In vitro translation of the major capsid polypeptide from Ustilago maydis virus stain P1

The H1 double-stranded RNA genome of Ustilago maydis virus-H1 encodes a polyprotein that contains structural motifs for capsid polypeptide, papain-like protease, and RNA-dependent RNA polymerase

The Ustilago maydis viral (UmV) genome consists of three distinct size groups of double-stranded RNA (dsRNA) segments: H (heavy), M (medium), and L (light). The H segments have been suggested to encode all essential viral proteins, but without any molecular evidences. As a preliminary step to understand viral genomic organization and the molecular mechanism governing gene expression in UmV, we determined the complete nucleotide sequence of the H1 dsRNA genome in P1 viral killer subtype. The H1 dsRNA genome (designated UmV-H1) contained a single open reading frame that encodes a polyprotein of 1820 residues, which is predicted to be autocatalytically processed by a viral papain-like protease to generate viral proteins. The amino-terminal region is the capsid polypeptide with a predicted molecular mass of 79.9 kDa. The carboxy-terminal region is the RNA-dependent RNA polymerase (RDRP) that has a high sequence homology to those of the totiviruses. The H2 dsRNA also encodes a distinct RDRP, suggesting that UmV is a complex virus system like the Saccharomyces cerevisiae viruses ScV-L1 and -La.

Detection of plasmid-like DNA and double-stranded RNA elements in some Canadian isolates of the oilseed rape pathogen Leptosphaeria maculans

Nine Canadian isolates of Leptosphaeria maculans were examined for the presence of plasmid-like elements. A single extrachromosomal DNA element, with an estimated size of 9 kb, was detected in undigested genomic DNA of a virulent isolate, Fairview 1. This element was susceptible to hydrolysis by DNAse I and exonuclease III, and it was shown to hybridize to DNA of all virulent isolates. In addition, another virulent strain, Saskatoon 8, was observed to contain four double-stranded RNA (dsRNA) segments, ranging from approximately 500 bp to 2.4 kb. The latter segments (dsRNA) were resistant to DNAse I and exonuclease III treatment, and to RNAse A in high-ionic-strength buffer, but were susceptible to RNAse A in the presence of low-ionic-strength buffer.Key words: Leptosphaeria maculans, Canola, double-stranded RNA, plasmids.

New developments in fungal virology

Although viruses are widely distributed in fungi, their biological significance to their hosts is still poorly understood. A large number of fungal viruses are associated with latent infections of their hosts. With the exception of the killer-immune character in the yeasts, smuts, and hypovirulence in the chestnut blight fungus, fungal properties that can specifically be related to virus infection are not well defined. Mycoviruses are not known to have natural vectors; they are transmitted in nature intracellularly by hyphal anastomosis and heterokaryosis, and are disseminated via spores. Because fungi have a potential for plasmogamy and cytoplasmic exchange during extended periods of their life cycles and because they produce many types of propagules (sexual and asexual spores), often in great profusion, mycoviruses have them accessible to highly efficient means for transmission and spread. It is no surprise, therefore, that fungal viruses are not known to have an extracellular phase to their life cycles. Although extracellular transmission of a few fungal viruses have been demonstrated, using fungal protoplasts, the lack of conventional methods for experimental transmission of these viruses have been, and remains, an obstacle to understanding their biology. The recent application of molecular biological approaches to the study of mycoviral dsRNAs and the improvements in DNA-mediated fungal transformation systems, have allowed a clearer understanding of the molecular biology of mycoviruses to emerge. Considerable progress has been made in elucidating the genome organization and expression strategies of the yeast L-A virus and the unencapsidated RNA virus associated with hypovirulence in the chestnut blight fungus. These recent advances in the biochemical and molecular characterization of the genomes of fungal viruses and associated satellite dsRNAs, as they relate to the biological properties of these viruses and to their interactions with their hosts are the focus of this chapter.

Sequences of three dsRNAs associated with La France disease of the cultivated mushroom (Agaricus bisporus)

La France disease of the cultivated mushroom, Agaricus bisporus, is known to be associated with the presence of a number of dsRNA segments. The nucleotide sequences of the dsRNAs M2 (1.3 kb), M1 (1.55 kb) and L3 (2.8 kb), invariably associated with the disease, were determined. Putative coding sequences for proteins with molecular weights of 38, 40 and 87 kDa were found for M2, M1 and L3 dsRNAs, respectively. The average G + C content of these dsRNAs was 43%, close to that of A. bisporus nuclear DNA. The nucleotide sequences, as well as the amino acid sequences, appear to be unique, as no matching sequences could be found among databases. S3 dsRNA (0.39 kb), which is occasionally found in large amounts in diseased mushrooms, is an internally deleted variant of M2 dsRNA and is largely composed of the non-coding ends of that dsRNA.

Ustilago maydis virus P4 killer toxin: characterization, partial amino terminus sequence, and evidence for glycosylation

The toxin from Ustilago maydis virus P4 was purified to homogeneity and characterized. The native molecular mass, using size-exclusion HPLC was estimated to be 7.2 kDa. The purified toxin was composed of a single subunit. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis under reduced and nonreduced conditions resulted in estimated molecular masses of 8.4 and 7.4 kDa, respectively. The purified toxin was found to be glycosylated when tested for carbohydrates using the phenol-sulfuric acid method, Schiff's base reagent, and a Glycan detection kit and when probed against different biotinylated lectins. Partial amino acid sequence analysis of the purified toxin indicated a free N-terminus, 16% glycine, and 23% basic amino acid residues. No homology was found to either the alpha or the beta subunit of the toxin encoded by U. maydis infected with the P6 virus.

A very small viral double-stranded RNA

UmV is a double-stranded RNA (dsRNA) virus of the corn fungal pathogen Ustilago maydis. UmV has no infectious cycle. Some UmV subtypes have viral dsRNAs encoding secreted toxins that kill sensitive cells of the same species and related species. There are three viral subtypes, P1, P4 and P6, which differ in the specificity of their secreted killer toxins. Each has three size classes of dsRNA: H (heavy), M (medium) and L (light). The L segments of UmV are unique in being derived from one end of the larger M segments. We have sequenced P1 L and placed it at the 3' end of the P1 M1 plus strand. In their overlapping regions, these dsRNAs are identical in sequence. In vitro translation of P1 M1 results in a peptide whose size is consistent with its being encoded by the non-L region of M1. P1 L is a very small dsRNA of 355 bp. It has no long open reading frames and produces no detectable in vitro translation product. The sequence of P1 L suggests that it is derived by a process unique among dsRNA viruses: replication and packaging of the 3' end fragment of a processed mRNA.

Semiconservative strand-displacement transcription of the M2 dsRNA segment of Ustilago maydis virus

The P1 strain of the Ustilago maydis virus (UmV) is a segmented dsRNA virus with segments designated H1, H2, M1, M2, and L. Incubation of purified virus with a mixture of nucleotides containing 32P-UTP resulted in labeled dsRNA which was retained in the capsid and labeled ssRNA which was released from the capsid. This in vitro transcription reaction was dependent on Mg2+ ion and the optimum concentration for maximum incorporation was 10 mM. The pH and temperature optima were 8.0 and 30 degrees C, respectively. The ssRNA transcripts were precipitated from the supernatant solution of the reaction mixture after ultracentrifugation to separate the virus. Transcription products from supernatant solution hybridized with all five virion dsRNAs. Further studies of the M2 segment indicated that it was labeled within 2 h and the label was completely chased out in 2 h. Analysis of the labeled M2 dsRNA segment by strand-separation gel showed that only one strand (slow moving) was labeled. When both strands were tested in an in vitro translation system, only the slow-moving strand was translated to produce a 24 kDa product. Thus the M2 dsRNA segment of UmV P1 transcribes by a semiconservative strand-displacement mechanism.

Double stranded RNA in natural isolates of Neurospora

Thirty-six wild type isolates of Neurospora were surveyed for the presence of dsRNA. The survey identified seven strains which contain dsRNA molecules. These seven strains are all from different geographic locations. The sizes of the dsRNAs range from 500 bp to 18 kb and a total of seven distinct dsRNA species was identified. Cross homologies of some of the dsRNAs were apparent. There was homology between the 9.0 kb dsRNA and genomic DNA prepared from all strains in the survey, indicating a possible cellular rather than viral origin for this dsRNA species. None of the other dsRNAs hybridized with genomic DNA suggesting a viral origin for these dsRNAs.

Transcription and in vitro translation of the dsRNA virus isolated from Rhizoctonia solani

A segmented double-stranded dsRNA virus has been isolated from virulent strains of Rhizoctonia solani. The dsRNA genome has mol. wts. of 1.45 and 1.32 X 10(6). Two full-size transcripts with mol. wts. of 0.74 and 0.66 X 10(6) (2.2 kb and 2 kb, respectively) were synthesized by the virus-associated RNA-dependent RNA polymerase and resolved by denaturing polyacrylamide gel electrophoresis. The transcripts cross-hybridized to the viral dsRNA isolated from a number of strains. The transcripts did not hybridize with the genomic DNA. An unencapsidated species of dsRNA with mol. wt. of 1.6 X 10(6) did not hybridize with the viral transcripts. No cross-hybridization between the two viral dsRNA segments was obtained. The viral-encoded proteins were studied by in vitro translation using the rabbit reticulocyte lysate system. The transcripts served as mRNA for the synthesis of the major capsid protein of 55 kD, and a number of other products. The viral coat protein was immunoprecipitated with antibodies against purified virus particles. Partial proteolysis of the major in vitro product and the authentic capsid protein using Staphylococcus aureus V8 protease produced similar peptide patterns. Denatured viral dsRNA also directed the synthesis of proteins identical to those translated from the transcripts in vitro.

Synthesis and processing of killer toxin from Ustilago maydis virus P4

The synthesis of toxin protein from Ustilago maydis virus (UmV) strain P4 was studied in vitro and in vivo. The protein synthesized in vitro and in vivo has a molecular weight of approximately 30 kd whereas the native toxin has a molecular weight of about 12 kd. In the presence of protease inhibitors and glycosylation inhibitors, toxin protein synthesized in vivo showed higher molecular weight products that could be immunoprecipitated with toxin antibodies. These results suggest that the UmV P4 toxin protein is synthesized as a preprotein, which upon processing results in the 12 kd secreted form toxin.

Virus-encoded toxin of Ustilago maydis: two polypeptides are essential for activity

Cells of Ustilago maydis containing double-stranded RNA viruses secrete a virus-encoded toxin to which other cells of the same species and related species are sensitive. Mutants affected in the expression of the KP6 toxin were characterized, and all were viral mutants. A temperature-sensitive nonkiller mutant indicated that the toxin consists of two polypeptides, 12.5K and 10K, that are essential for the toxic activity. The temperature-sensitive nonkiller mutant was affected in the expression of the 10K polypeptide, and its toxic activity was restored by the addition of the 10K polypeptide to its secreted inactive toxin. These results led to the reexamination of other mutants that were known to complement in vitro. Each was found to secrete one of the two polypeptides. Here we show for the first time that P6 toxin consists of two polypeptides that do not interact in solution, but both are essential for the toxic effect. Studies on the interaction between the two polypeptides indicated that there are no covalent or hydrogen bonds between the polypeptides. Toxin activity is not affected by the presence of 0.3 M NaCl in the toxin preparations and in the medium, suggesting that no electrostatic forces are involved in this interaction. Also, the two polypeptides do not share common antigenic determinants. The activity of the two polypeptides appears to be dependent on a sequential interaction with the target cell, and it is the 10K polypeptide that initiates the toxic effect. The similarity of the U. maydis virus-encoded toxin to that of Saccharomyces cerevisiae is discussed.

Virus-encoded toxin of Ustilago maydis: two polypeptides are essential for activity

Cells of Ustilago maydis containing double-stranded RNA viruses secrete a virus-encoded toxin to which other cells of the same species and related species are sensitive. Mutants affected in the expression of the KP6 toxin were characterized, and all were viral mutants. A temperature-sensitive nonkiller mutant indicated that the toxin consists of two polypeptides, 12.5K and 10K, that are essential for the toxic activity. The temperature-sensitive nonkiller mutant was affected in the expression of the 10K polypeptide, and its toxic activity was restored by the addition of the 10K polypeptide to its secreted inactive toxin. These results led to the reexamination of other mutants that were known to complement in vitro. Each was found to secrete one of the two polypeptides. Here we show for the first time that P6 toxin consists of two polypeptides that do not interact in solution, but both are essential for the toxic effect. Studies on the interaction between the two polypeptides indicated that there are no covalent or hydrogen bonds between the polypeptides. Toxin activity is not affected by the presence of 0.3 M NaCl in the toxin preparations and in the medium, suggesting that no electrostatic forces are involved in this interaction. Also, the two polypeptides do not share common antigenic determinants. The activity of the two polypeptides appears to be dependent on a sequential interaction with the target cell, and it is the 10K polypeptide that initiates the toxic effect. The similarity of the U. maydis virus-encoded toxin to that of Saccharomyces cerevisiae is discussed.

In vitro translation of mushroom tyrosinase

Mushroom tyrosinase was purified and antibodies prepared against the holo enzyme and a protein of 26,000 daltons. Both antibodies recognized the large subunit of the enzyme but only one recognized the 26,000 dalton protein. Poly A+ mRNA was isolated from mushrooms, translated in vitro, and a 41,000 dalton protein immunoprecipitated from the translation mix with either antibody. This 41,000 dalton protein presumably corresponds to the large subunit of the holoenzyme. Antibodies against the holoenzyme also immunoprecipitated another translation product with a molecular weight of 15,000 daltons corresponding to the small subunit of the holoenzyme. These results suggest that each subunit may be coded for by different genes and undergo posttranslational processing.