Temperature-sensitive mutants of fowl plague virus: isolation and genetic characterization

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.

Defective RNA replication and late gene expression in temperature-sensitive influenza viruses expressing deleted forms of the NS1 protein

Influenza A virus mutants expressing C-terminally deleted forms of the NS1 protein (NS1-81 and NS1-110) were generated by plasmid rescue. These viruses were temperature sensitive and showed a small plaque size at the permissive temperature. The accumulation of virion RNA in mutant virus-infected cells was reduced at the restrictive temperature, while the accumulation of cRNA or mRNA was not affected, indicating that the NS1 protein is involved in the control of transcription versus replication processes in the infection. The synthesis and accumulation of late virus proteins were reduced in NS1-81 mutant-infected cells at the permissive temperature and were essentially abolished for both viruses at the restrictive temperature, while synthesis and accumulation of nucleoprotein (NP) were unaffected. Probably as a consequence, the nucleocytoplasmic export of virus NP was strongly inhibited at the restrictive temperature. These results indicate that the NS1 protein is essential for nuclear and cytoplasmic steps during the virus cycle.

Amino acid replacements leading to temperature-sensitive defects of the NS1 protein of influenza A virus

The nonstructural (NS) genes of two influenza virus temperature-sensitive (ts) reassortants have been sequenced and compared with the corresponding wild type sequences. Ts 412 has a single base substitution (G100-->A) leading to an amino acid replacement (Arg 25-->Lys) in the NS1 protein. Ts 451 also has a single base substitution (U273-->C) leading to an amino acid replacement (Ser 83-->Pro) in the NS1 protein. In ts 412 infected cells at the nonpermissive temperature very little M1 and HA mRNA and proteins are synthesized, suggesting that NS1 is involved in a transcriptional regulation process. The ts mutation in ts 451 could be extragenically suppressed by replacement of the PB1 and/or PA protein genes of the mutant by the allelic genes of PR8. Both observations suggest that NS1 cooperates with the polymerase complex.

The critical cut-off temperature of avian influenza viruses

We have measured the pathogenicity for 6-week-old chicks of infection by H7 avian influenza viruses. One virus, strain S3 from A/FPV/Rostock/34(H7N1) showed a temperature sensitive phenotype at 41.5 degrees C and reduced pathogenicity. By analysis of reassortants made between virus S3 and A/FPV/Dobson/27(H7N7), a fully pathogenic virus, two conclusions arise. (1) The critical cut-off temperature for avian influenza virus in 6-week-old chicks is 41.5 degrees. (2) RNA segment 1 of virus S3 is responsible for the lack of pathogenicity in reassortant viruses. Nucleotide sequencing of RNA segment 1 from S3 and its parent, A/FPV/Rostock/34 has revealed a single mutation at nucleotide 1561. This results in a substitution of isoleucine for leucine at amino acid position 512 in the cap-binding protein, PB2.

The role of RNA segment 1 in an in vitro host restriction occurring in an avian influenza virus mutant

A temperature sensitive mutant, ts C47, derived from A/FPV/Rostock/34 and with a ts mutation in RNA segment 8, fails to form plaques in MDCK cells. From data obtained with reassortant viruses using the human influenza isolate A/FM/1/47 it was apparent that more than one mutation contributed to the temperature-sensitive (ts) and host range (hr) phenotypes of ts C47, and the phenotype of reassortants containing RNA segment 1 from A/FM/1/47 indicated that this segment was involved. A single nucleotide substitution at nucleotide 1961, resulting in valine instead of methionine in the predicted amino acid sequence of polypeptide PB2, was found in RNA segment 1 of ts C47, but this mutation did not segregate with the attenuated phenotype on gene reassortment. The following conclusions are drawn: (a) that ts C47 has at least two mutations in addition to that already known to exist in RNA segment 8, one of which (that in RNA segment 1) does not contribute to the observed ts hr phenotypes and (b) that the hr phenotype can be suppressed by substitution of RNA segment 1 by that of another strain.

Novel polypeptides encoded by influenza virus subgenomic (DI type) virion RNAs

We have isolated a ts mutant of influenza A/FPV/Rostock/34 that induces the synthesis of a novel small polypeptide in infected cells. This polypeptide is encoded by a subgenomic virion RNA derived from RNA segment 3, apparently by internal deletion. A second polypeptide, similarly derived from RNA segment 1, was found only after in vitro translation of infected cell RNA. The subgenomic vRNAs we describe are probably similar to those found in influenza DI virus preparations. The possible role of 'subgenomic' polypeptides in DI virus-mediated interference is discussed.

Capped mRNAs may stimulate the influenza virion polymerase by allosteric modulation

Analogues of the mRNA 5'-terminal methyl cap structure were found to stimulate the influenza virion RNA-dependent RNA polymerase. The single nucleotide analogue m7GMP was incorporated into RNA during transcription in vitro, and the stimulatory effect was not additive with the primer ApG, suggesting that m7GMP stimulates the virion polymerase by priming virus-specific mRNA synthesis, as has been shown for ApG. By contrast, stimulation by m7G(5')ppp(5')m6AM2-O was additive with that by ApG, and we could not demonstrate incorporation of the similar analogue m7G(5')ppp(5')Am2-O into RNA during transcription. We propose that these dinucleotide cap analogues stimulate the virion polymerase by allosteric modulation, independent of priming. This stimulation can be abolished by mutation, without loss of other activities associated with the cap-dependent endonuclease.

Characterization of an influenza A host range mutant

A mixed infection of primary chick kidney cells at 38 degrees with A/Ann Arbor/6/60 cold adapted virus and A/Alaska/6/77 wt virus yielded a cold-reassortant virus, CR43-clone 3, which had a host range different from that of either parent. It does not produce detectable virus when grown in Madin-Darby canine kidney cells, while growing normally in primary chick kidney cells at 33 degrees. Both parents, however, grow well in either cell type at 33 degrees C. Genotypic analysis of viral RNA electrophoresed in polyacrylamide gels has shown that CR43-clone 3 virus has an aberrant NS gene different from the NS gene of either parent virus. Reassortant viruses made between CR43-clone 3 virus and A/California/10/78 (H1N1) virus in primary chick kidney cells at 33 degrees showed the same host range restriction only if the NS gene was derived from the CR43-clone 3 virus. A mixed infection with these same parents, but in Madin-Darby canine kidney cells at 33 degrees C, produced reassortants that always contained the A/California/10/78 NS gene instead of the CR43-clone 3 NS gene. Ferrets inoculated intranasally with the CR43-clone 3 reassortant do not become sick or infected, based on the lack of symptoms: no rhinitis, coryza, or fever; and no detectable virus recovered from nasopharyngeal swabs, turbinate, or lung tissues at 48 hr after infection. Thus, CR43-clone 3 virus contains an aberrant NS gene and manifests a restricted host range phenotype in Madin-Darby canine kidney cells and ferrets.

Sequence analysis of fowl plague virus mutant ts47 reveals a nonsense mutation in the NS1 gene

A mutant of fowl plague virus, ts47, induces the synthesis in infected cells of a truncated NS1 polypeptide at both permissive and restrictive temperatures. Nucleotide sequence analysis of the segment coding for the NS1 polypeptide, segment 8, indicates that this aberration is due to a nonsense mutation. This mutation occurs in the region of the NS1 gene which overlaps with the NS2 gene and there is a corresponding amino acid substitution in the NS2 polypeptide. While it is not clear which polypeptide is responsible for the thermal instability of ts47, the loss of the COOH-terminal 28 amino acid residues from the NS1 polypeptide does not affect replication of the virus at permissive temperatures.

Genetic effects of N-methyl-N'-nitro-N-nitrosoguanidine and its homologs

Since the discovery of the mutagenic activity of N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) in 1960, this compound has become one of the most widely used chemical mutagens. The present paper gives a survey on the chemistry, metabolism, and mode of interaction of MNNG with DNA and proteins, and of the genotoxic effects of this agent on microorganisms, plants, and animals, including human cells cultured in vitro. Data on the carcinogenicity and teratogenicity of MNNG as well as on the genotoxic effects of homologs of MNNG are also presented.

Nucleotide sequence of human influenza A/PR/8/34 segment 2

The nucleotide sequence of RNA segment 2 of human influenza strain A/PR/8/34 has been determined. Segment 2 in 2341 nucleotides long and encodes a protein of 757 amino acids (86,500 daltons molecular weight) which is involved in RNA synthesis. Although segment 2 is identical in size to segment 1, which encodes a protein of related function, neither the nucleotide sequences of these two RNA segments nor the amino acid sequences of the encoded proteins appear to be homologous. The sequence of segment 2 completes the sequence of the virus (total 13,588 nucleotides).

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.