynthesis. Iron, added as siderophores, resulted in increased oxygen uptake and amino acid transport, which was soon followed by decreased ligand biosynthesis, suggesting that regulation may be indirect and related to oxidative metabolism. Uptake experiments were used to rule out a ligand-exchange mechanism for ferrichrome A-iron transport. The data suggest that ferrichrome A-iron is taken up at a specific site that results in a rapid distribution of iron inside the cell.     

Role of two siderophores in Ustilago sphaerogena. Regulation of biosynthesis and uptake mechanisms

Under iron-deficient conditions the smut fungus Ustilago sphaerogena produces two kinds of siderophores, ferrichrome and ferrichrome A. Regulation of ligand biosyntheses and uptake mechanisms of the iron chelates were studied to determine the role of each chelate in U. sphaerogena. The biosynthesis of each ligand was differentially regulated. Ferrichrome A, the more effective chelate, was preferentially synthesized under more extreme conditions of iron stress, but completely repressed when the cell was supplied with sufficient iron. In contrast, biosynthesis of ferrichrome was strongly but not completely repressed by iron. The mechanism of repression was examined using a newly developed in vivo synthesis assay. Chromium and gallium-containing siderophore analogs had no effect on siderophore ligand biosynthesis. Iron, added as siderophores, resulted in increased oxygen uptake and amino acid transport, which was soon followed by decreased ligand biosynthesis, suggesting that regulation may be indirect and related to oxidative metabolism. Uptake experiments were used to rule out a ligand-exchange mechanism for ferrichrome A-iron transport. The data suggest that ferrichrome A-iron is taken up at a specific site that results in a rapid distribution of iron inside the cell.     

Structural changes induced by the deamidation and isomerization of asparagine revealed by the crystal structure of Ustilago sphaerogena ribonuclease U2B

Under physiological conditions, the deamidation and isomerization of asparagine to isoaspartate (isoAsp) proceeds nonenzymatically via succinimide. Although a large number of proteins have been reported to contain isoAsp, information concerning the three‐dimensional structure of proteins containing isoaspartate is still limited. We have crystallized isoAsp containing Ustilago sphaerogena ribonuclease U2B, and determined the crystal structure at 1.32 Å resolution. The structure revealed that the formation of isoAsp32 induces a single turn unfolding of the α‐helix from Asp29 to Asp34, and the region from Asp29 to Arg35 forms a U‐shaped loop structure. The electron density map shows that isoAsp32 retained the L‐configuration at the C_α atom. IsoAsp32 is in gauche conformation about a C_α? C_β bond, and the polypeptide chain bends by ～90° at isoAsp32. IsoAsp32 protrudes from the surface of the protein, and the abnormal β‐peptide bond in the main‐chain and α‐carboxylate in the side‐chain is fully exposed. The structure suggests that the deamidation of the Asn and the isoAsp formation in proteins could confer immunogenicity. © 2010 Wiley Periodicals, Inc. Biopolymers 93: 1003–1010, 2010.     

The amino acid sequence of ribonuclease U1, a guanine-specific ribonuclease from the fungus Ustilago sphaerogena

The complete amino acid sequence of ribonuclease U1 (RNase U1), a guanine-specific ribonuclease from a fungus, Ustilago sphaerogena, was determined by conventional protein sequencing, using peptide fragments obtained by several enzymatic cleavages of the performic acid-oxidized protein. The oxidized protein was first cleaved by trypsin and the resulting peptides were purified and their amino acid sequences were determined. These tryptic peptides were aligned with the aid of overlapping peptides isolated from a chymotryptic digest of the oxidized protein. The amino acid sequence thus deduced was further confirmed by isolation and analysis of peptides obtained by digestion of the oxidized protein with lysyl endopeptidase. The location of the disulfide bonds was deduced by isolation and analysis of cystine-containing peptides from a chymotryptic digest of heat-denatured RNase U1. These results showed that the protein is composed of a single polypeptide chain of 105 amino acid residues cross-linked by two disulfide bonds, having a molecular weight of 11,235, and that the NH2-terminus is blocked by a pyroglutamate residue. It has an overall homology with other guanine-specific or related ribonucleases, and shows 48% identity with RNase T1 and 38% identity with RNase U2.     

Coordination isomers of biological iron transport compounds. III. (1) Transport of lambda-cis-chromic desferriferrichrome by Ustilago sphaerogena

Microbial iron transport studies of the structure and conformation dependent ferrichrome uptake system in Ustilago sphaerogena have been limited previously to kinetically labile metal ions such as the native ferrichrome complex and the aluminum(III) and gallium(III) analogs. Although two coordination isomers are possible (λ-cis amd δ-cis), no information can be obtained concerning their biological activity using kinetically labile complexes. In this report, both the ligand and chromic ion moieties of kinetically inert λ-cis-chromic [¹⁴C]-desferriferrichrome are shown to be taken up in Ustilago sphaerogena at rates comparable to that of ferrichrome. The λ-cis coordination isomer must be therefore at least one of the biologically active isomers and the transport system cannot rely on the rapid isomerization or dissociation of the labile ferric complex.     

Promotion of hyphal growth in Ustilago violacea by host factors from Silene alba

Ustilago violacea specifically parasitizes susceptible members of the Caryophyllaceae. We isolated water-soluble compounds from leaves of Silene alba which promoted hyphal development in the dimorphic pathogen. We also isolated hyphal growth promoting -tocopherol from S. alba. The water-soluble activity, which we term hyphal growth factor, or HGF, separated into four bands with gel filtration chromatography and represented over 40% of the total hyphal growth promoting activity isolated from S. alba. The water-soluble HGF activity may be host-specific and may function as a determinant of the host-parasite specificity between U. violacea and caryophyllaceous host plants.Abbreviations HGF Hyphal growth factor - BHT butylated hydroxytoluene     

Studies on extracellular ribonucleases of Ustilago sphaerogena. Characterization of substrate specificity with special reference to purine-specific ribonucleases

1. Ribonuclease U(1) splits only the phosphodiester bonds of guanosine 3'-phosphates in RNA. It may be regarded as a guanyloribonuclease [ribonucleate (guanine nucleotide)-2'-transferase (cyclizing), EC 2.7.7.26] similar to ribonuclease T(1) (Egami, Takahashi & Uchida, 1964). It seems to be identical with the extracellular ribonuclease described by Glitz & Dekker (1963, 1964a,b). 2. Ribonucleases U(2) and U(3) are novel enzymes with a strict specificity. They split the internucleotide bonds between purine 3'-nucleotides and 5'-hydroxy groups of adjacent nucleotides in RNA with the intermediary formation of purine nucleoside 2',3'-(cyclic)-phosphates, which are slowly hydrolysed to purine 3'-nucleotides. So they may be classified as ;puryloribonucleases [ribonucleate (purine nucleotide)-2'-transferase (cyclizing)]'. Double-stranded RNA is scarcely split by ribonucleases U(2) and U(3). 3. Ribonuclease U(4) has no absolute base specificity, and produces the mononucleotides 3'-adenylate, 3'-guanylate, 3'-cytidylate and 3'-uridylate from RNA.     

Formation of protoplasts from Ustilago maydis

Protoplasts of Ustilago maydis were obtained by incubating sporidia of the fungus with a combination of Helicase and a commercial Onozuka R-10 enzyme preparation of Trichoderma harzianum in the presence of 0.6 m (NH₄)₂ SO₄ as an osmotic stabilizer. In the presence of the organic stabilizers sorbitol and sucrose, however, the release of protoplasts was inhibited. Combinations of Helicase with other lytic enzymes such as cellulase from Aspergillus niger, cellulase and hemicellulase from Rhizopus, and Driselase or Nagarse were inactive.