Isolation and characterization of temperature-sensitive mutants of fowl plague virus

Studies of an influenza A virus temperature-sensitive mutant identify a late role for NP in the formation of infectious virions

The influenza A virus nucleoprotein (NP) is a single-stranded RNA-binding protein that encapsidates the virus genome and has essential functions in viral-RNA synthesis. Here, we report the characterization of a temperature-sensitive (ts) NP mutant (US3) originally generated in fowl plague virus (A/chicken/Rostock/34). Sequence analysis revealed a single mutation, M239L, in NP, consistent with earlier mapping studies assigning the ts lesion to segment 5. Introduction of this mutation into A/PR/8/34 virus by reverse genetics produced a ts phenotype, confirming the identity of the lesion. Despite an approximately 100-fold drop in the viral titer at the nonpermissive temperature, the mutant US3 polypeptide supported wild-type (WT) levels of genome transcription, replication, and protein synthesis, indicating a late-stage defect in function of the NP polypeptide. Nucleocytoplasmic trafficking of the US3 NP was also normal, and the virus actually assembled and released around sixfold more virus particles than the WT virus, with normal viral-RNA content. However, the particle/PFU ratio of these virions was 50-fold higher than that of WT virus, and many particles exhibited an abnormal morphology. Reverse-genetics studies in which A/PR/8/34 segment 7 was swapped with sequences from other strains of virus revealed a profound incompatibility between the M239L mutation and the A/Udorn/72 M1 gene, suggesting that the ts mutation affects M1-NP interactions. Thus, we have identified a late-acting defect in NP that, separate from its function in RNA synthesis, indicates a role for the polypeptide in virion assembly, most likely involving M1 as a partner.

Involvement of influenza virus PA subunit in assembly of functional RNA polymerase complexes

The RNA-dependent RNA polymerase of influenza virus consists of three subunits, PB1, PB2, and PA, and synthesizes three kinds of viral RNAs, vRNA, cRNA, and mRNA. PB1 is a catalytic subunit; PB2 recognizes the cap structure for generation of the primer for transcription; and PA is thought to be involved in viral RNA replication. However, the process of polymerase complex assembly and the exact nature of polymerase complexes involved in synthesis of the three different RNA species are not yet clear. ts53 virus is a temperature-sensitive (ts) mutant derived from A/WSN/33 (A. Sugiura, M. Ueda, K. Tobita, and C. Enomoto, Virology 65:363-373, 1975). We confirmed that the mRNA synthesis level of ts53 remains unaffected at the nonpermissive temperature, whereas vRNA synthesis is largely reduced. Sequencing of the gene encoding ts53 PA and recombinant virus rescue experiments revealed that an amino acid change from Leu to Pro at amino acid position 226 is causative of temperature sensitivity. By glycerol density gradient analyses of nuclear extracts prepared from wild-type virus-infected cells, we found that polymerase proteins sediment in three fractions: one (H fraction) consists of RNP complexes, another (M fraction) contains active polymerases but not viral RNA, and the other (L fraction) contains inactive forms of polymerases. Pulse-chase experiments showed that polymerases in the L fraction are converted to those in the M fraction. In ts53-infected cells, polymerases accumulated in the L fraction. These results strongly suggest that PA is involved in the assembly of functional viral RNA polymerase complexes from their inactive intermediates.

Genetics of influenza viruses

Influenza A viruses contain genomes composed of eight separate segments of negative-sense RNA. Circulating human strains are notorious for their tendency to accumulate mutations from one year to the next and cause recurrent epidemics. However, the segmented nature of the genome also allows for the exchange of entire genes between different viral strains. The ability to manipulate influenza gene segments in various combinations in the laboratory has contributed to its being one of the best characterized viruses, and studies on influenza have provided key contributions toward the understanding of various aspects of virology in general. However, the genetic plasticity of influenza viruses also has serious potential implications regarding vaccine design, pathogenicity, and the capacity for novel viruses to emerge from natural reservoirs and cause global pandemics.

Fine mapping of the subunit binding sites of influenza virus RNA polymerase

Influenza virus RNA polymerase consists of three subunits, PB1, PB2 and PA, and catalyzes both transcription and replication of the RNA genome. PB1 is a catalytic subunit of RNA polymerization and a core of the subunit assembly. The subunit binding sites were mapped at about several hundred amino-acid size. Fine mapping of the subunit binding sites was determined. The PB1-PA binding regions were mapped within in the N-terminal 25 amino acids of PB1 and 668-692 of PA. PB1 and PB2 interacted within wider regions, 600-757 of PB1 and 51-259 of PB2. In these amino-acid spans, 206-259 of PB2 may be the most important region of PB1 binding and 718-732 of PB1 may be the most important region of PB2 binding because the binding activity was lost when the regions were lost in the subunits. The additional regions contributed to strong binding of these subunits.

Influenza virus RNA polymerase PA subunit is a novel serine protease with Ser624 at the active site

BACKGROUND

Influenza virus RNA polymerase is a multifunctional enzyme that catalyses both transcription and replication of the RNA genome. The function of the influenza virus RNA polymerase PA subunit in viral replication is poorly understood, although the enzyme is known to be required for cRNA --> vRNA synthesis. The protease related activity of PA has been discussed ever since protease-inducing activity was demonstrated in transfection experiments.

RESULTS

PA protein was highly purified from insect cells infected with the recombinant baculovirus carrying PA cDNA, and a novel chymotrypsin-type serine protease activity was identified with the synthetic peptide, Suc-LLVY-MCA, in the PA protein. [3H]DFP was crosslinked with PA and a mutational analysis revealed that serine624 was as an active site for the protease activity.

CONCLUSIONS

These results constitute the demonstration of protease activity in PA subunit of the influenza virus RNA polymerase complexes.

Molecular mapping of influenza virus RNA polymerase by site-specific antibodies

Influenza virus RNA polymerase with the subunit structure PB1-PB2-PA is involved in both transcription and replication of the RNA genome, including the unique cap-I-dependent RNase activity. To map the important domains for RNA polymerization, cap-I-dependent RNase, and cap-I-binding activity, we generated site-specific antibodies against overlapping 150-amino-acid peptides that cover each entire subunit. Monospecific antibodies against each subunit inhibited RNA synthesis in vitro. Those against PB1 and PB2 inhibited the cap-I-dependent RNase activity, but those against PB2 alone slightly inhibited the cap-I-binding activity. Antibodies against the N-terminal amino acids 1-159 of PB2 that overlap the PB1-binding site on PB2 and the C-terminal amino acids 501-617 of PA that overlap the putative nucleotide-binding site and PB1-binding site on PA inhibited RNA polymerizing activity as well as monospecific antibodies. Those against the N-terminal (amino acids 1-159); the central region (amino acids 305-559) of PB2, where a part of the cap-binding domain predicted previously is localized; the N-terminal (amino acids 1-222) of PB1; and amino acids 301-517 and 601-716 of PA inhibited the cap-I-dependent RNase activity. The cap-binding domain on PB2 could be mapped in amino acids 402-559, where one of the cap-binding domains mapped previously overlapped.

Mutational analysis identifies functional domains in the influenza A virus PB2 polymerase subunit

A collection of influenza virus PB2 mutant genes was prepared, including N-terminal deletions, C-terminal deletions, and single-amino-acid insertions. These mutant genes, driven by a T7 promoter, were expressed by transfection into COS-1 cells infected with a vaccinia virus encoding T7 RNA polymerase. Mutant proteins accumulated to levels similar to that of wild-type PB2. Immunofluorescence analyses showed that the C-terminal region of the protein is essential for nuclear transport and that internal sequences affect nuclear localization, confirming previous results (J. Mukaijawa and D. P. Nayak, J. Virol. 65:245-253, 1991). The biological activity of these mutants was tested by determining their capacity to (i) reconstitute RNA polymerase activity in vivo by cotransfection with proteins NP, PB1, and PA and a virion-like RNA encoding the cat gene into vaccinia virus T7-infected COS-1 cells and (ii) complete with the wild-type PB2 activity. In addition, when tested at different temperatures in vivo, two mutant PB2 proteins showed a temperature-sensitive phenotype. The lack of interference shown by some N-terminal deletion mutants and the complete interference obtained with a C-terminal deletion mutant encoding only 124 amino acids indicated that this protein domain is responsible for interaction with another component of the polymerase, probably PB1. To further characterize the mutants, their ability to induce in vitro synthesis of viral cRNA or mRNA was tested by using ApG or beta-globin mRNA as a primer. One of the mutants, 1299, containing an isoleucine insertion at position 299, was able to induce cRNA and mRNA synthesis in ApG-primed reactions but required a higher beta-globin mRNA concentration than wild-type PB2 for detection of in vitro synthesis. This result suggested that mutant I299 has diminished cap-binding activity.

Structure and assembly of hemagglutinin mutants of fowl plague virus with impaired surface transport

Five temperature-sensitive mutants of influenza virus A/FPV/Rostock/34 (H7N1), ts206, ts293, ts478, ts482, and ts651, displaying correct hemagglutinin (HA) insertion into the apical plasma membrane of MDCK cells at the permissive temperature but defective transport to the cell surface at the restrictive temperature, have been investigated. Nucleotide sequence analysis of the HA gene of the mutants and their revertants demonstrated that with each mutant a single amino acid change is responsible for the transport block. The amino acid substitutions were compared with those of mutants ts1 and ts227, which have been analyzed previously (W. Schuy, C. Will, K. Kuroda, C. Scholtissek, W. Garten, and H.-D. Klenk, EMBO J. 5:2831-2836, 1986). With the exception of ts206, the changed amino acids of all mutants and revertants accumulate in three distinct areas of the three-dimensional HA model: (i) at the tip of the 80-A (8-nm)-long alpha helix, (ii) at the connection between the globular region and stem, and (iii) in the basal domain of the stem. The concept that these areas are critical for HA assembly and hence for transport is supported by the finding that the mutants that are unable to leave the endoplasmic reticulum at the nonpermissive temperature do not correctly trimerize. Upon analysis by density gradient centrifugation, cross-linking, and digestion with trypsin and endoglucosaminidase H, two groups can be discriminated among these mutants: with ts1, ts227, and ts478, the HA forms large irreversible aggregates, whereas with ts206 and ts293, it is retained in the monomeric form in the endoplasmic reticulum. With a third group, comprising mutants ts482 and ts651 that enter the Golgi apparatus, trimerization was not impaired.

Genetic relatedness of the nucleoprotein (NP) of recent swine, turkey, and human influenza A virus (H1N1) isolates

The sequences of nucleoprotein (NP) genes of recent human and turkey isolates of influenza A viruses, which serologically could be correlated to contemporary swine viruses, were determined. These sequences were closely related to the NPs of these swine viruses and they formed a separate branch on the phylogenetic tree. While the early swine virus from 1931 resembled the avian strains in consensus amino acids of the NP and in its ability to rescue NP ts mutants of fowl plague virus in chicken embryo cells, the later strains on that branch were different: at 15 positions they have their own amino acids and they rescued the NP ts mutants only poorly. Of the NPs of the human New Jersey/76 isolates analysed, one clustered with the recent H1N1 swine viruses of the U.S.A., the other one with contemporary human strains. Since the NP is one of the main determinants of species specificity it is concluded that, although the H1N1 swine isolates from the U.S.A. form their own branch in the phylogenetic tree, they can be transmitted to humans and turkeys, but they do not spread further in these populations and so far have not contributed to human pandemics. It is not very likely that they will do so in future, since its branch in the phylogenetic tree develops further away from the human and avian branch.

Mutants and revertants of an avian influenza A virus with temperature-sensitive defects in the nucleoprotein and PB2

ts19 is a temperature-sensitive (ts) mutant of the influenza A fowl plague virus with a defect in the nucleoprotein (NP). In ts19-infected chicken embryo cells all viral components are synthesized in normal yields at the nonpermissive temperature, but infectious virus is not formed. Under these conditions the migration of the NP and M of ts19 from the cell nucleus to the cytoplasm is affected. This ts defect is due to a single amino acid replacement (R162K) in a completely conserved region of the NP. Another mutant with a different defect in the NP is ts81. After infection with ts81 at 40 degrees no vRNA is being synthesized. By backcross of a revertant derived from ts81 many isolates with a ts defect in the PB2 protein were obtained. This ts defect seems to extragenically suppress the ts defect in the NP gene and to be dominant in a wild-type background.

Studies on the regulation of influenza virus RNA replication: a differential inhibition of the synthesis of vRNA segments in shift-up experiments with ts mutants

The regulation of influenza virus vRNA synthesis in the course of the reproduction cycle was studied with the use of a series of ts mutants in shift-up experiments. The synthesis of vRNA segments was registered by means of polyacrylamide gel electrophoresis of nucleocapsid-associated RNA isolated from the infected cells labelled with [3H]uridine after the shift-up to a semi-permissive temperature. Each mutant exhibited a specific differential pattern of vRNA synthesis inhibition after the shift-up. The most affected segments were either vRNA 4, vRNAs 4 and 7, or vRNAs 4, 6, and 7 in cells infected, respectively, with ts mutants C15 (ts lesion in PB1 gene), C45 (ts lesion in PA gene) and CmN3 (ts lesion in NS gene). The synthesis of vRNAs 1, 2, and 3 was relatively resistant to the shift-up in the cells infected with C15 or C45 and more sensitive in the cells infected with C44 (ts lesion in PB2 gene) or CmN3. The replication of the "early" genes (vRNAs 5 and 8) was generally least affected by the shift-up. The results are discussed in connection with the "early-late" transition of vRNA synthesis pattern in the course of infection.

Characterization of a temperature-sensitive mutant in the RNA polymerase PB2 subunit gene of influenza A/WSN/33 virus

The temperature-sensitive mutant ts-1 of influenza virus A/WSN/33 carries mutations in the gene encoding RNA polymerase PB2 subunit. Effect of temperature on various steps of viral RNA synthesis was examined using disrupted virions of ts-1 mutant. The initiation of RNA synthesis with dinucleotide ApG primer was not affected by elevated temperature, whereas that with primer RNA containing 5'-terminal cap-1 structure was temperature-sensitive. The result supports the previous notion deduced from the UV-crosslinking experiments, that PB2 is involved in the cap-1 dependent initiation of RNA synthesis. In addition, the ts-1 mutant showed a defect in RNA chain elongation. Nucleotide sequence analysis of RNA segment 1 of ts-1 mutant revealed that the amino acid number 417 is essential for the recognition of cap-1 structures and/or the interaction with catalytic unit of the RNA polymerase.

Molecular cloning and sequencing of influenza virus A/Victoria/3/75 polymerase genes: sequence evolution and prediction of possible functional domains

The influenza virus A/Victoria/3/75 (H3N2) polymerase genes encoding PB1, PB2 and PA have been cloned by cDNA synthesis and insertion into bacterial vectors. The complete sequence for each polymerase gene has been obtained from random M13 subclones and compared to other influenza virus polymerase genes. A total of 45, 74 and 78 nucleotide changes were fixed in the period 1968-1975, corresponding to 10, 12 and 9 amino acid changes, for PB1, PB2 and PA genes, respectively. The amino acid sequence of PB1 polypeptide contains motifs found in a series of positive- and negative-RNA virus polymerase genes and that of PA polypeptide share invariant residues common to DNA and presumptive RNA helicases.

Localisation of the temperature-sensitive defect in the nucleoprotein of an influenza A/FPV/Rostock/34 virus

The nucleotide sequences of the nucleoprotein (NP) genes of fowl plague virus (FPV) and of a temperature-sensitive (ts) mutant (ts81) derived therefrom have been determined. The ts81-NP nucleotide sequence possesses a single nucleotide substitution in comparison to the wild type. This causes an amino acid exchange at position 332 of the NP. An alanine in the wild type-NP is substituted by a threonine in ts81-NP. This substitution leads to a significant difference in the secondary structure prediction. Although this mutation is located within the karyophilic region of the NP, the accumulation of the NP in ts81-infected cells is not significantly affected at 40 degrees C. Therefore, we assume that the cooperation with one of the polymerase proteins (P) is interfered with at 40 degrees C, leading to the loss of viral vRNA or replicative cRNA synthesis. The comparison of the FPV-NP nucleotide sequence to a previously published sequence of the same strain (Tomley and Roditi, 1984) highlights ten nucleotide differences, four of them leading to amino acid substitutions.

Influenza virus RNA replication in vitro: synthesis of viral template RNAs and virion RNAs in the absence of an added primer

The two steps in influenza virus RNA replication are (i) the synthesis of template RNAs, i.e., full-length copies of the virion RNAs, and (ii) the copying of these template RNAs into new virion RNAs. We prepared nuclear extracts from infected HeLa cells that catalyzed both template RNA and virion RNA synthesis in vitro in the absence of an added primer. Antibody depletion experiments implicated nucleocapsid protein molecules not associated with nucleocapsids in template RNA synthesis for antitermination at the polyadenylation site used during viral mRNA synthesis. Experiments with the WSN influenza virus temperature-sensitive mutant ts56 containing a defect in the nucleocapsid protein proved that the nucleocapsid protein was indeed required for template RNA synthesis both in vivo and in vitro. Nuclear extracts prepared from mutant virus-infected cells synthesized template RNA at the permissive temperature but not at the nonpermissive temperature, whereas the synthesis of mRNA-size transcripts was not decreased at the nonpermissive temperature. Antibody depletion experiments showed that nucleocapsid protein molecules not associated with nucleocapsids were also required for the copying of template RNA into virion RNA. In contrast to the situation with the synthesis of transcripts complementary to virion RNA, no discrete termination product(s) were made during virion RNA synthesis in vitro in the absence of nucleocapsid protein molecules.

Viral RNA polymerases

Recent progress in molecular biological techniques revealed that genomes of animal viruses are complex in structure, for example, with respect to the chemical nature (DNA or RNA), strandedness (double or single), genetic sense (positive or negative), circularity (circle or linear), and so on. In agreement with this complexity in the genome structure, the modes of transcription and replication are various among virus families. The purpose of this article is to review and bring up to date the literature on viral RNA polymerases involved in transcription of animal DNA viruses and in both transcription and replication of RNA viruses. This review shows that the viral RNA polymerases are complex in both structure and function, being composed of multiple subunits and carrying multiple functions. The functions exposed seem to be controlled through structural interconversion.

Extragenic and intragenic suppression of a transport mutation in the hemagglutinin gene of an influenza A virus as revealed by backcross and sequence determination

Cooperation of viral proteins, or functional domains within a protein, can be studied by analyzing temperature-sensitive (ts) mutants and revertants carrying suppressor mutations. Accordingly, we have sequenced the hemagglutinin (HA) genes of a ts mutant of fowl plague virus (FPV), with a transport defect in the HA, and of five independent ts+ revertants (R1, R3, R4, R5, and R9). The amino acid replacement in position 480 from Thr to Ile, leading to the loss of a complex carbohydrate side chain, is responsible for the ts phenotype. R3, R4, and R5 are true revertants in that they have Thr in position 480, while R1 and R9 have kept Ile. The sequence of the HA of R1 is exactly the same as that of the ts mutant, while the R9 HA has two additional amino acid replacements in positions 91 (Lys-Thr) and 104 (Gly-Val). By doing a backcross with wild-type virus, it was shown that R1 carries an extragenic suppressor mutation, while R9 is intragenically suppressed. We conclude that the HA is transported from the site of its synthesis in the rough endoplasmic reticulum (RER) to the plasma membrane along with another viral gene product, which by mutation can complement the ts defect. An alternative interpretation is that the ts mutation results from a change in HA which allows an interacting protein to bind HA too soon, holding it back in the RER. The suppressor mutation may remove this premature interaction.

Mutants of an influenza A reassortant which are cold-sensitive (cs) as well as temperature-sensitive (ts): on the role of the neuraminidase activity for influenza virus infection

Temperature-sensitive (ts) mutants were obtained by undiluted passage of the cold-sensitive (cs) influenza A reassortant 113/Ho. This reassortant produces normal yields of infectious virus with negligible neuraminidase (NA) at 40 degrees. The mutants obtained from it had a narrow temperature optimum for plaque formation in chick embryo cells, since they were cs as well as ts. Such cs/ts mutants have not been described before. In contrast to mutants derived from FPV, most of the mutants derived from 113/Ho carried a ts defect in the NA gene. NA activity was not detectable after infection with these mutants at 40 degrees. The results are interpreted to mean that, although NA activity is not completely dispensible for influenza A virus replication in tissue cultures, the viruses possess a surplus of NA activity. The normally high activity of NA of influenza viruses seems to be necessary only for the natural infection of the respiratory tract.

Temperature-sensitive mutants of fowl plague virus defective in the intracellular transport of the hemagglutinin

Nine mutants of fowl plague virus with temperature-sensitive defects in the biosynthesis of the hemagglutinin have been characterized by analyzing the processing and the intracellular location of this glycoprotein in MDCK and chick embryo cells. It was found that with all of these mutants the transport of the hemagglutinin to the cell surface was impeded at the non-permissive temperature. There were differences, however, in the site of the block. With mutants tsl, ts227, ts478 and ts658 the precursor HA was not cleaved and the oligosaccharide side chains remained sensitive to endoglucosaminidase H. When the hemagglutinin was analyzed in permeabilized cells by immunofluorescence, usually only cytoplasmic labeling was seen. Immunofluorescence of non-permeabilized cells and hemadsorption revealed that the hemagglutinin did not reach the cell surface. In contrast, the hemagglutinin of mutants ts79, ts482, ts532, ts546 and ts651 was cleaved and oligosaccharides were processed to the endoglucosaminidase H-resistant form at non-permissive temperature. In permeabilized cells, the cytoplasm and juxtanuclear regions typical for the Golgi apparatus were labeled by immunofluorescence. Except for ts482, ts532 and ts546 which were leaky, hemagglutinin could not be detected at the cell surface. These observations indicate that, with the first group of mutants, hemagglutinin transport is usually arrested already in the rough endoplasmic reticulum, whereas with the second group it is inhibited at a late stage between the Golgi apparatus and the plasma membrane.

Studies on the temperature sensitivity of influenza A virus reassortants nonpathogenic for chicken

Influenza A virus reassortants which are nonpathogenic for chickens are like mammalian influenza A viruses in that they are temperature sensitive for growth at 41 degrees C. We have investigated the mechanism of this temperature sensitivity using reassortants between the two highly pathogenic strains A/FPV/Rostock/34 (FPV, H7N1) and A/turkey/England/63 (TE, H7N3). These reassortants show a strict correlation between the pathogenicity for chickens and the constellation of the genes coding for the ribonucleoprotein complex, RNP. Evidence is presented which shows that all viral components are synthesized in sufficient amounts and that the block in the viral replication cycle at the nonpermissive temperature is a late one affecting virus maturation. It is suggested that the RNP, although still enzymatically functional, may lose its ability to interact normally with viral surface components, thus interfering with the process of virus maturation. Some of the nonpathogenic reassortants which possessed the neuraminidase of TE showed an interesting temperature-dependent phenomenon: the haemagglutinin synthesized at the elevated temperature could only agglutinate erythrocytes at 20 degrees C, when the neuraminidase was inhibited or the infected cells vigorously disrupted by ultrasonication. This phenomenon is possibly not directly related to the temperature-sensitive block.

A reassortant between influenza A viruses (H7N2) synthesizing an enzymatically inactive neuraminidase at 40 degrees which is not incorporated into infectious particles

Cells infected with a reassortant (113/Ho, H7N2) between A/fowl plague/Rostock/34 (FPV, H7N1) and A/Hong Kong/1/68 (H3N2) carrying RNA segments 1 and 6 of the Hong Kong virus and the residual genes of FPV, synthesized at 40 degrees a neuraminidase (NA) which is enzymatically not active and which is not incorporated into infectious particles. At 40 degrees NA accumulates in the rough endoplasmic reticulum. It contains mainly carbohydrate side chains of the mannose type, and fucose is only scarcely incorporated. At 33 degrees NA of the reassortant is overproduced, and at least some of it is active and is incorporated into viral particles. Under nonreducing conditions during PAGE its NA migrates to the same position as after heating with mercaptoethanol, in contrast to the Hong Kong parent virus. It is speculated that at 40 degrees the tetramerization of the NA in the rough endoplasmic reticulum does not function, and in this way its migration to the cytoplasmic membrane and its incorporation into infectious particles does not occur. Since 113/Ho is as pathogenic for the chicken (body temperature of 41 degrees) as is FPV, the question arises which role the NA plays in virus replication and spread in the infected organism.

The nucleoprotein as a possible major factor in determining host specificity of influenza H3N2 viruses

In an attempt to assess the importance of the nucleoprotein (NP) in the determination of host specificity, a series of experiments was performed on influenza A viruses of the H3N2 subtype. We have examined rescue of mutants of A/FPV/Rostock/34 with temperature-sensitive (ts) lesions in the nucleoprotein (NP) gene by double infection of chick embryo cells with H3N2 strains isolated from different species. The ts mutants could be rescued by all avian H3N2 strains but not by any of the human H3N2 isolates. Only two of the swine H3N2 strains tested were able to rescue our mutants. The NP gene of these two swine isolates resembled the NP gene of the avian strains genetically in the hybridization test. However, their NPs reacted differently with a set of monoclonal antibodies when compared with NPs of avian H3N2 strains. Concerning multiplication in ducks they behaved like the other swine and human strains. The phosphopeptide fingerprints of all swine isolates tested were alike and were different from those of human or avian origin. Our observations are compatible with the idea that human H3N2 strains might not be able to cross the species barrier to birds directly, and possibly also not the other way around, without prior reassortment in pigs, which seem to have a broader host range concerning the compatibility of the NP gene in reassortants.

Oriented synthesis and cloning of influenza virus nucleoprotein cDNA that leads to its expression in mammalian cells

The influenza virus nucleoprotein gene has been cloned by a procedure that involves direct cDNA synthesis onto the primer-vector pBSV9, a pBR322-SV40 recombinant plasmid. dT-tailed pBSV9 was used to prime the synthesis of cDNA on a template of in vitro synthesized viral mRNA. The synthesis of ds-cDNA was initiated by a specific oligodeoxynucleotide and the resulting recombinant was circularized by intramolecular ligation. Recombinant pSVa963 contained the viral nucleoprotein gene directly fused to the SV40 early promoter region included in pBSV9 and followed by a dA:dT tail and the SV40 polyadenylation signal. When pSVa963 was used to transfect COS-1 cells, the presence of three NP-specific mRNAs of 1600, 1900 and 2500 nucleotides in length could be detected. Pulse labelling experiments of COS-1 transfected cells and immunobinding to a nucleoprotein monoclonal antibody indicated the synthesis of nucleoprotein. This nucleoprotein accumulated in the nucleus of transfected cells at a level similar to that found in infected cells. The vector and method described may be useful for the specific cloning and expression of any mRNA for which a 5'-terminal sequence is known.

Haemagglutinin transport mutants

Two mutants (ts1 and ts651) with a temperature sensitive defect in the intracellular transport of the haemagglutinin from the rough endoplasmic reticulum to the plasma membrane have been analysed. Nucleotide sequencing of the haemagglutinin revealed with each mutant two point mutations that are located in the stem region of the molecule.

Purification and enzymatic properties of an RNA polymerase-RNA complex from influenza virus

An RNA polymerase-viral RNA complex was purified from influenza A/PR/8 virions by combination of cesium trifluoroacetate centrifugation and phosphocellulose column chromatography. Surface proteins were removed from the detergent-treated virions by the centrifugation. Starting from the M protein-free ribonucleoprotein (RNP) fraction, an RNA polymerase-RNA complex lacking NP protein was isolated by repeated chromatography on phosphocellulose columns. The isolated RNA polymerase-RNA complex, which is composed of PB1, PB2, PA and vRNA, cleaved capped poly(A) endonucleolytically at 10-12 nucleotides from the 5' end and incorporated GMP into the 3' end of the resulting capped fragments. In the presence of all four ribonucleotide triphosphate substrates, the cleaved fragments were elongated to polynucleotides in the absence of exogenous vRNA. The RNA synthesis was primed not only by capped polynucleotides but also dinucleotide ApG. These results indicate that the purified RNA polymerase-RNA complex is as active in viral mRNAs synthesis as native RNP and that NP protein is not required for the catalytic function.

Sequence of the nucleoprotein gene of influenza A/parrot/Ulster/73

The nucleotide sequence of the nucleoprotein (NP) gene of the avian influenza A virus strain A/parrot/Ulster/73 (H7N1) has been determined. The gene (RNA segment 5) consists of 1565 bases. The only large open reading frame of the complementary RNA codes for a protein of 498 amino acids. A comparison of its sequence with that of three other influenza virus NPs shows that the NP of the parrot Ulster strain, although closely related to the NP of the other avian strain (A/FPV/Rostock/34), is definitely more closely related genetically to the NPs of the two human influenza strains, A/PR/8/34 and A/NT/60/68 than that of FPV. This raises the question how far the NP gene can cross the species barrier by reassortment and become adapted by mutation to the new host.

A mutant of fowl plague virus (influenza A) with an altered glycosylation pattern in its hemagglutinin

A temperature-sensitive mutant (ts 1/1) with a defect in the hemagglutinin (HA) gene, which was obtained by undiluted passage of fowl plague virus (FPV) at 33 degrees, is described. At 33 degrees proteolytic cleavage of the abnormal HA yielded an altered HA2 (XHA2) which migrated ahead of the NS1 protein and lacked the complex oligosaccharide side chain. At the nonpermissive temperature of 40 degrees, the migration of the HA of ts 1/1 from the rough endoplasmic reticulum (RER) via the Golgi apparatus to the cell surface was rate limiting for virus maturation. The HA was only slowly cleaved and migrated during polyacrylamide gel electrophoresis ahead of the HA of wild type FPV. Some revertants of ts 1/1 exhibited the same protein pattern as the mutant, others resembled wild type FPV, while one revertant gave rise to a mixture of HA2 and XHA2 at 40 degrees. These results suggest that (1) the loss of the complex oligosaccharide side chain is not responsible for the ts phenotype, (2) the mutation is presumably not at the site where the oligosaccharide side chain is linked to the protein backbone, and (3) ts 1/1 presumably carries a mutation located in RNA segment 4, which by pseudoreversion (suppressor mutation) in the same gene leads to different ts+ phenotypes.

Intracellular transport of influenza virus hemagglutinin to the apical surface of Madin-Darby canine kidney cells

The intracellular pathway followed by the influenza virus hemagglutinin (HA) to the apical surface of Madin-Darby canine kidney cells was studied by radioimmunoassay, immunofluorescence, and immunoelectron microscopy. To synchronize the migration, we used a temperature-sensitive mutant of influenza WSN, ts61, which, at the nonpermissive temperature, 39.5 degrees C, exhibits a defect in the HA that prevents its exit from the endoplasmic reticulum. Upon transfer to permissive temperature, 32 degrees C, the HA appeared in the Golgi apparatus after 10 min, and on the apical surface after 30-40 min. In the presence of cycloheximide, the expression was not inhibited, indicating that the ts defect is reversible; a wave of HA migrated to the cell surface, where it accumulated with a half time of 60 min. After passage through the Golgi apparatus the HA was detected in a population of smooth vesicles, about twice the size of coated vesicles, located in the apical half of the cytoplasm. These HA-containing vesicles did not react with anti-clathrin antibodies. Monensin (10 microM) delayed the surface appearance of HA by 2 h, but not the transport to the Golgi apparatus. Incubation at 20 degrees C retarded the migration to the Golgi apparatus by approximately 30 min and blocked the surface appearance by acting at a late stage in the intracellular pathway, presumably at the level of the post-Golgi vesicles. The initial appearance of HA on the apical surface was in the center; no preference was observed for the tight-junctional regions.

Correlation between loss of the temperature-sensitive phenotype and pathogenicity of fowl plague virus mutants in the chicken

The reversion of temperature-sensitive (ts) mutants of fowl plague virus to the ts+ phenotype was correlated with pathogenicity for chicken. Two types of ts mutants were investigated: those obtained by mutagenesis with 5-fluorouracil and those obtained by undiluted passages at 33 degrees C. The reversion frequency of the former mutants depended on the RNA segment in which the ts defect was located, mutations in RNA segments 1 and 2 having the highest reversion frequency, those in the RNA segments coding for the glycoproteins the lowest. ts mutants obtained by undiluted passages behaved differently in this respect. There was an approximate correlation between frequency of reversion and pathogenicity for chicken. Double mutants induced by 5-fluorouracil, having one tight and one leaky mutation, reverted easily without loss of the leaky mutation. These double mutants were still to a limited extent pathogenic for the chicken. Only one double mutant with two tight mutations (ts 293) was completely nonpathogenic after intramuscular inoculation. Two ts mutants with multiple tight defects (ts 1/1 and ts 3/18) obtained by undiluted passage did not revert to wild-type after injection into embryonated eggs and incubation at 33 degrees C, but they were still slightly pathogenic for the chicken. There was no obvious correlation between the shut-off temperature and pathogenicity of mutants carrying a single ts defect. However, for mutants with multiple tight mutations a high shut-off temperature seemed to be essential for reversion during serial passages as well as for pathogenicity in the chicken, when different routes of inoculation were examined. ts mutants seem to be safe as live vaccines only, (1) if they carry at least two tight ts defects, (2) if they have a relatively low shut-off temperature, and (3) if they could be administered other than via the respiratory tract.

Functional defects of fowl plague virus temperature-sensitive mutant having mutation in the neuraminidase

A fowl plague virus (FPV) temperature-sensitive mutant ts 5 having mutation lesions in the gene coding for the neuraminidase has been obtained. The mutant induced synthesis of cRNA, vRNA and proteins in cells under non-permissive conditions, but formation of virions including non-infectious ones was defective. The neuraminidase and haemagglutinin synthesized under non-permissive conditions possessed functional activity and could migrate from the rough endoplasmic reticulum into plasma membranes; however, cleavage of the haemagglutinin was reduced. In ts 5-infected cells under non-permissive conditions the synthesis of segments 5 and 8 of cRNA and vRNA was predominant both early and late in the reproduction cycle, and the synthesis of P1, P2, P3, HA and M proteins was reduced after approximately 3 hours. The data obtained suggest that involvement of the neuraminidase in the formation of infectious virions may have no direct association with the enzymatic activity of this protein, and that the mutation in the neuraminidase may affect regulation of replication and transcription processes.

Molecular biology of rotaviruses. III. Isolation and characterization of temperature-sensitive mutants of bovine rotavirus

Twenty-six temperature-sensitive (ts) mutants of United Kingdom tissue culture-adapted bovine rotavirus were isolated and characterized. Fourteen of these mutants were determined to be ts both by efficiency of plating and by virus yield at the nonpermissive temperature of 39.5 degrees C as compared with that at the permissive temperature of 32 degrees C. The remaining mutants were only ts by the criterion of efficiency of plating. High-frequency recombination (gene reassortment) was observed when some pairs of mutants were crossed, and this allowed the classification of the mutants into five separate recombination groups. Groups III and V have prototype ts mutants (ts34 and ts115, respectively) that do not synthesize RNA or polypeptides at 39.5 degrees C. The other groups, I, II, and IV, have prototype mutants (ts17, ts7, and ts6, respectively) that synthesize both RNA and polypeptides at 39.5 degrees C, although ts17 does so only at a reduced level.

Intrinsic interference between swine influenza and fowl plague virus

Multiplication of swine influenza (SW) virus is inhibited by fowl plaque virus (FPV) at the level of RNA synthesis when host cells are infected with both viruses at a high multiplicity of infection. Under these conditions reassortment between the two viruses cannot be detected. The inhibitory effect of FPV is highly reduced and recombinants between the two viruses could be obtained when the cells were superinfected with FPV 1--2 hours after infection with SW virus, or after simultaneous infection with a low multiplicity of infection. The phenomenon is compatible with the intrinsic interference.

On the origin of the gene coding for an influenze A virus nucleocapsid protein

The gene coding for the nucleocapsid protein NP of the influenza A virus recombinant strain 413 1,1 was characterized biochemically by molecular hybridization and fingerprint analysis. The data presented suggest that this NP gene has evolved by intracistronic recombination between NP genes of virus N and the fowl plague virus temperature-sensitive mutants ts 19.

Influenza virus-specific RNA and protein syntheses in cells infected with temperature-sensitive mutants defective in the genome segment encoding nonstructural proteins

Virus-specific protein and RNA syntheses have been analyzed in chicken embryo fibroblast cells infected with two group IV temperature-sensitive (ts) mutants of influenza A (fowl plague) virus in which the ts lesion maps in RNA segment 8 (J. W. Almond, D. McGeoch, and R. D. Barry, Virology 92:416-427, 1979), known to code to code for two nonstructural proteins, NS1 and NS2. Both mutants induced the synthesis of similar amounts of all the early virus-specific proteins (P1, P2, P3, NP, and NS1) at temperatures that were either permissive (34 degrees C) or nonpermissive (40.5 degrees C) for replication. However, the synthesis of M protein, which normally accumulates late in infection, was greatly reduced in ts mutant-infected cells at 40.5 degrees C compared to 34 degrees C. The NS2 protein was not detected at either temperature in cells infected with one mutant (mN3), and was detected only at the permissive temperature in cells infected with mutant ts47. There was no overall reduction in polyadenylated (A+) complementary RNA, which functions as mRNA, in cells infected with these mutants at 40.5 degrees C compared to 34 degrees C, nor was there any evidence of selective accumulation of this type of RNA within the nucleus at the nonpermissive temperature. No significant differences in ts mutant virion RNA transcriptase activity were detected by assays in vitro at 31 and 40.5 degrees C compared to wild-type virus. Virus-specific non-polyadenylated (A-) complementary RNA, which is believed to act as the template for new virion RNA production, accumulated normally in cells at both 34 and 40.5 degrees C, but at 40.5 degrees C accumulation of new virion RNA was reduced by greater than 90% when compared to accumulation at 34 degrees C.