Biochemical, biophysical, and biological properties of densonucleosis virus (parvovirus). III. common sequences of structural proteins

16 Methods for Detecting Microbial Pathogens in Food and Water

Newly developed methods for the detection of bacteria and viruses have provided microbiologists with the means to rapidly identify and monitor specific microorganisms in food and water. Traditional methods of testing involve culture techniques to increase the numbers of the organism to a detectable level, followed by isolation and biochemical identification. This chapter focuses on the methodologies to detect pathogens and indicator organisms; however, the methods described are applicable to most bacteria. As detection and isolation methods have improved, a growing number of pathogens have been identified as important food- and waterborne pathogens. This chapter describes the use of nucleic acid and antibody probes that have the potential to circumvent the need to culture the organism prior to identification. Nucleic acid probes have become a valuable diagnostic reagent in the identification of human and animal pathogens and have made possible the identification of viruses and bacteria that are difficult, if not impossible, to cultivate. DNA probes have also proved to be a useful tool for identifying and monitoring the organisms in food and the environment.

Expression strategy of densonucleosis virus from Mythimna loreyi

The genome of Mythimna loreyi densovirus (MlDNV) was cloned into the pEMBL(19)+ vector. This clone was infectious upon transfection, both in LD cells and larvae. The genome possessed ITRs of 543 nucleotides of which the distal 126 nucleotides could form a hairpin. The nonstructural (NS) and structural (VP) genes were located on the 5'-halves of the complementary strands and their transcripts started 27 nts downstream of the ITRs. These transcripts had an overlap of 57 nucleotides in middle of the genome. The NS cassette consisted of three genes with NS1 and the overlapping NS2 downstream of NS3. The NS3 gene was spliced out from a fraction of the NS transcripts to allow leaky scanning translation of the downstream bicistronic NS1 and NS2 genes. The four VPs were similarly generated by leaky scanning translation of unspliced mRNA. The 5'-untranslated region of the VP transcript was only seven nucleotides long.

Organization and expression strategy of the ambisense genome of densonucleosis virus of Galleria mellonella

The expression strategy of parvoviruses of the Densovirus genus has as yet not been reported. Clones were obtained from the densonucleosis virus of Galleria mellonella (GmDNV) that yielded infectious virus upon transfection into LD652 cells. Its genome was found to be the longest (6,039 nucleotides [nt]), with the largest inverted terminal repeats (ITRs) (550 nt) among all parvoviruses. The distal 136 nt could be folded into hairpins with flop or flip sequence orientations. In contrast to vertebrate parvoviruses, the gene cassettes for the nonstructural (NS) and structural (VP) proteins were found on the 5' halves of the opposite strands. The transcripts for both cassettes started 23 nt downstream of the ITRs. The TATA boxes, as well as all upstream promoter elements, were localized in the ITRs and, therefore, identical for the NS and VP transcripts. These transcripts overlapped for 60 nt at the 3' ends (antisense RNAs) at 50 m.u. The NS cassette consisted of three genes of which NS2 was contained completely within NS1 but from a different reading frame. Most of the NS transcripts were spliced to remove the upstream NS3, allowing leaky scanning translation of NS1 and NS2, similar to the genes of RNA-6 of influenza B virus. NS3 could be translated from the unspliced transcript. The VP transcript was not spliced and generated four VPs by a leaky scanning mechanism. The 5'-untranslated region of the VP transcript was only 5 nt long. Despite the transcription and translation strategies being radically different from those of vertebrate parvoviruses, the capsid was found to have phospholipase A(2) activity, a feature thus far unique for parvoviruses.

Disulfide bonding among micro 1 trimers in mammalian reovirus outer capsid: a late and reversible step in virion morphogenesis

We examined how a particular type of intermolecular disulfide (ds) bond is formed in the capsid of a cytoplasmically replicating nonenveloped animal virus despite the normally reducing environment inside cells. The micro 1 protein, a major component of the mammalian reovirus outer capsid, has been implicated in penetration of the cellular membrane barrier during cell entry. A recent crystal structure determination supports past evidence that the basal oligomer of micro 1 is a trimer and that 200 of these trimers surround the core in the fenestrated T=13 outer capsid of virions. We found in this study that the predominant forms of micro 1 seen in gels after the nonreducing disruption of virions are ds-linked dimers. Cys679, near the carboxyl terminus of micro 1, was shown to form this ds bond with the Cys679 residue from another micro 1 subunit. The crystal structure in combination with a cryomicroscopy-derived electron density map of virions indicates that the two subunits that contribute a Cys679 residue to each ds bond must be from adjacent micro 1 trimers in the outer capsid, explaining the trimer-dimer paradox. Successful in vitro assembly of the outer capsid by a nonbonding mutant of micro 1 (Cys679 substituted by serine) confirmed the role of Cys679 and suggested that the ds bonds are not required for assembly. A correlation between micro 1-associated ds bond formation and cell death in experiments in which virions were purified from cells at different times postinfection indicated that the ds bonds form late in infection, after virions are exposed to more oxidizing conditions than those in healthy cells. The infectivity measurements of the virions with differing levels of ds-bonded micro 1 showed that these bonds are not required for infection in culture. The ds bonds in purified virions were susceptible to reduction and reformation in situ, consistent with their initial formation late in morphogenesis and suggesting that they may undergo reduction during the entry of reovirus particles into new cells.

Expression of the insect parvovirus GmDNV in vivo: the structural and nonstructural proteins are encoded by opposite DNA strands

The nucleotide sequence of GmDNV, an insect parvovirus, reveals large open reading frames (ORFs) on both strands of the viral replicative form DNA. Previously, we identified two viral transcripts within the polyadenylated RNA fraction of infected host larvae (Gross et al., 1990, J. Invertebr. Pathol. 56, 175-180). In this work we used hybridization of single-stranded, unidirectional probes to RNA blots to show that the two transcripts, synthesized in vivo in GmDNV-infected Galleria mellonella larvae, are of antiparallel orientation. To determine their coding specificities, polyadenylated RNAs were isolated from hybrids with DNA from the left and right halves of the viral genome and translated in a rabbit reticulocyte system. The "right," 2.4-kb hybrid-selected RNA was shown to direct the synthesis of four polypeptides that comigrated with the four viral capsid proteins and were immunoprecipitated with anti-GmDNV serum. Translation of the "left," 1.8-kb RNA yielded three polypeptides, none of which was detected among the viral capsid proteins. This type of expression strategy is unique among vertebrate and most invertebrate parvoviruses, which use only one DNA strand to encode all their proteins. On the other hand, the basic organization of parvoviruses, in which the regulatory and structural proteins are encoded, respectively, by two clusters of ORFs located at the left and right halves of the genome, is conserved.

The structure of an insect parvovirus (Galleria mellonella densovirus) at 3.7 A resolution

BACKGROUND

Parvoviruses infect vertebrates, insects and crustaceans. Many arthropod parvoviruses (densoviruses) are highly pathogenic and kill approximately 90% of the host larvae within days, making them potentially effective as selective pesticides. Improved understanding of densoviral structure and function is therefore desirable. There are four different initiation sites for translation of the densovirus capsid protein mRNA, giving rise to the viral proteins VP1 to VP4. Sixty copies of the common, C-terminal domain make up the ordered part of the icosahedral capsid.

RESULTS

The Galleria mellonella densovirus (GMDNV) capsid protein consists of a core beta-barrel motif, similar to that found in many other viral capsid proteins. The structure most closely resembles that of the vertebrate parvoviruses, but it has diverged beyond recognition in many of the long loop regions that constitute the surface features and intersubunit contacts. The N termini of twofold-related subunits have swapped their positions relative to those of the vertebrate parvoviruses. Unlike in the vertebrate parvoviruses, in GmDNV there is no continuous electron density in the channels running along the fivefold axes of the virus. Electron density corresponding to some of the single-stranded DNA genome is visible in the crystal structure, but it is not as well defined as in the vertebrate parvoviruses.

CONCLUSIONS

The sequence of the glycine-rich motif, which occupies each of the channels along the fivefold axes in vertebrate viruses, is conserved between mammalian and insect parvoviruses. This motif may serve to externalize the N-terminal region of the single VP1 subunit per particle. The domain swapping of the N termini between insect and vertebrate parvoviruses may have the effect of increasing capsid stability in GmDNV.

Polychlorinated and polybrominated biphenyl congeners and retinoid levels in rat tissues: structure-activity relationships

The purpose of this study was to examine the structural requirements of polychlorinated and polybrominated biphenyls (PCBs and PBBs) for altering tissue levels of retinoids. Seven congeneric PCBs and PBBs were studied: 3,3',4,4'-tetrachlorobiphenyl (TCB), 2',3,3',4,5- and 3,3',4,4',5-pentachlorobiphenyls (-PeCBs), 3,3',4,4'- and 3,3',5,5'-tetrabromobiphenyls (-TBBs), 2,2',3,3',5,5'-hexachlorobiphenyl (-HCB), and 3,3',4,4',5,5'-hexabromobiphenyl (-HBB). Male Sprague-Dawley rats were fed a vitamin A-adequate diet (1.3 mg/kg) for 30 days before being given a single IP injection of one of seven polyhalogenated biphenyls (150 mumol/kg) in corn oil (10 ml/kg) or vehicle alone. Rats were killed 1 week later. Except for 3,3',4,4',5,5'-HBB, all PCBs and PBBs studied significantly decreased serum retinol levels and, except for 3,3',4,4',5,5'-HBB and 2,2',3,3',5,5'-HCB, all PCBs and PBBs also lowered the serum retinol-binding-protein (RBP) content. The activity of hepatic retinyl ester hydrolase (REH) was reduced by the treatment of 3,3',4,4',5-PeCB, 3,3',4,4'-TBB, and 3,3',4,4',5,5'-HBB. The levels of hepatic retinol were decreased by 2,2',3,3',5,5'-HCB, 2',3,3',4,5-PeCB, and 3,3',4,4',5-PeCB, while levels of hepatic retinyl palmitate were decreased by 2',3,3',4,5-PeCB, 3,3',4,4',5-PeCB, 3,3',4,4'-TCB, 3,3',4,4'-TBB, and 3,3',4,4',5,5'-HBB. The substantial decreases in hepatic retinyl palmitate levels could not be explained solely on the basis of hepatomegaly caused by acutely toxic PCBs and PBBs. All halogenated biphenyls which caused a decrease in hepatic retinyl palmitate also caused an increase in renal retinyl palmitate except 3,3',4,4',5-PeCB. In summary, the acutely toxic (nonortho substituted) congeners had pronounced effects on hepatic, renal, and serum retinoids whereas other biphenyls only decreased serum retinol levels. The effects of these seven compounds on REH activity were not correlated with the effects on serum retinol or RBP levels. Therefore, this study shows that the structure-activity relationships for altering hepatic retinoids differ from those for serum retinol, implying the involvement of multiple mechanisms.

The densovirus of Junonia coenia (Jc DNV) as an insect cell expression vector

An infectious genome of the Junonia coenia densovirus (Jc DNV) has been recently cloned and sequenced. We investigated the ability of this cloned genome to be used as expression vector by inserting the lacZ gene of Escherichia coli as fusion gene in the major open reading frame (ORF 1) of the viral sequence. The resulting recombinant plasmid designated pBRJlac Z was transfected into insect SPC-SL 52 cells and the expression of beta-galactosidase (beta-gal) was detected qualitatively or quantitatively by using Xgal or ONPG as chromogenic substrates. Western blot analysis revealed that beta-gal was expressed as chimeric capsid-beta-gal polypeptides. This provided evidence that ORF1 codes for structural polypeptides which share a common C-terminal sequence. Construction of plasmids with alterations or deletions in ORF2, 3 or 4, allowed us to implicate nonstructural (NS) functions in viral DNA replication. Deletions in inverted terminal repeats or in NS functions did not abolish expression of capsid polypeptides but reduced it dramatically. Encapsidation of Jlac Z recombinant genome was achieved by trans-complementation with plasmids bearing intact structural and nonstructural functions. Detection of a beta-gal activity in SPC-SL 52 cells following several subcultures post-transfection suggests that Jlac Z recombinant genome could be maintained in an integrative or episomal state.

Expression of densonucleosis virus GmDNV in Galleria mellonella larvae: size analysis and in vitro translation of viral transcription products

The RNA of densonucleosis virus type 1 (GmDNV), isolated from GmDNV-infected Galleria mellonella larvae, was shown by Northern blotting to contain five polyadenylated, viral-specific RNA species with sizes of 1.8, 2.4, 3.5, 4.0, and 5.0 kb. Poly(A)-containing RNA from whole larvae and hybrid-selected viral RNA were translated in a rabbit reticulocyte lysate system and the translation products were coelectrophoresed with proteins extracted from CsCl-purified virus. All four virion-associated proteins, namely p49, p55, p65, and p94, were present in the in vitro-translated products. However, in the majority of the experiments the most abundant translation product was a 30K polypeptide which is absent from virion extracts. The most abundant viral protein is p49, and the 49K polypeptide was also the most abundant translation product in about 30% of the preparations. The 1.8 kb transcript, which constitutes about half of the total viral RNA, is only slightly larger than the template required for a 30K polypeptide, suggesting that the latter may be a primary translation product of the smallest RNA transcript. The similarities in gene expression between densoviruses and mam malian parvoviruses are discussed.

Molecular cloning and characterization of Bombyx densovirus genomic DNA. Brief report

A map of the Bombyx densovirus (DNV) genomic DNA has been constructed by cleaving it with several restriction endonucleases. This map shows that this virus and the mammalian parvoviruses are distinct. A restriction fragment containing more than 85 per cent of the complete viral genome has been cloned in a bacterial plasmid.

Molecular homology among the structural proteins of densonucleosis virus from silkworm, Bombyx mori

Similarities among the four structural proteins of Bombyx densonucleosis virus (DNV) were examined by four independent techniques, peptide mapping, immunodiffusion tests, amino acid analysis and enzyme linked immunosorbent (ELISA) technique. The peptide maps produced by Staphylococcus aureus V8 protease, or chymotrypsin indicated the existence of sequence homology among the proteins. The results of immunodiffusion tests revealed that these four structural proteins share common antigens. The amino acid compositions of these proteins were also very similar to each other. However, the amount of amino acid residues (e.g., serine) in VP3 was not always sufficient to account for the amount found in the smaller structural protein, VP2. These results indicate that the four structural proteins of Bombyx DNV probably originate, at least partially, from a common DNA sequence, and that VP2 is not a direct cleavage product of VP3.

Rapid and sensitive heterologous enzyme immunoassays for densonucleosis virus (Parvoviridae)

There exists a serious lack of rapid and sensitive methods to identify densonucleosis viruses and to discriminate among them. Two different enzyme-linked immunosorbent assays (ELISA) were adapted for this purpose, which were both significantly faster and more sensitive than currently used ELISA procedures. This increase in sensitivity was due to an improvement in the conjugation procedure of peroxidase to antibody, the establishment of the optimum conditions for the various incubations, an optimisation of the substrate (H2O2) concentration, and the use of a new H-donor. The speed of the assay could be considerably shortened by the use of polyethylene glycol-6000 (i.e. the total time of the assay needed for maximum sensitivity of the indirect assay was only 2 hours). The assays using peroxidase conjugates were found considerably more sensitive than those using alkaline phosphatase, which is very probably due to a more efficient and better controlled conjugation procedure for peroxidase. The virus could be detected in the pg to ng range in a large excess of nonspecific antigens and titers for the antisera usually exceeded 10(6). Small differences in the viruses could be detected. Several factors, which may influence the sensitivity and specificity of these densonucleosis virus assays, were further investigated.