Patent Publication Number: US-2006019350-A1

Title: Recombinant negative strand RNA virus expression system and vacccines

Description:
This is a continuation of application Ser. No. 09/724,427 filed Nov. 28, 2000, which is a continuation-in-part of application Ser. No. 08/190,698, filed Feb. 1, 1994, which is a continuation of application Ser. No. 07/925,061, filed Aug. 4, 1992 (abandoned), which is a divisional of application Ser. No. 07/527,237, filed May 22, 1990 (issued as U.S. Pat. No. 5,166,057), which is a continuation-in-part of application Ser. No. 07/440,053, filed Nov. 21, 1989 (abandoned), which is a continuation-in-part of application Ser. No. 07/399,728, filed Aug. 28, 1989 (now abandoned). 
    
    
     1. INTRODUCTION  
      The present invention relates to recombinant negative strand virus RNA templates which may be used to express heterologous gene products in appropriate host cell systems and/or to construct recombinant viruses that express, package, and/or present the heterologous gene product. The expression products and chimeric viruses may advantageously be used in vaccine formulations.  
      The invention is demonstrated by way of examples in which recombinant influenza virus RNA templates containing a heterologous gene coding sequences in the negative-polarity were constructed. These recombinant templates, when combined with purified viral RNA-directed RNA polymerase, were infectious, replicated in appropriate host cells, and expressed the heterologous gene product at high levels. In addition, the heterologous gene was expressed and packaged by the resulting recombinant influenza viruses.  
     2. BACKGROUND OF THE INVENTION  
      A number of DNA viruses have been genetically engineered to direct the expression of heterologous proteins in host cell systems (e.g., vaccinia virus, baculovirus, etc.). Recently, similar advances have been made with positive-strand RNA viruses (e.g., poliovirus). The expression products of these constructs, i.e., the heterologous gene product or the chimeric virus which expresses the heterologous gene product, are thought to be potentially useful in vaccine formulations (either subunit or whole virus vaccines). One drawback to the use of viruses such as vaccinia for constructing recombinant or chimeric viruses for use in vaccines is the lack of variation in its major epitopes. This lack of variability in the viral strains places strict limitations on the repeated use of chimeric vaccinia, in that multiple vaccinations will generate host-resistance to the strain so that the inoculated virus cannot infect the host. Inoculation of a resistant individual with chimeric vaccina will, therefore, not induce immune stimulation.  
      By contrast, influenza virus, a negative-strand RNA virus, demonstrates a wide variability of its major epitopes. Indeed, thousands of variants of influenza have been identified; each strain evolving by antigenic drift. The negative-strand viruses such as influenza would be attractive candidates for constructing chimeric viruses for use in vaccines because its genetic variability allows for the construction of a vast repertoire of vaccine formulations which will stimulate immunity without risk of developing a tolerance. However, achieving this goal has been precluded by the fact that, to date, it has not been possible to construct recombinant or chimeric negative-strand RNA particles that are infectious.  
     2.1. The Influenza Virus  
      Virus families containing enveloped single-stranded RNA of the negative-sense genome are classified into groups having non-segmented genomes (Paramyxoviridae, Rhabdoviridae) or those having segmented genomes (Orthomyxoviridae, Bunyaviridae and Arenaviridae). The Orthomyxoviridae family, described in detail below, and used in the examples herein, contains only the viruses of influenza, types A, B and C.  
      The influenza virions consist of an internal ribonucleoprotein core (a helical nucleocapsid) containing the single-stranded RNA genome, and an outer lipoprotein envelope lined inside by a matrix protein (M). The segmented genome of influenza A consists of eight molecules (seven for influenza C) of linear, negative polarity, single-stranded RNAs which encode ten polypeptides, including: the RNA-directed RNA polymerase proteins (PB2, PB1 and PA) and nucleoprotein (NP) which form the nucleocapsid; the matrix proteins (M1, M2); two surface glycoproteins which project from the lipoprotein envelope: hemagglutinin (HA) and neuraminidase (NA); and nonstructural proteins whose function is unknown (NS1 and NS2). Transcription and replication of the genome takes place in the nucleus and assembly occurs via budding on the plasma membrane. The viruses can reassort genes during mixed infections.  
      Influenza virus adsorbs via HA to sialyloligosaccharides in cell membrane glycoproteins and glycolipids. Following endocytosis of the virion, a conformational change in the HA molecule occurs within the cellular endosome which facilitates membrane fusion, thus triggering uncoating. The nucleocapsid migrates to the nucleus where viral mRNA is transcribed as the essential initial event in infection. Viral mRNA is transcribed by a unique mechanism in which viral endonuclease cleaves the capped 5′-terminus from cellular heterologous mRNAs which then serve as primers for transcription of viral RNA templates by the viral transcriptase. Transcripts terminate at sites 15 to 22 bases from the ends of their templates, where oligo(U) sequences act as signals for the template-independent addition of poly(A) tracts. Of the eight viral mRNA molecules so produced, six are monocistronic messages that are translated directly into the proteins representing HA, NA, NP and the viral polymerase proteins, PB2, PB1 and PA. The other two transcripts undergo splicing, each yielding two mRNAs which are translated in different reading frames to produce M1, M2, NS1 and NS2. In other words, the eight viral mRNAs code for ten proteins: eight structural and two nonstructural. A summary of the genes of the influenza virus and their protein products is shown in Table I below.  
               TABLE I                          INFLUENZA VIRUS GENOME RNA SEGMENTS AND CODING       ASSIGMENTS a                                               En-                       Length b     coded       Mole-           (Nu-   Poly-   Length d     cules       Seg-   cleo-   pep-   (Amino   Per       ment   tides)   tide c     Acids)   Virion   Comments                                             1   2341   PB2   759   30-60   RNA transcriptase                           component; host cell RNA                           cap binding       2   2341   PB1   757   30-60   RNA transcriptase                           component; initiation of                           transcription;                           endonuclease activity?       3   2233   PA   716   30-60   RNA transcriptase                           component; elongation of                           mRNA chains?       4   1778   HA   566    500   Hemagglutinin; trimer;                           envelope glycoprotein;                           mediates attachment to                           cells       5   1565   NP   498   1000   Nucleoprotein; associated                           with RNA; structural                           component of RNA                           transcriptase       6   1413   NA   454    100   Neuraminidase; tetramer;                           envelope glycoprotein       7   1027   M 1     252   3000   Matrix protein; lines                           inside of envelope               M 2     96       Structural protein in                           plasma membrane; spliced                           mRNA               ?   ?9       Unidentified protein       8   890   NS 1     230       Nonstructural protein;                           function unknown               NS 2     121       Nonstructural protein;                           function unknown; spliced                           mRNA                   a Adapted from R. A. Lamb and P. W. Choppin (1983), Reproduced from the Annual Review of Biochemistry, Volume 52, 467-506.              b For A/PR/8/34 strain              c Determined by biochemical and genetic approaches              d Determined by nucleotide sequence analysis and protein sequencing             
 
      Following transcription, virus genome replication is the second essential event in infection by negative-strand RNA viruses. As with other negative-strand RNA viruses, virus genome replication in influenza is mediated by virus-specified proteins. It is hypothesized that most or all of the viral proteins that transcribe influenza virus mRNA segments also carry out their replication. All viral RNA segments have common 3′ and 5′ termini, presumably to enable the RNA-synthesizing apparatus to recognize each segment with equal efficiency. The mechanism that regulates the alternative uses (i.e., transcription or replication) of the same complement of proteins (PB2, PB1, PA and NP) has not been clearly identified but appears to involve the abundance of free forms of one or more of the nucleocapsid proteins, in particular, the NP. The nucleus appears to be the site of virus RNA replication, just as it is the site for transcription.  
      The first products of replicative RNA synthesis are complementary copies (i.e., plus-polarity) of all influenza virus genome RNA segments (cRNA). These plus-stranded copies (anti-genomes) differ from the plus-strand mRNA transcripts in the structure of their termini. Unlike the mRNA transcripts, the anti-genomic cRNAs are not capped and methylated at the 5′ termini, and are not truncated and polyadenylated at the 3′ termini. The cRNAs are coterminal with their negative strand templates and contain all the genetic information in each genomic RNA segment in the complementary form. The cRNAs serve as templates for the synthesis of genomic negative-strand vRNAs.  
      The influenza virus negative strand genomes (vRNAs) and antigenomes (cRNAs) are always encapsidated by nucleocapsid proteins; the only unencapsidated RNA species are virus mRNAs. In contrast to the other enveloped RNA viruses, nucleocapsid assembly appears to take place in the nucleus rather than in the cytoplasm. The virus matures by budding from the apical surface of the cell incorporating the M protein on the cytoplasmic side or inner surface of the budding envelope. The HA and NA become glycosylated and incorporated into the lipid envelope. In permissive cells, HA is eventually cleaved, but the two resulting chains remain united by disulfide bonds.  
      It is not known by what mechanism one copy of each of the eight genomic viral RNAs is selected for incorporation into each new virion. Defective interfering (DI) particles are often produced, especially following infection at high multiplicity.  
     2.2. RNA Directed RNA Polymerase  
      The RNA-directed RNA polymerases of animal viruses have been extensively studied with regard to many aspects of protein structure and reaction conditions. However, the elements of the template RNA which promote optimal expression by the polymerase could only be studied by inference using existing viral RNA sequences. This promoter analysis is of interest since it is unknown how a viral polymerase recognizes specific viral RNAs from among the many host-encoded RNAs found in an infected cell.  
      Animal viruses containing plus-sense genome RNA can be replicated when plasmid-derived RNA is introduced into cells by transfection (for example, Racaniello et al., 1981, Science 214:916-919; Levis, et al., 1986, Cell 44: 137-145). In the case of poliovirus, the purified polymerase will replicate a genome RNA in in vitro reactions and when this preparation is transfected into cells it is infectious (Kaplan, et al., 1985, Proc. Natl. Acad. Sci. USA 82:8424-8428). However, the template elements which serve as transcription promoter for the poliovirus-encoded polymerase are unknown since even RNA homopolymers can be copied (Ward, et al., 1988, J. Virol. 62: 558-562). SP6 transcripts have also been used to produce model defective interfering (DI) RNAs for the Sindbis viral genome. When the RNA is introduced into infected cells, it is replicated and packaged. The RNA sequences which were responsible for both recognition by the Sindbis viral polymerase and packaging of the genome into virus particles were shown to be within 162 nucleotides (nt) of the 5′ terminus and 19 nt of the 3′ terminus of the genome (Levis, et al., 1986, Cell 44: 137-145). In the case of brome mosaic virus (BMV), a positive strand RNA plant virus, SP6 transcripts have been used to identify the promoter as a 134 nt tRNA-like 3′ terminus (Dreher, and Hall, 1988, J. Mol. Biol. 201: 31-40). Polymerase recognition and synthesis were shown to be dependent on both sequence and secondary structural features (Dreher, et al., 1984, Nature 311: 171-175).  
      The negative-sense RNA viruses have been refractory to study of the sequence requirements of the replicase. The purified polymerase of vesicular stomatitis virus is only active in transcription when virus-derived ribonucleoprotein complexes (RNPs) are included as template (De and Banerjee, 1985, Biochem. Biophys. Res. Commun. 126: 40-49; Emerson and Yu, 1975, J. Virol. 15: 1348-1356; Naito, and Ishihama, 1976, J. Biol. Chem. 251: 4307-4314). RNPs have been reconstituted from naked RNA of VSV DI particles using infected cell extracts as protein source. These RNPs were then replicated when added back to infected cells (Mirakhur, and Peluso, 1988, Proc. Natl. Acad. Sci. USA 85: 7511-7515). With regard to influenza viruses, it was recently reported that naked RNA purified from virus was used to reconstitute RNPs. The viral nucleocapsid and polymerase proteins were gel-purified and renatured on the viral RNA using thioredoxin (Szewczyk, et al., 1988, Proc. Natl. Acad. Sci. USA, 85: 7907-7911). However, these authors did not show that the activity of the preparation was specific for influenza viral RNA, nor did they analyze the signals which promote transcription.  
      During the course of influenza virus infection the polymerase catalyzes three distinct transcription activities. These include the synthesis of (a) subgenomic mRNA, which contains a 5′ cap and a 3′ poly-A tail; (b) a full length plus-strand or anti-genome (cRNA) copied from the genome RNA; and (c) genomic vRNA synthesized from the full length cRNA (reviewed in Ishihama and Nagata, 1988, CRC Crit. Rev. Biochem. 23: 27-76; and Krug, Transcription and replication of influenza viruses. In: Genetics of influenza viruses, Ed., Palese, P. and Kingsbury, D. W. New York, Springer-Verlag, 1983, p. 70-98). Viral proteins PB2, PB1 and PA are thought to catalyze all influenza virus-specific RNA synthesis when in the presence of excess nucleocapsid protein (NP; see above reviews). These polymerase functions have been studied using RNP cores derived from detergent-disrupted virus, and RNPs from the nuclear extracts of infected cells. Transcription from the RNPs derived from disrupted virus occurs when primed with either dinucleotide adenylyl-(3′-5′)-guanosine (ApG) or capped mRNAs. The plus sense mRNA products have terminated synthesis 17-20 nucleotides upstream of the 5′ terminus of the RNA template and have been processed by the addition of poly A tails. These products cannot serve as template for the viral-sense genome since they lack terminal sequences (Hay, et al., 1977, Virology 83: 337-355). RNPs derived from nuclear extracts of infected cells also synthesize polyadenylated mRNA in the presence of capped RNA primers. However, if ApG is used under these conditions, both RNAs, polyadenylated and full length cRNA, can be obtained (Beaton and Krug, 1986, Proc. Natl. Acad. Sci. USA 83: 6282-6286; Takeuchi, et al., 1987, J. Biochem. 101: 837-845). Recently it was shown that replicative synthesis of cRNA could occur in the absence of exogenous primer if the nuclear extract was harvested at certain times post infection. In these same preparations the synthesis of negative-sense vRNA from a cRNA template was also observed (Shapiro and Krug, 1988, J. Virol. 62: 2285-2290). The synthesis of full length cRNA was shown to be dependent upon the presence of nucleocapsid protein (NP) which was free in solution (Beaton and Krug, 1986, Proc. Natl. Acad. Sci. USA 83: 6282-6286; Shapiro and Krug, 1988, J. Virol. 62: 2285-2290). These findings led to the suggestion that the regulatory control between mRNA and cRNA synthesis by the RNP complex is based on the requirement for there being an excess of soluble NP (Beaton and Krug, 1986, Proc. Natl. Acad. Sci. USA 83: 6282-6286).  
      Another line of investigation has focused on the preparation of polymerase-RNA complexes derived from RNPs from detergent-disrupted virus. When the RNP complex is centrifuged through a CsCl-glycerol gradient, the RNA can be found associated with the three polymerase (P) proteins at the bottom of the gradient. Near the top of the gradient, free NP protein can be found (Honda, et al., 1988, J. Biochem. 104: 1021-1026; Kato, et al., 1985, Virus Research 3, 115-127). The purified polymerase-RNA complex (bottom of gradient), is active in initiating ApG-primed synthesis of RNA, but fails to elongate to more than 12-19 nucleotides. When fractions from the top of the gradient containing the NP protein are added back to the polymerase-RNA complex, elongation can ensue (Honda, et al., 1987, J. Biochem. 102: 41-49). These data suggest that the NP protein is needed for elongation, but that initiation can occur in the absence of NP.  
      It has been shown that the genomic RNA of influenza viruses is in a circular conformation via base-pairing of the termini to form a panhandle of 15 to 16 nt (Honda, et al., 1988, J. Biochem. 104: 1021-1026; Hsu, et al., 1987, Proc. Natl. Acad. Sci. USA 84: 8140-8144). Since the viral polymerase was found bound to the panhandle, this led to the suggestion that a panhandle structure was required for recognition by the viral polymerase (Honda, et al., 1988, J. Biochem. 104: 1021-1026.) Therefore, it was hypothesized in these two reports that the promoter for the viral RNA polymerase was the double stranded RNA in panhandle conformation.  
     3. SUMMARY OF THE INVENTION  
      Recombinant negative-strand viral RNA templates are described which may be used with purified RNA-directed RNA polymerase complex to express heterologous gene products in appropriate host cells and/or to rescue the heterologous gene in virus particles. The RNA templates are prepared by transcription of appropriate DNA sequences with a DNA-directed RNA polymerase. The resulting RNA templates are of the negative-polarity and contain appropriate terminal sequences which enable the viral RNA-synthesizing apparatus to recognize the template. Bicistronic mRNAs can be constructed to permit internal initiation of translation of viral sequences and allow for the expression of foreign protein coding sequences from the regular terminal initiation site, or vice versa.  
      As demonstrated by the examples described herein, recombinant negative-sense influenza RNA templates may be mixed with purified viral polymerase proteins and nucleoprotein (i.e., the purified viral polymerase complex) to form infectious recombinant RNPs. These can be used to express heterologous gene products in host cells or to rescue the heterologous gene in virus particles by cotransfection of host cells with recombinant RNPs and virus. Alternatively, the recombinant RNA templates or recombinant RNPs may be used to transfect transformed cell lines that express the RNA dependent RNA-polymerase and allow for complementation. Additionally, a non-virus dependent replication system for influenza virus is also described. Vaccinia vectors expressing influenza virus polypeptides were used as the source of proteins which were able to replicate and transcribe synthetically derived RNPs. The minimum subset of influenza virus protein needed for specific replication and expression of the viral RNP was found to be the three polymerase proteins (PB2, PB1 and PA) and the nucleoprotein (NP). This suggests that the nonstructural proteins, NS1 and NS2, are not absolutely required for the replication and expression of viral RNP.  
      The expression products and/or chimeric virions obtained may advantageously be utilized in vaccine formulations. The use of recombinant influenza for this purpose is especially attractive since influenza demonstrates tremendous strain variability allowing for the construction of a vast repertoire of vaccine formulations. The ability to select from thousands of influenza variants for constructing chimeric viruses obviates the problem of host resistance encountered when using other viruses such as vaccinia. In addition, since influenza stimulates a vigorous secretory and cytotoxic T cell response, the presentation of foreign epitopes in the influenza virus background may also provide for the induction of secretory immunity and cell-mediated immunity.  
     3.1. Definitions  
      As used herein, the following terms will have the meanings indicated:  
      cRNA=anti-genomic RNA  
      HA=hemagglutinin (envelope glycoprotein)  
      M=matrix protein (lines inside of envelope)  
      MDCK=Madin Darby canine kidney cells  
      MDBK=Madin Darby bovine kidney cells  
      moi=multiplicity of infection  
      NA=neuraminidase (envelope glycoprotein)  
      NP=nucleoprotein (associated with RNA and required for polymerase activity)  
      NS=nonstructural protein (function unknown)  
      nt=nucleotide  
      PA, PB1, PB2=RNA-directed RNA polymerase components  
      RNP=ribonucleoprotein (RNA, PB2, PB1, PA and NP)  
      rRNP=recombinant RNP  
      vRNA=genomic virus RNA viral polymerase complex ═PA, PB1, PB2 and NP  
      WSN=influenza A/WSN/33 virus  
      WSN-HK virus: reassortment virus containing seven genes from WSN virus and the NA gene from influenza A/HK/8/68 virus  
    
    
     4. DESCRIPTION OF THE FIGURES  
       FIG. 1 . Purification of the polymerase preparation. RNP cores were purified from whole virus and then subjected to CsCl-glycerol gradient centrifugation. The polymerase was purified from fractions with 1.5 to 2.0 M CsCl. Samples were then analyzed by polyacrylamide gel electrophoresis on a 7-14% linear gradient gel in the presence of 0.1% sodium dodecylsulfate followed by staining with silver. Protein samples contained 1.4 μg whole virus (lane 1), 0.3 μg whole virus (lane 2), 5 μl of RNP cores (lane 3) and 25 μl RNA polymerase (lane 4). Known assignments of the proteins are indicated at the left.  
       FIG. 2 . Plasmid constructs used to prepare RNA templates. The plasmid design is depicted with the solid box representing pUC-19 sequences, the hatched box represents the truncated promoter specifically recognized by bacteriophage T7 RNA polymerase, the solid line represents the DNA which is transcribed from plasmids which have been digested with MboII. The white box represents sequences encoding the recognition sites for MboII, EcoRI and PstI, in that order. Sites of cleavage by restriction endonucleases are indicated. Beneath the diagram, the entire sequences of RNAs which result from synthesis by T7 RNA polymerase from MboII-digested plasmid are given. The V-wt RNA (SEQ ID NO.: 49) has the identical 5′ and 3′ termini as found in RNA segment 8 of influenza A viruses, separated by 16 “spacer” nucleotides. The RNA, M-wt (SEQ ID NO.: 50), represents the exact opposite stand, or “message-sense”, of V-wt (SEQ ID NO.: 49). Restriction endonuclease sites for DraI, EcoRI, PstI and SmaI are indicated. T7 transcripts of plasmids cleaved by these enzymes result in, respectively, 32, 58, 66 and 91 nucleotide long RNAs. The sequences of V-d5′ (SEQ ID NO.: 51) RNA are indicated. The plasmid design is essentially the same as that used for the V-wt RNA (SEQ ID NO.: 49) except for the minor changes in the “spacer” sequence. The point mutants of V-d5′ RNAs which were studied are indicated in Table I.  
       FIG. 3 . Analysis of products of influenza viral polymerase.  FIG. 3A : Polymerase reaction mixtures containing 0.4 mM ApG (lane 2) or no primer (lane 3) were electrophoresed on 8% polyacrylamide gels containing 7.7 M urea.  FIG. 3B : The nascent RNA is resistant to single-stranded specific nuclease S1. Following the standard polymerase reaction, the solutions were diluted in nuclease S1 buffer (lane 1) and enzyme was added (lane 2). As control for S1 digestion conditions, radioactively labeled single-stranded V-wt RNA was treated with nuclease S1 (lane 3) or with buffer alone (lane 4).  FIG. 3C : Ribonuclease T1 analayis of gel-purified reaction products. The reaction products of the viral polymerase using the V-wt RNA template was subjected to electrophoresis on an 8% polyacrylamide gel. The 53 nt band and the smaller transcript were excised and eluted from the gel matrix. These RNAs were digested with RNAse T1 and analyzed by electrophoresis on a 20% polyacrylamide gel containing 7.7 M urea. For comparison, T7 transcripts of M-wt and V-wt RNAs which had been synthesized in the presence of a α- 32 P-UTP were also analyzed with RNAse T1. The predicted radiolabeled oliognucleotides of the control RNAs are indicated. Lane 1, 53 nucleotide full length (FL) product; lane 2, 40-45 nucleotide smaller (Sm) RNA product; lane 3, M-wt RNA labeled by incorporation of  32 P-UMP; and lane 4, V-wt RNA labeled as in lane 3.  
       FIG. 4 . Optimal reaction conditions for the viral polymerase.  FIG. 4A : Reactions with V-wt template were assembled on ice and then incubated at the indicated temperatures for 90 minutes.  FIG. 4B : Reactions with the V-wt template were prepared in parallel with the indicated NaCl or KCl concentrations and were incubated at 30° C. for 90 minutes.  FIG. 4C : A single reaction with the V-wt template was incubated at 30° C., and at the indicated times, samples were removed and immediately processed by phenol-chloroform extraction. All gels contained 8% polyacrylamide with 7.7 M urea.  
       FIG. 5 . Template specificity of the viral polymerase.  FIG. 5A : The viral polymerase reaction requires 3′ terminal promoter sequences. Different template RNAs were used in reactions under standard conditions. Lane 1, the V-Pst RNA, which is identical to V-wt except it has a 13 nt extension at the 3′ end; lane 2, V-Sma RNA, which has a 38 nt extension at the 3′ end; lane 3, V-wt RNA; lane 4, a DNA polynucleotide with identical sequence as the V-wt RNA; lane 5, an 80 nt RNA generated by bacteriophage T3 RNA polymerase transcription of a pIBI-31 plasmid digested with HindIII. The autoradiograph was overexposed in order to emphasize the absence of specific reaction products when these other templates were used.  FIG. 5B : 10 ng of each template RNA were incubated with the viral polymerase and the products were then subjected to electrophoresis on 8% polyacrylamide gels containing 7.7 M urea. Lane 1, V-wt RNA; lane 2, V-Dra RNA; lane 3, V-Eco RNA; lane 4, M-wt RNA are shown; and lane 5, a 53 nt marker oligonucleotide. For the exact sequence differences refer to  FIG. 2  and Section 6.1 et seq.  
       FIG. 6 . The RNA promoter does not require a terminal panhandle. Polymerase reaction using two template RNAs. Each reaction contained 5 ng of V-wt RNA. As a second template the reactions contained 0 ng (lane 1), 0.6 ng (lane 2), and 3.0 ng (lane 3) of V-d5′ RNA. The resulting molar ratios are as indicated in the figure. The reaction products were analyzed on an 8% polyacrylamide gel in the presence of 7.7 M urea. Following densitometry analysis of autoradiographs, the relative intensity of each peak was corrected for the amount of radioactive UMP which is incorporated in each product.  
       FIG. 7 . Specificity of promoter sequences. RNAs which lacked the 5′ terminus and contained point mutations (Table II) were compared with V-d5′ RNA in standard polymerase reactions. The right panel is from a separate reaction set. Quantitative comparisons is outlined in Table II.  
       FIG. 8 . High concentration polymerase preparations are active in cap-endonuclease primed and in primerless RNA synthesis reactions.  FIG. 8A : Primer specificty of the high concentration enzyme. Radioactively synthesized 30 nt template is in lane 1. Reactions using 20 ng of V-d5′ RNA and 5 μl of viral polymerase contained as primer: no primer (lane 2); 100 ng BMV RNA (De and Banerjee, 1985, Biochem. Biophys. Res. Commun. 6:40-49) containing a cap 0 structure (lane 3); 100 ng rabbit globin mRNA, containing a cap 1 structure, (lane 4); and 0.4 mM ApG (lane 5). A lighter exposure of lane 5 is shown as lane 6.  FIG. 8B : Nuclease S1 analysis of gel-purified RNAs. Products from reactions using as primer ApG (lanes 1 and 2); no primer (lanes 3 and 4); or globin mRNA (lanes 5 and 6) were electrophoresed in the absence of urea and the appropriate gel piece was excised and the RNA was eluted. This RNA was then digested with nuclease S1 (lanes 2, 4, and 6) and the products were denatured and analyzed on an 8% polyacrylamide gel containing 7.7 M urea.  
       FIG. 9 . Genomic length RNA synthesis from reconstitituted RNPs. Reaction products using 10 μl of polymerase and as template 890 nt RNA identical to the sequence of segment 8 of virus A/WSN/33 and RNA extracted from A/PR/8/34 virus were analyzed on a 4% polyacrylamide gel containing 7.7 M urea. In lane 1, the 890 nt template synthesized radioactively by T7 RNA polymerase is shown. The 890 nt plasmid-derived RNA was used as template in lanes 2, 3, 8 and 9. RNA extracted from virus was used as template in lanes 4, 5, 10 and 11. No template was used in lanes 6 and 7. No primer was used in lanes 2 to 5, and ApG was used as primer in lanes 6 to 11. Reaction products were treated with nuclease S1 in lanes 3, 5, 7, 9 and 11.  
       FIG. 10 . Diagrammatic representation of a PCR-directed mutagenesis method which can be used to replace viral coding sequences within viral gene segments.  
       FIG. 11 .(A). Diagrammatic representation of relevant portions of pIVCAT1. The various domains are labeled and are, from left to right; a truncated T7 promoter; the 5′ nontranslated end of influenza A/PR/8/34 virus segment 8 (22 nucleotides); 8 nucleotides of linker sequence; the entire CAT gene coding region (660 nucleotides) the entire 3′ nontranslated end of influenza A/PR/8/34 virus segment 8 (26 nucleotides); and linker sequence containing the HgaI restriction enzyme site. Relevant restriction enzyme sites and start and stop sites for the CAT gene are indicated. (B) The 716 base RNA product obtained following HgaI digestion and transcription of pIVACAT1 by T7 RNA polymerase. Influenza viral sequences are indicated by bold letters, CAT gene sequences by plain letters, and linker sequences by italics. The triplets—in antisense orientation—representing the initiation and termination codons of the CAT gene are indicated by arrow and underline, respectively (SEQ ID NOS.: 52 and 58).  
       FIG. 12 . RNA products of T7 polymerase transcription and in vitro influenza virus polymerase transcription. Lanes 1-4: polyacrylamide gel analysis of radiolabeled T7 polymerase transcripts from pIVACAT1, and pHgaNS. Lanes 5 and 6: Polyacrylamide gel analysis of the radiolabeled products of in vitro transcription by purified influenza A polymerase protein using unlabeled IVACAT1 RNA and HgaNS RNA templates. Lane 1: HgaNS RNA of 80 nt. Lanes 2-4: different preparations of IVACAT1 RNA. Lane 5: viral polymerase transcript of IVACAT1 RNA. Lane 6: viral polymerase transcript of HgaNS RNA.  
       FIG. 13 . Schematic of the RNP-transfection and passaging experiments.  
       FIG. 14 . CAT assays of cells RNP-transfected with IVACAT1 RNA. (A) Time course of RNP-transfection in 293 cells. Cells were transfected at −1 hour with the recombinant RNP and infected with virus at 0 hour. Cells were harvested at the indicated time points and assayed for CAT activity. (B) Requirements for RNP-transfection of 293 cells Paramaeters of the reaction mixtures were as indicated. (C) RNP-transfection of MDCK cells. MDCK cells were transfected with IVACAT1 RNA-polymerase at either −1 hour or +2 hours relative to virus infection. Cells were harvested and CAT activity assayed at the indicated times. Components/conditions of the reaction were as indicated. “Time” indicates the time point of harvesting the cells. T=0 marks the time of addition of helper virus. “RNA” represents the IVACAT1 RNA. “Pol” is the purified influenza A/PR/8/34 polymerase protein complex. “WSN” indicates the influenza A/WSN/33 helper virus. “Pre-Inc.” indicates preincubation of RNA and polymerase in transcription buffer at 30° C. for 30 min. “RNP transfection” indicates the time of RNP transfection relative to virus infection. “+/−” indicate presence or absence of the particular component/feature. “C” indicates control assays using commercially available CAT enzyme (Boehringer-Mannheim).  
       FIG. 15 . CAT activity in MDCK cells infected with recombinant virus. Supernatant from RNP-transfected and helper virus-infected MDCK cells was used to infect fresh MDCK cells. The inoculum was removed 1 hour after infection, cells were harvested 11 hours later and CAT activity was assayed. Lane 1: extract of cells infected with helper virus only. Lane 2: extract of cells infected with 100 μl of supernatant from RNP-transfected and helper virus-infected MDCK cells. Lane 3: Supernatant (80 μl) of cells from lane 2. Lane 4: Same as lane 2 except that helper virus (MOI 4) was added to inoculum. In contrast to experiments shown in  FIG. 4 , the assays contained 20 μl of  14 C chloramphenicol.  
       FIG. 16 . Diagram of relevant portions of the neuraminidase (NA) gene contained in plasmids used for transfection experiments. The pUC19 derived plasmid pT3NAv contains the influenza A/WSN/33 virus NA gene and a truncated promoter specifically recognized by bacteriophage T3 RNA polymerase. The T3 promoter used is truncated such that the initial transcribed nucleotide (an adenine) corresponds to the 5′ adenine of the WSN NA gene. At the 3′ end of the cDNA copy of the NA gene, a Ksp632I restriction enzyme site was inserted such that the cleavage site occurs directly after the 3′ end of the NA gene sequence (SEQ ID NO.: 55). A 1409 nucleotide long transcript was obtained following Ksp632I digestion and transcription by T3 RNA polymerase of PT3NAv (as described in Section 8.1, infra). The 15 5′ terminal nucleotides, the 52 nucleotides corresponding to the region between the restriction endonuclease sites NcoI and PstI and the 12 3′ terminal nucleotides are shown (SEQ ID NOS.: 53, 59 and 55). The transcript of pT3NAv mut 1 (SEQ ID NO.: 54) is identical to that of pT3NAv (SEQ ID NO.: 53) except for a single deletion, eleven nucleotides downstream from the 5′ end of the wild type RNA (SEQ ID NO.: 53). The transcript of the pT3NAv mut 2 is identical to that of pT3NAv except for 5 mutations located in the central region (indicated by underline) (SEQ ID NO.: 61). These five mutations do not change the amino acid sequence in the open reading frame of the gene. The serine codon UCC at position 887-889 (plus sense RNA) was replaced with the serine codon AGU in the same frame. The numbering of nucleotides follows Hiti et al., 1982, J. Virol. 41:730-734.  
       FIG. 17 . Polyacrylamide gel electrophoresis of RNAs purified from rescued influenza viruses. RNA transcripts of pT3NAs ( FIG. 16 ) of phenol-extracted RNA derived from influenza A/WSN/33 virus was mixed with purified polymerase preparations following the protocol described in Section 6.1.1, infra. These reconstituted RNPs were then transfected into MDBK cells which had been infected one hour earlier with WSN-HK helper virus. The medium, containing 28 μg/ml plasminogen, was harvested after 16 hours and virus was amplified and plaqued on MDBK cells in the absence of protease. Virus obtained from plaques was then further amplified in MDBK cells and RNA was phenol-extracted from purified virus preparations as described in Sections 6.1 et seq. and 7.1 et seq. RNAs were separated on 2.8% polyacrylamide-0.075% bisacrylamide gels containing 7.7 M urea in TBE buffer and visualized by silverstaining as described in Section 6.1 et seq. Lanes 1 and 6: WSN-HK virus RNA. Lane 2: RNA of virus which was rescued from MDBK cells following RNP-transfection with pT3NAv derived NA RNA and infection with helper virus WSN-HK. Lane 3: NA RNA transcribed in vitro from pT3NAv. Lane 4: RNA of control WSN virus. Lane 5: RNA of virus which was rescued from MDBK cells following RNP-transfection with phenol-extracted WSN virus RNA and infection with helper virus WSN-HK.  
       FIG. 18 . Sequence analysis of RNA obtained from rescued influenza virus containing five site-specific mutations. Following infection with the WSN-HK helper virus, MDBK cells were RNP-transfected with T3NAv mut 2 RNA which was obtained by transcription from pT3NAv mut 2. Following overnight incubation in the presence of 28 μg/ml plasminogen, medium was used for propagation and plaquing on MDBK cells in the absence of protease. Virus from plaques was then amplified and RNA was obtained following phenol-extraction of purified virus. Rescue of the mutant NA gene into virus particles was verified through direct RNA sequencing using 5′-TACGAGGAAATGTTCCTGTTA-3′ (SEQ ID NO.: 1) as primer (corresponding to position 800-819; Hiti et al., J. Virol. 41:730-734) and reverse transcriptase (Yamashita et al., 1988, Virol. 163:112-122). Sequences shown correspond to position 878-930 in the NA gene (SEQ ID NO.: 56)(Hiti et al., J. Virol. 41:730-734). The arrows and the underlined nucleotides indicate the changes in the mutant RNA compared to the wild type RNA. Left: Control RNA obtained from influenza A/WSN/33 virus. Right: RNA of mutant virus rescued from MDBK cells which were RNP-transfected with T3NAv mut 2 RNA and infected with helper virus WSN-HK.  
       FIG. 19 . CAT expression in vaccinia virus-infected/IVACAT-1 RNP transfected cells. Approximately 10 6  mouse C127 cells in 35 mm dishes were infected with mixtures of recombinant vaccinia viruses (Smith et al., 1986) at an M.O.I. of approximately 10 for each vector. After 1.5 hours, synthetic IVACAT-1 RNP was transfected into the virus-infected cells as described (Lutjyes et al., 1989). Cells were incubated overnight, harvested and assayed for CAT activity according to standard procedures (Gorman et al., 1982). The assays contained 0.05 uCl [ 14 C] chloramphenicol, 20 ul of 40 mM acetyl-CoA (Boehringer and 50 ul of cell extracts in 0.25 M Tris buffer (pH 7.5). Incubation times were approximately 4 hours. The labels under the lane numbers indicate the treatment of cells. Lanes 1-control; 2-naked RNA transfection (no polymerase added), no helper virus infection; 3-RNP transfection, no helper virus; 4-RNP transfection, influenza virus as helper; Lanes 5-11-RNP transfection, vaccinia virus vectors as helper viruses express the indicated influenza virus proteins.  
       FIG. 20 . Test of various cell lines. A) Cells were infected with vaccinia vectors expressing the PB2, PB1 and PA proteins (Lanes 1,3,5,7) or the PB2, PB1, PA and NP proteins (Lanes 2,4,6,8), transfected with IVACAT-1 RNP and examined for CAT activity as described. Lanes 1,2: Maden-Darby Canine Kidney (MDCK) cell; 3,4: Hela cells, 5,6: 293 cells (Graham et al., 1977 J. gen. Virol 36: 59-72); 7,8 L cells. B) Cell line 3 PNP-4 was used as host cell. Shown under each lane is the influenza viral proteins expressed in each sample. C) 293 cells were infected with the four required vaccinia and transfected with synthetic RNP made using IVA-CAT-1 (lane 1) or IVA-CAT-2 (lane 2) RNA. After overnight incubation, cells were harvested and CAT assayss were performed.  
       FIG. 21 . Schematic representation of plasmids used in RNP-transfection experiments. All plasmids are pUC19 derivatives. The arrows indicate selected restriction sites of the constructs. Asterisks represent the first ATG at the 5′ end of the encoded mRNA. Closed squares represent the first ATG downstream of the BIP-IRES sequences. Domains of the plasmids are indicated as follows: 3′NA N.C. and 5′NA N.C.: 3′ and 5′ noncoding regions of the influenza A/WSN/33 NA gene (Hiti, A. L. and Nayak, D. P., 1982, J. Virology 41: 730-734); deINA: small ORF (110 aa) derived from the influenza A/WSN/33 NA coding sequences; BIP: IRES sequences derived from the 5′ untranslated region of the BiP mRNA; CAT ORF: coding sequences of the CAT gene; T3: truncated T3 RNA polymerase promoter; NA coding sequence: NA coding sequence of the influenza A/WSN/33 virus NA gene; X: coding sequences of the foreign recombinant proteins GP2 or HGP2. ORFs 1 and 2 are indicated by a line below the pT3GP2/BIP-NA and pT3HGP2/BIP-NA plasmid representations. GP2 and HGP2 polypeptide domains are indicated at the bottom. L: leader peptide (15 aa) derived from the HA protein of influenza A/Japan/305/57 virus; GP41: ectodomain derived from the ectodomain of the gp41 protein of HIV-1; TM and TM′: transmembrane domains derived from the gp41 protein (22 aa) of HIV-1 and from the HA protein (27 aa) of influenza A/WSN/33 virus, respectively; CT and CT′: cytoplasmic tails derived from the truncated cytoplasmic domain of gp41 (2 aa) of HIV-1 and from the HA protein (10 aa) of influenza A/WSN/33 virus, respectively. The total length in amino acids of the encoded GP2 and HGP2 proteins, including the leader peptide, are indicated on the right.  
       FIG. 22 . CAT assays of MDBK cells that were RNP-transfected with NACAT(wt) or BIP-NA RNA. Cells were RNP-transfected 1 h after virus infection, harvested 16 h posttransfection, and assayed for CAT activity. Mock sample represents mock-transfected cells.  
       FIG. 23 . Polyacrylamide gel electrophoresis of RNAS extracted from BIP-NA transfectant influenza viruses. RNAs were visualized by silver staining. RNAs that encode polymerase proteins (P), hemagglutinin (HA), nucleoprotein (NP), neuraminidase (NA), matrix proteins (M), and nonstructural proteins (NS) are indicated on the left. BIP-NA RNA is indicated by the arrow. 18S ribosomal RNA is also indicated. Lane 1: influenza A/WSN/33 virus RNA; lane 2: BIP-NA virus RNA; lane 3: RNA transcribed in vitro from pT3BIP-NA. In order to facilitate the comparison of the RNA patterns, lane 2 has a shorter photographic exposure time than lanes 1 and 3.  
       FIG. 24 . Polyacrylamide gel electrophoresis of RNAs extracted from GP2/BIP-NA and HGP2/BIP-NA transfectant viruses. RNAs were visualized by silver staining. RNAs that encode polymerase proteins (P), hemagglutinin (HA), nucleoprotein (NP), neuraminidase (NA), matrix proteins (M), and nonstructural proteins (NS) are indicated on the right. The positions of GP2/BIP-NA and HGP2/BIP-NA RNAs are indicated by arrows. 18S ribosomal RNA is also indicated. Lanes 1 and 3: RNAs transcribed in vitro from pT3HGP2/BIP-NA and pT3GP2/BIP-NA, respectively; lane 2: HGP2/BIP-NA virus RNA; lane 4: GP2/BIP-NA virus RNA; lane 5: influenza A/WSN/33 virus RNA. Lane 5 has a shorter photographic exposure time than lanes 1-4.  
       FIG. 25 . Immunostaining of influenza virus-infected MDCK cells. MDCK monolayers were infected at an MOI≧2 with influenza A/WSN/33 virus or with the transfectant viruses GP2/BIP-NA or HGP2/BIP-NA. 9 h postinfection, cells were fixed and stained with a specific monocional antibody (2F5) directed against gp41 as described in Materials and Methods, Section 10.1. below.  
       FIG. 26A -B. Western blot analysis of the GP2 and HGP2 proteins in infected cell extracts and in purified virions. A. MDBK cells were infected at an MOI≧2 with influenza A/WSN/33 virus, GP2/BIP-NA or HGP2/BIP-NA transfectant viruses. 8 h postinfection, cells were lysed in NP-40 lysis buffer, and cellular extracts were subjected to SDS-PAGE. The monocional antibody 2F5 was used to detect the recombinant proteins GP2 and HGP2 in the western blot analysis. Lane 1: influenza A/WSN/33 virus-infected cells; lane 2: GP2/BIP-NA virus-infected cells; lane 3: HGP2/BIP-NA virus-infected cells. B. 2 μg of purified virus was analyzed by the same technique. Lane 1: influenza A/WSN/33 virus; lane 2: GP2/BIP-NA virus; lane 3: HGP2/BIP-NA virus.  
    
    
     5. DESCRIPTION OF THE INVENTION  
      This invention relates to the construction and use of recombinant negative strand viral RNA templates which may be used with viral RNA-directed RNA polymerase to express heterologous gene products in appropriate host cells and/or to rescue the heterologous gene in virus particles. The RNA templates may be prepared by transcription of appropriate DNA sequences using a DNA-directed RNA polymerase such as bacteriophage T7, T3 or the Sp6 polymerase. Using influenza, for example, the DNA is constructed to encode the message-sense of the heterologous gene sequence flanked upstream of the ATG by the complement of the viral polymerase binding site/promoter of influenza, i.e., the complement of the 3′-terminus of a genome segment of influenza. For rescue in virus particles, it may be preferred to flank the heterologous coding sequence with the complement of both the 3′-terminus and the 5′-terminus of a genome segment of influenza. After transcription with a DNA-directed RNA polymerase, the resulting RNA template will encode the negative polarity of the heterologous gene sequence and will contain the vRNA terminal sequences that enable the viral RNA-directed RNA polymerase to recognize the template.  
      The recombinant negative sense RNA templates may be mixed with purified viral polymerase complex comprising viral RNA-directed RNA polymerase proteins (the P proteins) and nucleoprotein (NP) which may be isolated from RNP cores prepared from whole virus to form “recombinant RNPs” (rRNPs). These rRNPs are infectious and may be used to express the heterologous gene product in appropriate host cells or to rescue the heterologous gene in virus particles by cotransfection of host cells with the rRNPs and virus. Alternatively, the recombinant RNA templates may be used to transfect transformed cell lines that express the RNA-directed RNA polymerase proteins allowing for complementation.  
      The invention is demonstrated by way of working examples in which RNA transcripts of cloned DNA containing the coding region—in negative sense orientation—of the chloramphenicol acetyltransferase (CAT) gene, flanked by the the 22 5′ terminal and the 26 3′ terminal nucleotides of the influenza A/PR/8/34 virus NS RNA were mixed with isolated influenza A virus polymerase proteins. This reconstituted ribonucleoprotein (RNP) complex was transfected into MDCK (or 293) cells, which were infected with influenza virus. CAT activity was negligible before and soon after virus infection, but was demonstrable by seven hours post virus infection. When cell supernatant containing budded virus from this “rescue” experiment was used to infect a new monolayer of MDCK cells, CAT activity was also detected, suggesting that the RNA containing the recombinant CAT gene had been packaged into virus particles. These results demonstrate the successful use of recombinant negative strand viral RNA templates and purified RNA-dependent RNA polymerase to reconstitute recombinant influenza virus RNP. Furthermore, the data suggest that the 22 5′ terminal and the 26 3′ terminal sequences of the influenza A virus RNA are sufficient to provide the signals for RNA tanscription, RNA replication and for packaging of RNA into influenza virus particles.  
      Using this methodology we also demonstrated the rescue of synthetic RNAs, derived from appropriate recombinant plasmid DNAs, into stable and infectious influenza viruses. In particular, RNA corresponding to the neuraminidase (NA) gene of influenza A/WSN/33 virus (WSN) was transcribed in vitro from plasmid DNA and, following the addition of purified influenza virus polymerase complex, was transfected into MDBK cells. Superinfection with helper virus lacking the WSN NA gene resulted in the release of virus containing the WSN NA gene. We then introduced five point mutations into the WSN NA gene by cassette mutagenesis of the plasmid DNA. Sequence analysis of the rescued virus revealed that the genome contained all five mutations present in the mutated plasmid. This technology can be used to create viruses with site-specific mutations so that influenza viruses with defined biological properties may be engineered.  
      The ability to reconstitute RNP&#39;s in vitro allows the design of novel chimeric influenza viruses which express foreign genes. One way to achieve this goal involves modifying existing influenza virus genes. For example, the HA gene may be modified to contain foreign sequences in its external domains. Where the heterologous sequence are epitopes or antigens of pathogens, these chimeric viruses may be used to induce a protective immune response against the disease agent from which these determinants are derived. In addition to modifying genes coding for surface proteins, genes coding for nonsurface proteins may be altered. The latter genes have been shown to be associated with most of the important cellular immune responses in the influenza virus system (Townsend et al., 1985, Cell 42:475-482). Thus, the inclusion of a foreign determinant in the NP or the NS gene of an influenza virus may—following infection—induce an effective cellular immune response against this determinant. Such an approach may be particularly helpful in situations in which protective immunity heavily depends on the induction of cellular immune responses (e.g., malaria, etc.).  
      Another approach which would permit the expression of foreign proteins (or domains of such proteins) via chimeric influenza viruses concerns the introduction of complete heterologous genes into the virus. Influenza virus preparations with more than eight RNA segments have previously been described (Nayak, D. et al. in Genetics of Influenza Virus, P. Palese and D. W. Kingsbury, eds., Springer-Verlag, Vienna, pp. 255-279). Thus, chimeric influenza viruses with nine or more RNA segments may be viable, and correct packaging of such chimeric viruses may readily occur.  
      The invention may be divided into the following stages solely for the purpose of description and not by way of limitation: (a) construction of recombinant RNA templates; (b) expression of heterologous gene products using the recombinant RNA templates; and (c) rescue of the heterologous gene in recombinant virus particles. For clarity of discussion, the invention is described in the subsections below using influenza. Any strain of influenza (e.g., A, B, C) may be utilized. However, the principles may be analagously applied to construct other negative strand RNA virus templates and chimeric viruses including, but not limited to paramyxoviruses, such as parainfluenza viruses, measles viruses, respiratory syncytial virus; bunyaviruses; arena viruses; etc. A particularly interesting virus system that can be used in accordance with the invention are the orthomyxo-like insect virus called Dhori (Fuller, 1987, Virology 160:81-87).  
     5.1. Construction of the Recombinant RNA Templates  
      Heterologous gene coding sequences flanked by the complement of the viral polymerase binding site/promoter, e.g, the complement of 3′-influenza virus terminus, or the complements of both the 3′- and 5′-influenza virus termini may be constructed using techniques known in the art. Recombinant DNA molecules containing these hybrid sequences can be cloned and transcribed by a DNA-directed RNA polymerase, such as bacteriophage T7, T3 or the Sp6 polymerase and the like, to produce the recombinant RNA templates which possess the appropriate viral sequences that allow for viral polymerase recognition and activity.  
      One approach for constructing these hybrid molecules is to insert the heterologous coding sequence into a DNA complement of an influenza virus genomic segment so that the heterologous sequence is flanked by the viral sequences required for viral polymerase activity; i.e., the viral polymerase binding site/promoter, hereinafter referred to as the viral polymerase binding site. In an alternative approach, oligonucleotides encoding the viral polymerase binding site, e.g., the complement of the 3′-terminus or both termini of the virus genomic segments can be ligated to the heterologous coding sequence to construct the hybrid molecule. The placement of a foreign gene or segment of a foreign gene within a target sequence was formerly dictated by the presence of appropriate restriction enzyme sites within the target sequence. However, recent advances in molecular biology have lessened this problem greatly. Restriction enzyme sites can readily be placed anywhere within a target sequence through the use of site-directed mutagenesis (e.g., see, for example, the techniques described by Kunkel, 1985, Proc. Natl. Acad. Sci. U.S.A. 82; 488). Variations in polymerase chain reaction (PCR) technology, described infra, also allow for the specific insertion of sequences (i.e., restriction enzyme sites) and allow for the facile construction of hybrid molecules. Alternatively, PCR reactions could be used to prepare recombinant templates without the need of cloning. For example, PCR reactions could be used to prepare double-stranded DNA molecules containing a DNA-directed RNA polymerase promoter (e.g., bacteriophase T3, T7 or Sp6) and the hybrid sequence containing the heterologous gene and the influenza viral polymerase binding site. RNA templates could then be transcribed directly from this recombinant DNA. In yet another embodiment, the recombinant RNA templates may be prepared by ligating RNAs specifying the negative polarity of the heterologous gene and the viral polymerase binding site using an RNA ligase. Sequence requirements for viral polymerase activity and constructs which may be used in accordance with the invention are described in the subsections below.  
     5.1.1. The Viral 3′-Terminus is Required for Polymerase Activity  
      The experiments described in Section 6 et seq., infra, are the first to define promoter sequences for a polymerase of a negative-sense RNA virus, and it was found that the specificity lies in the 3′ terminal 15 nucleotides. These viral polymerase binding site sequences, as well as functionally equivalent sequences may be used in accordance with the invention. For example, functionally equivalent sequences containing substitions, insertions, deletions, additions or inversions which exhibit similar activity may be utilized. The RNA synthesis by the viral polymerase described infra is a model for specific recognition and elongation by the influenza viral polymerase for the following reasons: (a) the polymerase has high activity when primed with ApG, a feature unique to influenza viral polymerase; (b) it has optimal activity at temperature and ionic conditions previously shown to be effective for the viral RNPs; (c) the polymerase is specific for influenza viral sequences on the model RNA templates; (d) the polymerase is active in the cap-endonuclease primed RNA synthesis which is the hallmark of the influenza viral polymerase; (e) recognition of cap donor RNA is specific to cap 1 structures; and (f) genomic RNA segments are specifically copied.  
     5.1.2. A Terminal Panhandle is not Required for Optimal Recognition and Synthesis by the Viral Polymerase  
      We had previously shown that the influenza viral segment RNAs base-pair at their termini to form panhandle structures. This was achieved by two methods. A cross-linking reagent derivative of psoralen covalently bound the termini of each segment in intact virus or in RNPs from infected cells (Hsu et al., 1987, Proc. Natl. Acad. Sci. USA 84: 8140-8144). The treated RNA was seen by electron microscopy to be circular, by virtue of the crosslinked termini. Similarly, the RNA termini in RNPs were found to be sensitive to ribonuclease V1, which recognizes and cleaves double-stranded RNA, and the viral polymerase was found to be bound to both termini in the panhandle conformation (Honda, et al., 1988, J. Biochem. 104: 1021-1026). In these studies the panhandle structure of the genomic RNA was shown to exist, and it was inferred to play a role in polymerase recognition. Although the template RNAs used in the examples described, were originally prepared to reveal panhandle-specific protein binding, it was found that the terminal panhandle had no obvious role in the polymerase reactions studied herein.  
     5.1.3. The RNA Polymerase Preparation Specifically Copies Negative Sense Templates  
      The viral polymerase was shown to synthesize RNA with optimal efficiency if the template had the “wild-type” negative sense 3′ terminus. It was shown that RNAs of unrelated sequence were not copied, and that those with extra polylinker sequences on the 3′ end were much less efficiently copied. A DNA of the correct sequence was similarly unsuitable as a template. The reaction was highly specific since the M-wt template was replicated only at very low levels. Even though our source of polymerase was intact virus, this finding was very surprising since it had never been suggested that the polymerase which recognizes the viral sense RNA would not efficiently copy the plus sense strand. Studies are underway to examine the specificity of the polymerase purified from infected cells at times post infection when the complementary RNA is copied into genomic templates. The present data support a model whereby the viral polymerase which copies vRNA is functionally different from that which synthesizes vRNA from cRNA by virtue of their promoter recognition. It is possible that by regulated modification of the polymerase in infected cells it then becomes capable of recognizing the 3′ terminus of plus sense RNA. By analyzing promoter mutants we investigated the fine specificity of the reaction and found that the only single mutation which generated a significantly lower level of synthesis was that of V-A 3  RNA. Furthermore, combinations of two or more point changes in positions 3, 5, 8 and 10 greatly lowered synthesis levels.  
     5.1.4. Insertion of the Heterologous Gene Sequence into the PB2, PB1, PA or NP Gene Segments  
      The gene segments coding for the PB2, PB1, PA and NP proteins contain a single open reading frame with 24-45 untranslated nucleotides at their 5′-end, and 22-57 untranslated nucleotides at their 3′-end. Insertion of a foreign gene sequence into any of these segments could be accomplished by either a complete replacement of the viral coding region with the foreign gene or by a partial replacement. Complete replacement would probably best be accomplished through the use of PCR-directed mutagenesis. The principle of this mutagenesis method is illustrated in  FIG. 10 . Briefly, PCR-primer A would contain, from 5′ to 3′, a unique restriction enzyme site, such as a class IIS restriction enzyme site (i.e., a “shifter” enzyme; that recognizes a specific sequence but cleaves the DNA either upstream or downstream of that sequence); the entire 3′ untranslated region of the influenza gene segment; and a stretch of nucleotides complementary to the carboxy-terminus coding portion of the foreign gene product. PCR-primer B would contain from the 5′ to 3′ end: a unique restriction enzyme site; a truncated but active phage polymerase sequence; the complement of the entire 5′ untranslated region of the influenza gene segment (with respect to the negative sense vRNA); and a stretch of nucleotides corresponding to the 5′ coding portion of the foreign gene. After a PCR reaction using these primers with a cloned copy of the foreign gene, the product may be excised and cloned using the unique restriction sites. Digestion with the class IIS enzyme and transcription with the purified phage polymerase would generate an RNA molecule containing the exact untranslated ends of the influenza viral gene segment with a foreign gene insertion. Such a construction is described for the chloramphenicol acetyltransferase (CAT) gene used in the examples described in Section 7 infra. In an alernate embodiment, PCR-primed reactions could be used to prepare double-stranded DNA containing the bacteriophage promoter sequence, and the hybrid gene sequence so that RNA templates can be transcribed directly without cloning.  
      Depending on the integrity of the foreign gene product and the purpose of the construction, it may be desirable to construct hybrid sequences that will direct the expression of fusion proteins. For example, the four influenza virus proteins, PB2, PB1, PA or NP are polymerase proteins which are directed to the nucleus of the infected cell through specific sequences present in the protein. For the NP this amino acid sequence has been found to be (single letter code) QLVWMACNSAAFEDLRVLS (SEQ ID NO.: 2) (Davey et al., 1985, Cell 40:667-675). Therefore, if it is desired to direct the foreign gene product to the nucleus (if by itself it would not ordinarily do so) the hybrid protein should be engineered to contain a domain which directs it there. This domain could be of influenza viral origin, but not necessarily so. Hybrid proteins can also be made from non-viral sources, as long as they contain the necessary sequences for replication by influenza virus (3′ untranslated region, etc.).  
      As another example, certain antigenic regions of the viral gene products may be substituted with foreign sequences. Townsend et al., (1985, Cell 42:475-482), identified an epitope within the NP molecule which is able to elicit a vigorous CTL (cytotoxic T cell) response. This epitope spans residues 147-161 of the NP protein and consists of the amino acids TYQRTRQLVRLTGMDP (SEQ ID NO.: 3). Substituting a short foreign epitope in place of this NP sequence may elicit a strong cellular immune response against the intact foreign antigen. Conversely, expression of a foreign gene product containing this 15 amino acid region may also help induce a strong cellular immune response against the foreign protein.  
     5.1.5. Insertion of the Heterologous Gene Sequence into the HA or NA Gene Segments  
      The HA and NA proteins, coded for by separate gene segments, are the major surface glycoproteins of the virus. Consequently, these proteins are the major targets for the humoral immune response after infection. They have been the most widely-studied of all the influenza viral proteins as the three-dimensional structures of both these proteins have been solved.  
      The three-dimensional structure of the H3 hemagglutinin along with sequence information on large numbers of variants has allowed for the elucidation of the antigenic sites on the HA molecule (Webster et al., 1983, In Genetics Of Influenza Virus, P. Palese and D. W. Kingsbury, eds., Springer-Verlag, Vienna, pp. 127-160). These sites fall into four discrete non-overlapping regions on the surface of the HA. These regions are highly variable and have also been shown to be able to accept insertions and deletions. Therefore, substitution of these sites within HA (e.g., site A; amino acids 122-147 of the A/HK/68 HA) with a portion of a foreign protein may provide for a vigorous humoral response against this foreign peptide. In a different approach, the foreign peptide sequence may be inserted within the antigenic site without deleting any viral sequences. Expression products of such constructs may be useful in vaccines against the foreign antigen, and may indeed circumvent a problem discussed earlier, that of propagation of the recombinant virus in the vaccinated host. An intact HA molecule with a substitution only in antigenic sites may allow for HA function and thus allow for the construction of a viable virus. Therefore, this virus can be grown without the need for additional helper functions. Of course, the virus should be attenuated in other ways to avoid any danger of accidental escape.  
      Other hybrid constructions may be made to express proteins on the cell surface or enable them to be released from the cell. As a surface glycoprotein, the HA has an amino-terminal cleavable signal sequence necessary for transport to the cell surface, and a carboxy-terminal sequence necessary for membrane anchoring. In order to express an intact foreign protein on the cell surface it may be necessary to use these HA signals to create a hybrid protein. Alternatively, if only the transport signals are present and the membrane anchoring domain is absent, the protein may be excreted out of the cell.  
      In the case of the NA protein, the three-dimensional structure is known but the antigenic sites are spread out over the surface of the molecule and are overlapping. This indicates that if a sequence is inserted within the NA molecule and it is expressed on the outside surface of the NA it will be immunogenic. Additionally, as a surface glycoprotein, the NA exhibits two striking differences from the HA protein. Firstly, the NA does not contain a cleavable signal sequence; in fact, the amino-terminal signal sequence acts as a membrane anchoring domain. The consequence of this, and the second difference between the NA and HA, is that the NA is orientated with the amino-terminus in the membrane while the HA is orientated with the carboxy-terminus in the membrane. Therefore it may be advantageous in some cases to construct a hybrid NA protein, since the fusion protein will be orientated opposite of a HA-fusion hybrid.  
     5.1.6. Insertion of the Heterologous Gene into the NS and M Gene Segments  
      The unique property of the NS and M segments as compared to the other six gene segments of influenza virus is that these segments code for at least two protein products. In each case, one protein is coded for by an mRNA which is co-linear with genomic RNA while the other protein is coded for by a spliced message. However, since the splice donor site occurs within the coding region for the co-linear transcript, the NS1 and NS2 proteins have an identical 10 amino acid amino terminus while M1 and M2 have an idential 14 amino acid amino terminus.  
      As a result of this unique structure, recombinant viruses may be constructed so as to replace one gene product within the segment while leaving the second product intact. For instance, replacement of the bulk of the NS2 or M2 coding region with a foreign gene product (keeping the splice acceptor site) could result in the expression of an intact NS1 or M1 protein and a fusion protein instead of NS2 or M2. Alternatively, a foreign gene may be inserted within the NS gene segment without affecting either NS1 or NS2 expression. Although most NS genes contain a substantial overlap of NS1 and NS2 reading frames, certain natural NS genes do not. We have analyzed the NS gene segment from A/Ty/or/71 virus (Norton et al., 1987, Virology 156:204-213) and found that in this particular gene, the NS1 protein terminates at nucleotide position 409 of the NS gene segment while the splice acceptor site for the NS2 is at nucleotide position 528. Therefore, a foreign gene could be placed between the termination codon of the NS1 coding region and the splice acceptor site of the NS2 coding region without affecting either protein. It may be necessary to include a splice acceptor site at the 5′ end of the foreign gene sequence to ensure protein production (this would encode a hybrid protein containing the amino-terminus of NS1). In this way, the recombinant virus should not be defective and should be able to be propagated without need of helper functions.  
      Although the influenza virus genome consists of eight functional gene segments it is unknown how many actual segments a virus packages. It has been suggested that influenza can package more than eight segments, and possibly up to 12 (Lamb and Choppin, 1983, Ann. Rev. Biochem. 52:467-506). This would allow for easier propagation of recombinant virus in that “ninth” gene segment could be designed to express the foreign gene product. Although this “ninth” segment may be incorporated into some viruses, it would soon be lost during virus growth unless some selection is supplied. This can be accomplished by “uncoupling” the NS or M gene segment. The NS2 coding portion could be removed from the NS gene segment and placed on the gene segment coding for the foreign protein (along with appropriate splicing signals). The resulting recombinant virus with the “uncoupled” NS or M gene would be able to propagate on its own and also would necessarily have to package the “ninth” gene segment, thus ensuring expression of the foreign gene.  
      Alternatively, a bicistronic mRNA could be constructed to permit internal initiation of translation of viral sequences and allow for the expression of foreign protein coding sequences from the regular terminal initiation site. Alternatively, a bicistronic mRNA sequence may be constructed wherein the viral sequence is translated from the regular terminal open reading frame, while the foreign sequence is initiated from an internal site. Certain internal ribosome entry site (IRES) sequences may be utilized. The IRES sequences which are chosen should be short enough to not interfere with influenza virus packaging limitations. Thus, it is prefereable that the IRES chosen for such a bicistronic approach be no more than 500 nucleotides in length, with less than 250 nucleotides being preferred. Further, it is preferable that the IRES utilized not share sequence or structural homology with picornaviral elements. Preferred IRES elements include, but are not limited to the mammalain BiP IRES (see Section 10, below) and the hepatitis C virus IRES.  
     5.2. Expression of Heterologous Gene Products Using Recombinant RNA Template  
      The recombinant templates prepared as described above can be used in a variety of ways to express the heterologous gene products in appropriate host cells or to create chimeric viruses that express the heterologous gene products. In one embodiment, the recombinant template can be combined with viral polymerase complex purified as described in Section 6, infra, to produce rRNPs which are infectious. To this end, the recombinant template can be transcribed in the presence of the viral polymerase complex. Alternatively, the recombinant template may be mixed with or transcribed in the presence of viral polymerase complex prepared using recombinant DNA methods (e.g. see Kingsbury et al., 1987, Virology 156:396-403). Such rRNPs, when used to transfect appropriate host cells, may direct the expression of the heterologous gene product at high levels. Host cell systems which provide for high levels of expression include continuous cell lines that supply viral functions such as cell lines superinfected with influenza, cell lines engineered to complement influenza viral functions, etc.  
      In an alternate embodiment of the invention, the recombinant templates or the rRNPs may be used to transfect cell lines that express the viral polymerase proteins in order to achieve expression of the heterologous gene product. To this end, transformed cell lines that express all three polymerase proteins such as 3P-38 and 3P-133 (Krystal et al., 1986, Proc. Natl. Acad. Sci. U.S.A. 83:2709-2713) may be utilized as appropriate host cells. Host cells may be similarly engineered to provide other viral functions or additional functions such as NP.  
     5.2.1. Purification of the Viral Polymerase  
      The viral polymerase proteins used to produce the rRNPs may be purified from dissociated RNP cores isolated from whole virus. In general, RNP cores may be prepared using standard methods (Plotch et al., 1981, Cell 23:847-858; Rochavansky, 1976, Virology 73:327-338). The pooled RNP cores may then be centrifuged on a second gradient of CsCl (1.5-3.0 M) and glycerol (30%-45%) as described by Honda et al., 1988, J. Biochem. 104:1021-1026. The active viral polymerase fractions may be isolated from top of the gradient, i.e. in the region of the gradient correlating with 1.5 to 2.0 M CsCl and corresponding to the fraction Honda et al. identified as “NP”. Surprisingly, this fraction contains all the viral polymerase proteins required for the active complex. Moreover, the P proteins which may be recovered from the bottom of the gradient are not required, and indeed do not provide for the transcription of full length viral RNA. Thus, it appears that the so-called “NP” fraction contains, in addition to NP, the active forms of the PB2, PB1, and PA proteins.  
     5.2.2. High Concentrations of Polymerase are Required for Cap-Primed RNA Synthesis  
      High concentrations of viral polymerase complex are able to catalyze this virus-specific cap-endonuclease primed transcription. Under the conditions specified in Section 6 infra, about 50 ng NP with 200 pg of the three P proteins were found to react optimally with 5 to 10 ng RNA reaction. The observation has been that although the NP selectively encapsidates influenza vRNA or cRNA in vivo, the NP will bind to RNA nonspecifically in vitro (Kingsbury, et al., 1987, Virology 156: 396-403; Scholtissek and Becht, 1971, J. Gen. Virol. 10: 11-16). Presumably, in order for the viral polymerase to recognize the viral template RNAs in our in vitro reaction, they have to be encapsidated by the NP. Therefore, the addition of a capped mRNA primer would essentially compete with the template RNA for binding of NP. Since the dinucleotide ApG would not be expected to bind NP, the low concentration polymerase was able to use only the short templates with ApG. Supporting this hypothesis is the observation that the higher concentration polymerase preparation is inhibited through the addition of progressively higher amounts of either template RNA or any non-specific RNA. It should also be noted that the unusual specificity for the m7GpppXm cap 1 structure previously shown with viral RNPs was also found with the reconstituted RNPs.  
     5.2.3. Genomic Length RNA Templates are Efficiently Copied  
      Plasmid-derived RNA identical to segment 8 of the A/WSN/33 virus was specifically copied by the polymerase (using the PCR method described in  FIG. 10 ). In reactions using RNA extracted from virus, all eight segments were copied, although the HA gene was copied at a lower level. The background in these reactions was decreased in comparison to the 30 to 53 nt templates, probably since the contaminating RNAs in the polymerase preparation were predominantly defective RNAs of small size. Recombinant templates encoding foreign genes transcribed in this system may be used to rescue the engineered gene in a virus particle.  
     5.3. Preparation of Chimeric Negative Strand RNA Virus  
      In order to prepare chimeric virus, reconstituted RNPs containing modified influenza virus RNAs or RNA coding for foreign proteins may be used to transfect cells which are also infected with a “parent” influenza virus. Alternatively, the reconstituted RNP preparations may be mixed with the RNPs of wild type parent virus and used for transfection directly. Following reassortment, the novel viruses may be isolated and their genomes be identified through hybridization analysis. In additional approaches described herein for the production of infectious chimeric virus, rRNPs may be replicated in host cell systems that express the influenza viral polymerase proteins (e.g., in virus/host cell expression systems; transformed cell lines engineered to express the polymerase proteins, etc.), so that infectious chimeric virus are rescued; in this instance, helper virus need not be utilized since this function is provided by the viral polymerase proteins expressed. In a particularly desirable approach, cells infected with rRNPs engineered for all eight influenza virus segments may result in the production of infectious chimeric virus which contain the desired genotype; thus eliminating the need for a selection system.  
      Theoretically, one can replace any one of the eight gene segments, or part of any one of the eight segments with the foreign sequence. However, a necessary part of this equation is the ability to propagate the defective virus (defective because a normal viral gene product is missing or altered). A number of possible approaches exist to circumvent this problem. We have shown that mutants of influenza virus defective in the PB2 and NP proteins can be grown to substantially higher titers in cell lines which were constructed to constitutively express the polymerase and NP proteins (Krystal et al., 1986 Proc. Natl. Acad. Sci. U.S.A. 83:2709-2813). Similar techniques may be used to construct transformed cell lines that constitutively express any of the influenza genes. These cell lines which are made to express the viral protein may be used to complement the defect in the recombinant virus and thereby propagate it. Alternatively, certain natural host range systems may be available to propagate recombinant virus. An example of this approach concerns the natural influenza isolate CR43-3. This virus will grow normally when passaged in primary chick kidney cells (PCK) but will not grow in Madin-Darby canine kidney cells (MDCK), a natural host for influenza (Maassab &amp; DeBorde, 1983, Virology 130:342-350). When we analyzed this virus we found that it codes for a defective NS1 protein caused by a deletion of 12 amino acids. The PCK cells contain some activity which either complements the defective NS1 protein or can completely substitute for the defective protein.  
      A third approach to propagating the recombinant virus may involve co-cultivation with wild-type virus. This could be done by simply taking recombinant virus and co-infecting cells with this and another wild-type virus (preferably a vaccine strain). The wild-type virus should complement for the defective virus gene product and allow growth of both the wild-type and recombinant virus. This would be an analagous situation to the propagation of defective-interfering particles of influenza virus (Nayak et al., 1983, In: Genetics of Influenza Viruses, P. Palese and D. W. Kingsbury, eds., Springer-Verlag, Vienna, pp. 255-279). In the case of defective-interfering viruses, conditions can be modified such that the majority of the propagated virus is the defective particle rather than the wild-type virus. Therefore this approach may be useful in generating high titer stocks of recombinant virus. However, these stocks would necessarily contain some wild-type virus.  
      Alternatively, synthetic RNPs may be replicated in cells co-infected with recombinant viruses that express the influenza virus polymerase proteins. In fact, this method may be used to rescue recombinant infectious virus in accordance with the invention. To this end, the influenza virus polymerase proteins may be expressed in any expression vector/host cell system, including but not limited to viral expression vectors (e.g., vaccinia virus, adenovirus, baculovirus, etc.) or cell lines that express the polymerase proteins (e.g., see Krystal et al., 1986, Proc. Natl. Acad. Sci. USA 83: 2709-2713). Moreover, infection of host cells with rRNPs encoding all eight influenza virus proteins may result in the production of infectious chimeric virus particles. This system would eliminate the need for a selection system, as all recombinant virus produced would be of the desired genotype. In the examples herein, we describe a completely synthetic replication system where, rather than infecting cells with influenza virus, synthetic RNP&#39;s are replicated in cells through the action of influenza virus proteins expressed by recombinant vaccinia vectors. In this way we show that the only influenza virus proteins essential for transcription and replication of RNP are the three polymerase proteins and the nucleoprotein.  
      It should be noted that it may be possible to construct a recombinant virus without altering virus viability. These altered viruses would then be growth competent and would not need helper functions to replicate. For example, alterations in the hemagglutinin gene segment and the NS gene segment discussed, supra, may be used to construct such viable chimeric viruses.  
      In the examples infra, the construction of a recombinant plasmid is described that, following transcription by T7 polymerase, yielded an RNA template which was recognized and transcribed by the influenza virus polymerase in vitro. This RNA template corresponds to the NS RNA of an influenza virus except that the viral coding sequences are replaced by those of a CAT gene. This recombinant negative strand viral RNA template was then mixed with purified influenza virus polymerase to reconstitute an RNP complex. The recombinant RNP complex was transfected into cells which were then infected with influenza virus, leading to expression of CAT activity.  
      A number of factors indicate that this system represents a biologically active recombinant RNP complex which is under tight control of the signals for transcription, replication and packaging of influenza virus RNAs. First, the CAT gene is of negative polarity in the recombinant viral RNA used for RNP transfection. Thus, the incoming RNA cannot be translated directly in the cell and must first be transcribed by the influenza virus polymerase to permit translation and expression of the CAT gene. Secondly, neither transfected naked recombinant RNA alone in the presence of infecting helper virus, nor recombinant RNP complex in the absence of infecting helper virus is successful in inducing CAT activity. This suggests that influenza viral proteins provided by the incoming RNP, as well as by the infecting helper virus, are necessary for the amplification of the recombinant RNA template. Finally, after RNP-transfection and infection by helper virus, virus particles emerge which apparently contain the recombinant RNA, since these particles again induce CAT activity in freshly infected cells. These results suggest that the 26 3′ terminal and the 22 5′ terminal nucleotides corresponding to the terminal nucleotides in the influenza A virus NS RNA are sufficient to provide the signals for polymerase transcription and replication, as well as for packaging of the RNA into particles.  
      The foregoing results, which defined the cis acting sequences required for transciption, replication and packaging of influenza virus RNAs, were extended by additional working examples, described infra, which demonstrate that recombinant DNA techniques can be used to introduce site-specific mutations into the genomes of infectious influenza viruses.  
      Synthetic RNAs, derived by transcription of plasmid RNA in vitro were used in RNP-transfection experiments to rescue infectious influenza virus. To enable selection of this virus, we chose a system that required the presence of a WSN-like neuraminidase gene in the rescued virus. Viruses containing this gene can grow in MDBK cells in the absence of protease in the medium (Schulman et al., 1977, J. Virol. 24:170-176). The helper virus WSN-HK does not grow under these circumstances. Clearly, alternative selection systems exist. For example, antibody screens or conditionally lethal mutants could be used to isolate rescued viruses containing RNAs derived from plasmid DNAs. In the experiments viruses described infra, viruses which were WSN virus-like were recovered. The WSN NA gene was derived from plasmid DNAs or from purified WSN virion RNA ( FIG. 17 , lanes 2 and 5). In the latter case, using whole virion RNA for the RNP-transfection, we do not know whether other genes were also transfered to the rescued virus, since the helper virus shares the remaining seven genes with WSN virus. The rescued viruses had the expected RNA patterns ( FIG. 17 ) and grew to titers in MDBK or MDCK cells which were indistinguishable from those of the wild type WSN virus. It should be noted that rescue of an NA RNA containing a single nucleotide deletion in the 5′ nontranslated region was not possible. This again illustrates the importance of regulatory sequences present in the non-translated regions of influenza virus RNAs. We also rescued virus using RNA that was engineered to contain 5 nucleotide changes in a 39 nucleotide long region ( FIG. 16 ). We verified the presence of these mutations in the rescued mutant virus by direct sequencing of the RNA ( FIG. 18 ). These mutations did not result in any amino acid change in the neuraminidase protein and thus were not expected to change the biological property of the virus. Although this virus was not extensively studied, its plaquing behavior and its growth characteristics were indistinguishable from that of wild type WSN virus. Using such technology, mutations may be introduced that will change the biological characteristics of influenza viruses. These studies will help in distinguishing the precise functions of all the viral proteins, including those of the nonstructural proteins. In addition, the nontranslated regions of the genome can be studied by mutagenesis, which should lead to a better understanding of the regulatory signals present in viral RNAs. An additional area of great interest concerns the development of the influenza virus system as a vaccine vector.  
     5.4. Vaccine Formulations Using the Chimeric Viruses  
      Virtually any heterologous gene sequence may be constructed into the chimeric viruses of the invention for use in vaccines. Preferably, epitopes that induce a protective immune response to any of a variety of pathogens, or antigens that bind neutralizing antibodies may be expressed by or as part of the chimeric viruses. For example, heterologous gene sequences that can be constructed into the chimeric viruses of the invention for use in vaccines include but are not limited to epitopes of human immunodeficiency virus (HIV) such as gp120; hepatitis B virus surface antigen (HBsAg); the glycoproteins of herpes virus (e.g. gD, gE); VP1 of poliovirus; antigenic determinants of non-viral pathogens such as bacteria and parasites, to name but a few. In another embodiment, all or portions of immunoglobulin genes may be expressed. For example, variable regions of anti-idiotypic immunoglobulins that mimic such epitopes may be constructed into the chimeric viruses of the invention.  
      Either a live recombinant viral vaccine or an inactiviated recombinant viral vaccine can be formulated. A live vaccine may be preferred because multiplication in the host leads to a prolonged stimulus of similar kind and magnitude to that occurring in natural infections, and therefore, confers substantial, long-lasting immunity. Production of such live recombinant virus vaccine formulations may be accomplished using conventional methods involving propagation of the virus in cell culture or in the allantois of the chick embryo followed by purification.  
      In this regard, the use of genetically engineered influenza virus (vectors) for vaccine purposes may require the presence of attenuation characteristics in these strains. Current live virus vaccine candidates for use in humans are either cold adapted, temperature sensitive, or passaged so that they derive several (six) genes from avian viruses, which results in attenuation. The introduction of appropriate mutations (e.g., deletions) into the templates used for transfection may provide the novel viruses with attenuation characteristics. For example, specific missense mutations which are associated with temperature sensitivity or cold adaption can be made into deletion mutations. These mutations should be more stable than the point mutations associated with cold or temperature sensitive mutants and reversion frequencies should be extremely low.  
      Alternatively, chimeric viruses with “suicide” characteristics may be constructed. Such viruses would go through only one or a few rounds of replication in the host. For example, cleavage of the HA is necessary to allow for reinitiation of replication. Therefore, changes in the HA cleavage site may produce a virus that replicates in an appropriate cell system but not in the human host. When used as a vaccine, the recombinant virus would go through a single replication cycle and induce a sufficient level of immune response but it would not go further in the human host and cause disease. Recombinant viruses lacking one or more of the essential influenza virus genes would not be able to undergo successive rounds of replication. Such defective viruses can be produced by co-transfecting reconstituted RNPs lacking a specific gene(s) into cell lines which permanently express this gene(s). Viruses lacking an essential gene(s) will be replicated in these cell lines but when administered to the human host will not be able to complete a round of replication. Such preparations may transcribe and translate—in this abortive cycle—a sufficient number of genes to induce an immune response. Alternatively, larger quantities of the strains could be administered, so that these preparations serve as inactivated (killed) virus vaccines. For inactivated vaccines, it is preferred that the heterologous gene product be expressed as a viral component, so that the gene product is associated with the virion. The advantage of such preparations is that they contain native proteins and do not undergo inactivation by treatment with formalin or other agents used in the manufacturing of killed virus vaccines.  
      In another embodiment of this aspect of the invention, inactivated vaccine formulations may be prepared using conventional techniques to “kill” the chimeric viruses. Inactivated vaccines are “dead” in the sense that their infectivity has been destroyed. Ideally, the infectivity of the virus is destroyed without affecting its immunogenicity. In order to prepare inactivated vaccines, the chimeric virus may be grown in cell culture or in the allantois of the chick embryo, purified by zonal ultracentrifugation, inactivated by formaldehyde or β-propiolactone, and pooled. The resulting vaccine is usually inoculated intramuscularly.  
      Inactivated viruses may be formulated with a suitable adjuvant in order to enhance the immunological response. Such adjuvants may include but are not limited to mineral gels, e.g., aluminum hydroxide; surface active substances such as lysolecithin, pluronic polyols, polyanions; peptides; oil emulsions; and potentially useful human adjuvants such as BCG and  Corynebacterium parvum.    
      Many methods may be used to introduce the vaccine formulations described above, these include but are not limited to oral, intradermal, intramuscular, intraperitoneal, intravenous, subcutaneous, and intranasal routes. It may be preferable to introduce the chimeric virus vaccine formulation via the natural route of infection of the pathogen for which the vaccine is designed. Where a live chimeric virus vaccine preparation is used, it may be preferable to introduce the formulation via the natural route of infection for influenza virus. The ability of influenza virus to induce a vigorous secretory and cellular immune response can be used advantageously. For example, infection of the respiratory tract by chimeric influenza viruses may induce a strong secretory immune response, for example in the urogenital system, with concomitant protection against a particular disease causing agent.  
     6. EXAMPLE  
     Promoter Analysis of the Influenza Viral RNA Polymerase  
      In the examples described below, polymerase which is depleted of genomic RNA was prepared from the upper fractions of the CsCl-glycerol gradient centrifugation. This polymerase is able to copy short model templates which are derived from transcription of appropriate plasmid DNA with bacteriophage T7 RNA polymerase in a sequence-specific manner. The termini of this model RNA are identical to the 3′ 15 and 5′ 22 nucleotides conserved in segment 8 from all influenza A viral RNAs. By manipulating the plasmid in order to prepare different RNAs to serve as template, we demonstrated that recognition of and synthesis from this model RNA was specific for the promoter at the 3′ terminal sequence and did not require the panhandle. In addition, site specific mutagenesis identified nucleotide positions responsible for the viral polymerase favoring synthesis from genomic sense templates over complementary sense RNA. Conditions were also found in which cap-endonuclease primed RNA synthesis could be observed using model RNAs. In addition, the reconstituted system permitted virus-specific synthesis from genomic length RNAs, derived either from plasmids or from RNA purified from virus through phenol extraction.  
     6.1. Materials and Methods  
     6.1.1. Purification of the Viral RNA Polymerase  
      RNP cores were prepared from whole virus using standard methods (Plotch, et al., 1981, Cell 23: 847-858; Rochavansky, 1976, Virology 73: 327-338). Two to three milligrams of virus were disrupted by incubating in 1.5% Triton N-101, 10 mg/ml lysolecithin, 100 mM tris-HCl, pH 8.0, 100 mM KCl, 5 mM MgCl 2 , 5% glycerol and 1.5 mM dithiothreitol. The sample was fractionated by centrifugation on a 30-70% glycerol (w/v) step gradient in the presence of 50 mM tris-HCl, pH 7.8 and 150 mM NaCl. The core preparation was centrifuged at 45,000 rpm in an SW50.1 rotor for 4 hours at 4° C. Fractions enriched in RNP were identified by SDS-polyacrylamide gel electrophoresis of protein samples from each fraction and staining with silver. The core fractions were then subjected to a second gradient centrifugation as was described in Honda et al. 1988, J. Biochem. 104: 1021-1026. This second gradient had steps of 0.5 ml 3.0 M CsCl and 45% (w/v) glycerol, 1.75 ml 2.5 M CsCl and 40% glycerol, 1.25 ml 2.0 M CsCl and 35% glycerol, and 1.0 ml of 1.5 M CsCl and 30% glycerol. All steps were buffered with 50 mM tris-HCl, pH 7.6 and 100 mM NaCl. 0.5 ml of RNP cores were layered on top and the sample was centrifuged at 45,000 rpm in an SW50.1 rotor for 25 hours at 4° C. Polymerase fractions were again identified by SDS-polyacrylamide electrophoresis of the protein samples and silver staining. Active polymerase fractions were generally found in the region of the gradient correlating with 1.5 to 2.0 M CsCl. These fractions were pooled and then dialyzed against 50 mM tri-HCl, pH 7.6, 100 mM NaCl and 10 mM MgCl 2  and concentrated in centricon-10 tubes (Amicon) or fractions were dialyzed in bags against 50 mM tris-HCl, pH 7.6, 100 mM NaCl, 10 mM MgCl 2 , 2 mM dithiothreitol, and 50% glycerol.  
     6.1.2. Preparation of Plasmid  
      The plasmid design is indicated in  FIG. 2 . Insert DNA for the pV-wt plasmid was prepared using an Applied Biosystems DNA synthesizer. The “top” strand was 5′-GAAGCTTAATACGACTCACTATAAGTAGAAACAAGGGTGTTTTTTCATATCATTTAAACTTCACCCTGCTTTTGCTGAATTCATTCTTCTGCAGG-3′ (SEQ ID NO.: 4). The “bottom” strand was synthesized by primer-extension with 5′-CCTGCAGAAGAATGA-3′ (SEQ ID NO.: 57) as primer. The 95 bp DNA was digested with HindIII and PstI and purified by extraction with phenol/chloroform, ethanol precipitation, and passage over a NACS-prepack ion exchange column (Bethesda Research Laboratories). This DNA was ligated into pUC-19 which had been digested with HindIII and PstI and then used to transform  E. coli  strain DH5-α which had been made competent using standard protocols. Bacteria were spread on agar plates containing X-gal and IPTG, and blue colonies were found to have the plasmid containing the predicted insert since the small insert conserved the lacZ reading frame and did not contain a termination codon. The pM-wt plasmid was prepared by a similar strategy except that both strands were chemically synthesized with the upper strand having the sequence 5′-GAAGCTTAATACGACTCACTATAAGCAAAAGCAGGGTGAAGTTTAAATGATATGAAAAAACACCCTTGTTTCTACTGAATTCATTCTTCTGCAGG-3′ (SEQ ID NO.: 5).  
      The pV-d5′ plasmid ( FIG. 2 ) was prepared using the oligonucleotides 5′-AGCTTAATACGACTCACTATAAGATCTATTAAACTTCACCCTGCTTTTGCTGAATTCATTCTTCTGCA-3′ (SEQ ID NO.: 6) and 5′-GAAGAATGAATTCAGCAAAAGCAGGGTGAAGTTTAATAGATCTTATAGTGAGTCGTATTA-3′ (SEQ ID NO.: 7). The DNAs were annealed and ligated into the HindIII/PstI digested pUC-19 and white colonies were found to contain the correct plasmid because this insert resulted in a frameshift in the lacZ gene. The point mutants were isolated following digestion of pV-d5′ with BglII and PstI and ligation of the linearized plasmid with a single stranded oligonucleotide of mixed composition. Since BglII laves a 5′ extension and PstI a 3′ extension, a single oligonucleotide was all that was necessary for ligation of insert. The host cell was then able to repair gaps caused by the lack of a complementary oligonucleotide. Oligonucleotides were designed to repair the frameshift in the lacZ gene so that bacteria which contained mutant plasmids were selected by their blue color.  
      Plasmid pHgaNS, which was used to prepare an RNA identical to segment 8 of A/WSN/33, was prepared using the primers 5′-CCGAATTCTTAATACGACTCACTATAAGTAGAAACAAGGGTG-3′ (SEQ ID NO.: 8) and 5′-CCTCTAGACGCTCGAGAGCAAAAGCAGGTG-3′ (SEQ ID NO.: 9) in a polymerase chain reaction off a cDNA clone. The product was then cloned into the XbaI/EcoRI window of pUC19.  
     6.1.3. Preparation of RNA Templates  
      Plasmid DNAs were digested with MboII or other appropriate endonucleases (see  FIG. 2 ), and the linearized DNA was transcribed using the bacteriophage T7 RNA polymerase. Run-off RNA transcripts were treated with RNAse-free DNAse 1 and then the RNA was purified from the proteins and free nucleotides using Qiagen tip-5 ion exchange columns (Qiagen, Inc.). Following precipitation in ethanol, purified RNAs were resuspended in water and a sample was analyzed by electrophoresis and followed by silver staining of the polyacrylamide gel in order to quantitate the yield of RNA.  
     6.1.4. Influenza Viral Polymerase Reactions  
      In a 25 μl total volume, about 30 μg of nucleoprotein and 200 pg total of the three polymerase proteins were mixed with 10 ng of template RNA and the solution was made up to a final concentration of: 50 mM Hepes pH 7.9, 50 mM NaCl, 5 mM MgCl 2 , 1 mM dithiothreitol, 0.05% NP-40, 0.4 mM adenylyl-(3′-5′)-guanosyl (ApG) dinucleotide (Pharmacia), 0.5 mM ATP, 0.5 mM GTP, 0.5 mM CTP and approximately 0.6 μM α- 32 P-UTP (40 μCi at 3000 Ci/mmole, New England Nuclear). Reactions were assembled on ice and then transferred to a 30° C. water bath for 90 minutes. Reactions were terminated by the addition of 0.18 ml ice-cold 0.3 M sodium acetate/10 mM EDTA and were then extracted with phenol/chloroform (1:1 volume ratio). Following the first extraction, 15 μg polyI-polyC RNA was added as carrier, and the sample was extracted again with phenol/chloroform. The samples were then extracted with ether and precipitated in ethanol. Following centrifugation, the RNA pellet was washed twice with 70% ethanol and then dried under vacuum.  
      In reactions using the high concentration polymerase, conditions were identical as above except that 20 ng of template RNA were added. In reactions using genomic length RNAs, the amount of polymerase used was doubled, 50 ng of template RNA was used, and the UTP concentration was raised to 2.6 μM.  
      The RNA was resuspended in a dye mix containing 78% formamide, 10 mM EDTA, 0.1% xylene cyanol and 0.05% bromophenol blue. Typically, a sample from this RNA was electrophoresed on an 8% polyacrylamide gel in the absence of urea, and the remainder was denatured by heating to 100° C. for 1.5 minutes and an aliquot was loaded on an 8% polyacrylamide gel containing 7.7 M urea. Gels were fixed by a two step procedure, first in 10% acetic acid, and then in 25% methanol/8% acetic acid. Gels were dried onto filter paper and then exposed to x-ray film.  
      When different RNAs were being tested for use as template, the different RNA preparations were always analyzed on polyacrylamide gels and stained with silver in order that equal amounts of each template were used. To quantitate the amount of product, gels were exposed to x-ray film in the absence of an intensifying screen in order to improve the linearity of the densitometer readings. Autoradiographs were analyzed using a FB910 scanning densitometer (Fisher Biotech) and peaks were evaluated using computer software from Fisher Biotech.  
     6.1.5. Nuclease Analysis of Reaction Products  
      For ribonuclease T1 analysis of the two principle RNA products, reaction products were analyzed by 8% polyacrylamide gel electrophoresis (without urea) and the gel was not treated with fixative. The wet gel was exposed to an x-ray film and the appropriate gel pieces were located and excised. The gel piece was crushed in 0.3 ml containing 10 mM tris pH 7.5, 1 mM EDTA, 0.1% sodium dodecyl sulfate, and 1 μg tRNA as carrier. The RNA diffused into this solution for 3 hours and then the gel was pelleted and the supernatant was made 0.3M in sodium acetate. The supernatant was then extracted twice in phenol/chloroform and once in ether and then precipitated in ethanol. The RNA pellet was resuspended in 5 μl formamide, denatured in boiling water for 1.5 minutes and then diluted by the addition of 0.1 ml 10 mM tris-HCl, pH 7.5, and 1 mM EDTA. Ribonuclease T1 (50 units, Boehringer Mannheim Biochemicals) was added and the samples were incubated for 60 minutes at 37° C. V-wt and M-wt RNAs synthesized with T7 RNA polymerase in the presence of α- 32 P_UTP were similarly digested with RNAse T1. Reaction products were extracted in phenol/chloroform and precipitated in ethanol and then were analyzed on 20% polyacrylamide gels containing 7.7 M urea.  
      Nuclease S1 analysis of reaction products was done on transcribed RNA by first terminating the standard polymerase reaction through the addition of S1 buffer to a volume of 0.2 ml with 0.26 M NaCl, 0.05 M sodium acetate, pH 4.6, and 4.5 mM zinc sulfate. The sample was divided into two 0.1 ml volumes and 100 units of S1 nuclease (Sigma Chemical Company) were added to one tube. The samples were incubated for 60 minutes at 37° C. Following the incubation, EDTA (10 mM final concentration) and 15 g polyI-polyC RNA was added and the sample was extracted with phenol/chloroform and precipitated in ethanol. The samples were then subjected to polyacrylamide gel electrophoresis.  
     6.2. Results  
     6.2.1. Preparation of Influenza Viral RNA Polymerase and of Template RNA  
      RNP cores of influenza virus A/Puerto Rico/8/34 were prepared by disruption of virus in lysolecithin and Triton N-101 followed by glycerol gradient centrifugation (Rochavansky, 1976, Virology 73: 327-338). Fractions containing cores were then subjected to a second centrifugation in a CsCl-glycerol step gradient (Honda, et al., 1988, J. Biochem. 104: 1021-1026). Fractions containing the polymerase were identified by gel electrophoresis of samples followed by silver-staining.  FIG. 1  shows the polymerase preparation after CsCl centrifugation. Bovine serum albumin (BSA) was added during dialysis to protect against protein loss. Densitometric scanning of lane 4 compared to known quantities of whole virus in lanes 1 and 2 allowed us to estimate that the proteins in lane 4 consist of 150 ng of NP and about 1 ng total of the three polymerase proteins. One fifth of the preparation used for this gel was used per reaction.  
      The overall design of the plasmids used to prepare template RNAs in this study is depicted in  FIG. 2 . The entire insert was prepared using oligonucleotides from a DNA synthesizer which were then cloned into the polylinker of pUC19. The insert contained a truncated promoter sequence recognized by the bacteriophage T7 RNA polymerase (Studier and Dunn, 1983, Cold Spring Harbor Symposia on Quantitative Biology, XLVII, 999-1007) so that the first nucleotides synthesized were the terminal 22 nucleotides (nt) of the conserved sequence from the 5′ end of the genome RNA. When the plasmid was cut with restriction endonuclease MboII (which cuts 7 bases upstream of its recognition site), the RNA which resulted from T7 RNA polymerase transcription ended with the terminal 3′ nucleotides of the influenza viral sequence. Included in the sequence was the poly-U stretch adjacent to the 5′ end of the conserved terminus which is thought to comprise at least part of the termination-polyadenylation signal (Robertson, et al., 1981, J. Virol. 38, 157-163). The total length of this model genomic RNA was 53 nt since a 16 nt spacer separated the terminal conserved sequences. The model RNA which contained both termini identical to those of vRNA was named V-wt. The RNA M-wt encoded the exact complementary strand of V-wt so that the termini match those of complementary RNA (cRNA). V-wt and M-wt were constructed to serve as models for influenza virus-specific vRNA and cRNA, respectively.  
     6.2.2. Viral Polymerase Catalyzes Synthesis of a Full Length Copy of the Template  
      In the reaction using the influenza viral polymerase, V-wt template and ApG primer, a product was obtained which comigrated with a 53 nt RNA on denaturing gels. RNA migrating as a doublet at a position of about 40 to 45 nucleotides ( FIG. 3A , lane 2) was also seen. This shorter product is shown below to be RNA which had terminated at a stretch of adenosines present between nucleotides 43-48 in the virion sense template. In addition to the template specific transcripts, a general background of light bands could be seen which correspond to truncated RNA products transcribed from viral genomic RNA not removed during the CsCl-glycerol centrifugation step. When no primer is used, there was no specific transcription product seen ( FIG. 3A , lane 3). Additional experiments showed globin mRNA, containing a terminal cap 1 structure, was inactive as primer using initial preparations of polymerase.  
      When the polymerase reaction was terminated by the addition of excess buffer favorable for nuclease S1 digestion and nuclease was added, the radioactively-labeled product was resistant to digestion ( FIG. 3B , lane 2). By contrast these conditions very efficiently digested the V-wt single-stranded RNA radioactively synthesized with T7 RNA polymerase (FIG.  3 B, lanes 3 and 4). These nuclease S1 data confirmed that the opposite strand was indeed being synthesized in these reactions. The product of the reaction might be a double stranded RNA, but it could not be ruled out that the product was in fact single stranded and later annealed to the template RNA in the presence of high salt used in the nuclease reaction.  
      The RNA products were purified by electrophoresis on an 8% gel, excised, eluted from the gel, and then digested by ribonuclease T1. Products were analyzed by electrophoresis and compared to the patterns generated by RNase T1 digestion of internally labeled M-wt and V-wt control probes. As can be seen in  FIG. 3C , the full length RNA (lane 1) has the identical pattern as does the plus sense RNA, M-wt (lane 3), and it does not have the pattern of the V-wt RNA (lane 4). The observed patterns were essentially identical to that which is predicted from the sequence of the RNA and thus showed that the polymerase faithfully copied the V-wt template. The smaller RNA product, a doublet with most templates, was also digested with RNase T1. Its pattern was similar to that of the full length RNA product ( FIG. 3C , lane 2) except the 14 base oligonucleotide was not present. Instead, a faint 13 base oligonucleotide was seen, thus mapping the termination of the short RNA to position 44, a site where two uridines would be incorporated. Since the amount of smaller RNA product decreased at higher UTP concentrations and disappeared when CTP was used as label, these bands appeared to be an artifact of low UTP concentrations in the polymerase reaction.  
     6.2.3. Conditions for the Polymerase Reactions Using Model RNA Templates  
      It was found that protein samples containing about 30 ng of NP protein and about 200 pg total of the three P proteins would react optimally with 5 to 10 ng of RNA. By using cold competitor RNA, polyI-polyC, it was found that excess RNA nonspecifically inhibited transcription, possibly via non-specific binding of the NP protein (Kingsbury, et al., 1987, Virology 156: 396-403; Scholtissek and Becht, 1971, J. Gen. Virol. 10: 11-16). In the absence of nonspecific competitor, variations in the amount of template between 1 and 10 ng produced little change in the efficiency of RNA synthesis. The NP protein and RNA were present at about equal molar concentrations and these were each about a thousand-fold in excess of the moles of the complex (assuming it to be 1:1:1) formed by the three P proteins in the typical reaction.  
      Since these reconstituted RNPs were able to use ApG but not globin mRNA as primer, we tested these model RNPs for other variables of the transcription reaction. In all other ways tested, the reconstituted RNPs behaved in solution similarly to those RNPs purified from detergent disrupted virus. The optimum temperature for RNA synthesis was 30° C. ( FIG. 4A , lane 2) as has been repeatedly found for the viral polymerase (Bishop, et al., 1971, J. Virol. 8: 66-73; Takeuchi, et al., 1987, J. Biochem. 101: 837-845; Ulmanen, et al., 1983, J. Virol. 45: 27-35). Also, the most active salt conditions were 60 mM NaCl ( FIG. 4B , lane 2), again consistent with conditions used by several groups (Bishop, et al., 1971, J. Virol. 8: 66-73; Honda, et al., 1988, J. Biochem. 104: 1021-1026; Shapiro, and Krug, 1988, J. Virol. 62: 2285-2290).  FIG. 4C  shows a time-course experiment. The amount of RNA synthesis appeared to increase roughly linearly for the first 90 minutes, as was found for viral RNPs (Takeguchi, et al., 1987, J. Biochem. 101: 837-845).  
     6.2.4. Specificity of the Elongation Reaction  
      Various RNAs were tested for suitability as templates for the RNA polymerase of influenza virus. The pV-wt plasmid clone was digested with either EcoRI, PstI or SmaI, and T7 polymerase was used to transcribe RNA. This resulted in RNAs identical to V-wt except for the addition of 5, 13 and 38 nt at the 3′ end. In  FIG. 5A  an overexposure of an autoradiograph is shown in order to demonstrate that no transcripts over background were observed in reactions which contained as template: two of the RNAs identical to V-wt except they contained 13 and 38 nt of extra sequence on the 3′ terminus (lanes 1 and 2); a single stranded DNA of identical sequence to that of V-wt (lane 4); and an unrelated 80 nt RNA generated by transcribing the polylinker of pIBI-31 with T3 RNA polymerase (lane 5). However, the V-Eco template, containing five extra nucleotides on the 3′ end, could be recognized and faithfully transcribed, although at approximately one-third the efficiency of the wild type V-wt RNA ( FIG. 5B , lane 3). It is interesting to note that initiation on the V-Eco RNA by the influenza viral polymerase appeared to occur at the correct base since the transcribed RNA was the same size as the product form the V-wt template.  
     6.2.5. Analysis of the Promoter Region for the Viral RNA Polymerase  
      The original construct used for these studies contained the sequences of both RNA termini of genomic RNAs which could base pair and thus form a panhandle. This was done since it was shown that the vRNA in virions and in RNPs in infected cells was in circular conformation via the 15 to 16 nt long panhandle (Honda, et al., 1988, J. Biochem. 104: 1021-1026; Hsu, et al., 1987, Proc. Natl. Acad. Sci. USA 84: 8140-8144). It was further shown that the viral polymerase was bound to the double stranded structure (Honda, et al., 1988, J. Biochem. 104: 1021-1026), thus leading to the suggestion that the promoter for RNA synthesis was the panhandle. In order to test whether the panhandle was an absolute requirement for recognition, the following templates were used: the plasmid pV-wt was digested with DraI prior to transcription by the T7 polymerase ( FIG. 2 ). This should result in an RNA molecule of 32 nt containing only virus-specific sequences from the 5′ end of the RNA. When this RNA was used as template, no apparent product was produced ( FIG. 5B , lane 2). Therefore the 3′ terminus of virion RNA was required for this reaction. This finding was consistent with the fact that the initiation site at the 3′ end of V-wt was not present in V-Dra. A second plasmid clone was produced which deleted the 5′ terminal sequences but kept intact the 3′ terminus. This clone, pV-d5′, when digested with MboII and used for transcription by T7 polymerase produced a major transcript of 30 nt and minor species of 29 and 31 nt. Surprisingly, this template was recognized and copied by the influenza viral polymerase.  FIG. 7 , lane 1, shows that the product of the viral RNA polymerase reaction with V-d5′ contains multiple bands reflecting the input RNA. When the products shown in  FIG. 7 , lane 1, were eluted from gels and subjected to RNase T1 analysis, the pattern expected of the transcription product of V-d5′ was observed. Since the V-d5′ RNA template was copied, the panhandle was not required for viral polymerase binding and synthesis.  
      Although the 5′ terminus was not required for synthesis by the polymerase, a distinct possibility was that V-wt RNA might be a preferred template as compared to V-d5′. In order to examine this, reactions were done in which the templates were mixed. The V-wt RNA was present at 5 ng in each reaction. The V-d5′ was absent ( FIG. 6 , lane 1) or was present at a 1/5 molar ratio ( FIG. 6 , lane 2) or a 1/1 molar ratio ( FIG. 6 , lane 3). The relative intensities of the bands from each RNA were determined by densitometry of the autoradiograph. The values were corrected for the amount of the radioactive nucleotide, UTP, which could be incorporated into each product, and the value was normalized so that the level of synthesis in each lane was set equal to one. The level of copying of V-wt decreased as V-d5′ was increased. When V-d5′ was present in one fifth molar ratio, its corrected level of synthesis was about one fourth of that from V-wt ( FIG. 6 , lane 2). When the two templates were present in equimolar amounts, the level of synthesis from V-wt was about 60% of the total ( FIG. 6 , lane 3) which might be within the expected range of experimental error for equivalent levels of synthesis. Similar results were obtained when V-d5′ RNA was kept constant and the V-wt RNA was varied. It was thus concluded that the panhandle-containing V-wt RNA was not greatly favored over the template RNA which only contained the proper 3′ terminus.  
     6.2.6. The Viral Polymerase does not Copy RNA Templates Containing Plus-Sense Termini  
      As described earlier, the influenza RNA polymerase performs three distinct activities during the course of an infection. Two activities involve the transcription of genome sense RNA and the third involves copying of the complementary sense RNA into vRNA. We therefore constructed an RNA template which contained the 5′ and 3′ termini of the complementary sense RNA of segment 8 (M-wt;  FIG. 2 ).  
      When the M-wt RNA was used as template, little synthesis was observed ( FIG. 5B , lane 4). In two experiments used for quantitation, the average level of synthesis from M-wt RNA was 4% that of V-wt. In comparing the V-wt and M-wt RNA promoters, the M-wt has only three transition changes and one point insertion within the 3′ 15 nucleotides. These include a G to A change at position 3, a U to C change at position 5, a C to U change at position 8 and an inserted U between the ninth and tenth nucleotides (see Table II, below). In order to determine which of the four point differences in the 3′ termini were responsible for the specificity, many combinations of these were prepared and assayed for efficiency as a template ( FIG. 7 ). These templates were derivatives of V-d5′ since they did not contain the 5′ terminus. The results of densitometry scans of several experiments are outlined in Table II.  
                   TABLE II                          QUANTITATIVE COMPARISON OF THE EFFECT OF           POINT MUTATIONS IN THE PROMOTER SEQUENCE*                                 RNA       Level of   SEQ ID                                 Template   3′ sequence   Synthesis   NO.                                         V-d5′   CACCCUGCUUUUGCU-OH   1   10                   V-A3   CACCCUGCUUUUACU-OH   0.4   11               V-C5   CACCCUGCUUCUGCU-OH   1.0   12               V-dU 25 U 8     CACCCUGUUUUUGCU-OH   1.0   13               V-U 8 A 3     CACCCUGUUUUUACU-OH   0.08   15               V-U 8 C 5     CACCCUGUUUCUGCU-OH   0.3   16               V-iU 10     CACCCUUGCUUUUGCU-OH   0.7   14               V-iU 10 A 3     CACCCUUGCUUUUACU-OH   0.06   17               V-iU 10 U 8 A 3     CACCCUUGUUUUUACU-OH   0.2   18               V-iU 10 U 8 C 5 A 3     CACCCUUGUUUCUACU-OH   0.2   19                 *Sequences of V-wt, M-wt and V-d5′ are shown in  FIG. 2 . All other RNAs are identical to V-d5′ except for the indicated positions. The subscripted number indicates the distance from the 3′ end of a change, and d and i refer to deleted or inserted nucleotides.             
 
      As shown in Table II, single point changes in V-d5′ were equally well copied as compared to V-d5′ itself, except for the V-A 3  RNA which was copied at 40% efficiency ( FIG. 7 , lane 10; Table II). When RNAs with two changes were tested, the activity generally dropped to very low levels ( FIG. 7 , lanes 3, 4, and 5). Therefore, these experiments confirmed that the specificity of the reactions for V-wt over M-wt was the result of the combination of the nucleotide changes present at the 3′ terminus of M-wt.  
     6.2.7. Cap-Endonuclease Primed RNA Synthesis  
      The method of purifying the viral polymerase was modified in order to decrease loss of protein during dialysis. Rather than using the Amicon centricon-10 dialysis system, the enzyme was dialyzed in standard membranes resulting in higher concentrations of all four viral core proteins. The pattern of the protein gel of this preparation was identical to that shown in  FIG. 1 , lane 4, except that there is no BSA-derived band. It was found that 5 μl of this preparation, containing 150 ng of NP and 5 ng total of the three polymerase proteins, reacted optimally with 10 to 40 ng of model RNA template. However, the use of higher levels of protein increased the background, possibly due to higher levels of contaminating RNAs (virion RNAs not removed by CsCl centrifugation) yielding products of the size class around 50-75 nt, complicating analysis of RNA templates containing a length of 50 nt.  
      This high concentration polymerase preparation was now active in cap-endonuclease primed RNA synthesis ( FIG. 8A , lane 4) and also in primer-independent replication of the template RNA ( FIG. 8A , lane 2). When globin mRNA was used as primer for transcription from the 30 nt V-d5′ template, a triplet of bands of size about 42 to 44 nt was apparent as product ( FIG. 8A , lane 4), consistent with cleavage of the cap structure at about 12 nt from the 5′ end of the mRNA and use of this oligonucleotide to initiate synthesis from the 30 nt model template. Since excess RNA inhibits RNA synthesis, probably via nonspecific binding of NP in vitro as discussed above, the optimal amount of cap donor RNA added to each reaction was found to be 100 ng, which is much lower than is usually used with preformed RNP structures (e.g. Bouloy, et al., 1980, Proc. Natl. Acad. Sci. USA 77:3952-3956). The most effective primer was ApG ( FIG. 8A , lane 5 and lighter exposure in lane 6). The product migrates slower than that of the input template ( FIG. 8A , lane 1) or the product in the absence of primer ( FIG. 8A , lane 2) probably since the 5′ terminus of the ApG product is unphosphorylated. The intensity of the ApG-primed product was about ten-fold higher than that of the cap-primed product, but at 0.4 mM, ApG was at a 60,000-fold molar excess of the concentration of the cap donors. Thus, although the intensity of the product band from cap-priming was about ten-fold lower than that from ApG priming, the cap-primed reaction was about 6000-fold more efficient on a molar basis. This value is similar to the approximately 4000-fold excess efficiency observed previously for the viral polymerase (Bouloy, et al., 1980, Proc. Natl. Acad. Sci. USA 77: 3952-3956). It has been previously shown that cap donor RNAs containing a cap 0 structure, as in BMV RNA, are about ten-fold less active in priming the influenza viral polymerase (Bouloy, et al., 1980, Proc. Natl. Acad. Sci. USA 77: 3952-3956). This unusual cap specificity was shared by the reconstituted RNPs studied here as the specific product from the model RNA was greatly decreased in reactions containing BMV RNA as cap donor. A 30 nt product was observed in lanes 2-4, probably due to primeness replication of the model template.  
      That the product RNAs were of the opposite sense of the input template V-d5′ was shown by nuclease S1 analysis ( FIG. 8B ). The ApG-primed ( FIG. 8B , lanes 1 and 2) and the primeness ( FIG. 8B , lanes 3 and 4) RNA products were essentially nuclease resistant. The product of the cap-primed reaction ( FIG. 8B , lanes 5 and 6) was partially sensitive to nuclease as about 12 nt were digested from the product. These results were most consistent with the 5′ 12 nt being of mRNA origin as has been shown many times for influenza virus-specific mRNA synthesis.  
      The promoter specificity of this polymerase preparation in reactions primed with ApG was found to be essentially identical to those for the lower concentration enzyme as shown earlier. However, attempts thus far to perform similar analyses of promoter specificity with the primeness and cap-primed reactions have been frustrated by the comparatively high levels of background, thus making quantitation difficult.  
     6.2.8. Replication of Genomic Length RNA Templates  
      A full length 890 nt RNA identical to the sequence of A/WSN/33 segment 8 was prepared by T7 RNA polymerase transcription of plasmid DNA, pHgaNS, which had been digested with restriction endonuclease HgaI. This RNA was copied in ApG-primed reactions containing 10 μl of the high concentration polymerase ( FIG. 9 , lane 8). That the RNA was in fact a copy of the template was demonstrated by its resistance to nuclease S1 ( FIG. 9 , lane 9). A similar product was observed in the absence of primer ( FIG. 9 , lanes 2 and 3). Confirmation that these product RNAs were full length copies of the template was done by RNase T1 analysis. Virion RNA purified from phenol-extracted A/PR/8/34 virus was similarly copied in ApG primed reaction ( FIG. 9 , lanes 10 and 11) and in the absence of primer ( FIG. 9 , lanes 4 and 5). Interestingly, the product from replication of the HA gene was at greatly reduced levels. The 3′ end of this RNA differs from that of segment 8 only at nucleotides 14 and 15, suggesting importance for these nucleotides in the promoter for RNA synthesis. In addition, we found that when whole viral RNA was used in the reconstituted RNPs, the level of acid precipitable counts was about 70% of that observed with native RNPs. The viral polymerase was also able to copy these full length RNAs when globin mRNA was used in cap-primed reaction.  
     7. EXAMPLE  
     Expression and Packaging of a Foreign Gene by Recombinant Influenza Virus  
      The expression of the chloramphenicol transferase gene (CAT) using rRNPs is described. The rRNPs were prepared using pIVACAT (originally referred to as pCATcNS), a recombinant plasmid containing the CAT gene. The pIVACAT plasmid is a pUC19 plasmid conaining in sequence: the T7-promoter; the 5′-(viral-sense) noncoding flanking sequence of the influenza A/PR8/34 RNA segment 8 (encodes the NS proteins); a BglII cloning site; the complete coding sequence of the chloramphenicol transferase (CAT) gene in the reversed and complemented order; the 3′-(viral-sense) noncoding NS RNA sequence; and several restriction sites allowing run-off transcription of the template. The pIVACAT can be transcribed using T7 polymerase to create an RNA with influenza A viral-sense flanking sequences around a CAT gene in reversed orientation.  
      The in vivo experiments described in the subsections below utilized the recombinant RNA molecule described containing sequences corresponding to the untranslated 3′ and 5′ terminal sequences of the NS RNA of influenza virus A/PR/8/34 flanking the antisense-oriented open reading frame of the CAT gene. This RNA was mixed with purified influenza virus polymerase complex and transfected into MDCK (or 293) cells. Following infection with influenza A/WSN/33 virus, CAT activity was measured in the RNP-transfected cells and amplification of the gene was indicated. In addition, the recombinant influenza virus gene was packaged into virus particles, since CAT activity was demonstrated in cells following infection with the recombinant virus preparation.  
     7.1. Materials and Methods  
      In order to get the flanking sequences of the NS RNA fused to the coding sequence of the CAT gene, the following strategy was used. Two suitable internal restriction sites were selected, close to the start and stop codon of the CAT gene, that would allow the replacement of the sequences flanking the CAT gene in the pCM7 plasmid with the 3′- and 5′-NS RNA sequences. At the 5′ and, a SfaNI site was chosen, (which generates a cut 57 nt from the ATG) and at the 3′-end a ScaI site which generates a cut 28 nt from the end of the gene (stop codon included). Next, four synthetic oligonucleotides were made using an Applied Biosystems DNA synthesizer, to generate two double-stranded DNA fragments with correct overhangs for cloning. Around the start codon these oligonucleotides formed a piece of DNA containing a XbaI overhang followed by a HgaI site and a PstI site, the 3′-(viral-sense) NS sequence immediately followed by the CAT sequence from start codon up to the SfaNI overhang (underscored). In addition a silent mutation was incorporated to generate an AccI site closer to the start codon to permit future modifications.  
                                   Xba I                   Hga I   Pst I                             Acc I         5′-ctagacgccctgcagcaaaagcagggtgacaaagacata atggagaaaaaaatcac                  SfaN   I         tgggtataccaccgttgatatatcccaatcgcatcgtaaa -3′ (SEQ ID NO.: 62) oligo2                       Xba I               Hga I   Pst I                             Acc I         3′ tgcgggacgtcgttttcgtcccactgtttctgtattacctctttttttagtg                  SfaN   I       acccatatggtggcaactatatagggttagcgtagcatttcttg-5′ (SEQ ID NO.: 21) oligo1          
 
      Around the stop codon the two other oligonucleotides generated a piece of DNA as follows: a blunt-ended ScaI site, the CAT sequence from this site up to and including the stop codon (underlined) followed by a BglII site and a Xba I overhang.  
                             Sca I                           Bgl II             5′- actgcgatgagtggcagggcggggcgtaa tagat-3′ (SEQ ID NO.: 22) oligo3               3′-tgacgctactcaccgtcccgccccgcattatct agatc -5′ (SEQ ID NO.: 25) oligo4                                          Xba  I          
 
      Using a single internal EcoRI site in the CAT sequence, the SfaNI/EcoRI and the EcoRI/ScaI fragment from pCM7 were independently cut out and purified from acrylamide gels. The SfaNI/EcoRI fragment was subsequently ligated with the synthetic DNA fragment obtained by annealing oligonucleotides 1 and 2 into a pUC19 plasmid that was cut with XbaI and EcoRI. The EcoRI/ScaI fragment was similarly cloned into an XbaI and EcoRI-digested pUC19 plasmid using oligonucleotides 3 and 4. The ligated DNA was transformed into competent DH5a bacteria, amplified, isolated and screened by means of restriction analysis using standard techniques.  
      The recombinants with the SfaNI containing insert were cut with XbaI and EcoRI and the plasmids with the ScaI insert were cut with EcoRI and BglII. The fragments were purified from acrylamide gel and cloned together into the pPHV vector which had been cut with XbaI and BglII. After transformation, white colonies were grown, analysed by endonuclease digestion and selected clones were sequenced. The final clone, pCATcNS2, was grown in large amounts and sequenced from the flanking pUC sequences up to 300 nt into the CAT gene, revealing no discrepancies with the intended sequence, with the exception of a G to A transition in the CAT gene, which appeared silent.  
     7.1.1. Viruses and Cells  
      Influenza A/PR/8/34 and A/WSN/33 viruses were grown in embryonated eggs and MDCK cells, respectively (Ritchey et al. 1976, J. Virol. 18: 736-744; Sugiura et al., 1972, J. Virol. 10: 639-647). RNP-transfections were performed on human 293 cells (Graham et al., 1977, J. Gen. Virol. 36:59-72) and on Madin-Darby canine kidney (MDCK) cells (Sugiura et al., 1972, supra).  
     7.1.2. Construction of Plasmids  
      Plasmid pIVACAT1, derived from pUC19, contains the coding region of the chloramphenicol acetyltransferase (CAT) gene flanked by the noncoding sequences of the influenza A/PR/8/34 RNA segment 8. This construct is placed under the control of the T7 polymerase promoter in such a way that the RNA transcript IVACAT1 contains in 5′ to 3′ order: 22 nucleotides derived from the 5′ terminus of the influenza virus NS RNA, an 8 nt linker sequence including a BglII restriction site, the CAT gene in negative polarity, and 26 nt derived from the 3′ end of the influenza virus NS RNA ( FIG. 11 ).  
      pIVACAT1 was constructed in the following way: In order to obtain the correct 5′-end in pIVACAT1, the EcoRI-ScaI fragment of the CAT gene derived from plasmid pCM7 (Pharmacia) was ligated to a DNA fragment formed by two synthetic oligonucleotides. The sequence of these oligonucleotides are: 5′-ACTGCGATGAGTGGCAGGGCGGGGCGTAATAGAT-3′ (top strand) (SEQ ID NO.: 22), and 5′-CTAGATCTATTACGCCCCGCCCTGCCACTCATCGCAGT-3′ (bottom strand) (SEQ ID NO.: 23). For the 3′-end of the insert in pIVACAT1 the SfaN 1-EcoRI fragment of the CAT gene was ligated to a DNA fragment made up of the synthetic oligonucleotdies: 5′-CTAGACGCCCTGCAGCAAAAGCAGGGTGACAAAGACATAATGGAGAAAAAAAATCACTGGGTATACCACCGTTGATATATCCCAATCGCATCGTAAA-3′ (top strand) (SEQ ID NO.: 26), and GTTCTTTACGATGCGATTGGGATATATCAACGGTGGTATACCCAGTGATTTTTTTCTCCATTATGTCTTTGTCACCCTGCTTTTGCTGCAGGGCGT-3′ (bottom strand) (SEQ ID NO.: 27). Oligonucleotides were synthesized on an Applied Biosystems DNA synthesizer. These 5′ and 3′ constructs were ligated into pUC19 shuttle vectors digested with XbaI and EcoRI, grown up, cut out with EcoRI/BglII (5′ region) and XbaI/EcoRI (3′ region) and ligated into BglII/XbaI cut pPHV. The latter plasmid is similar to pV-WT described in Section 6, supra, except that it contains a BglII site which separates the noncoding terminal sequences of the influenza A virus NS RNA segment. The final clone pIVACAT1 ( FIG. 1 ) was grown up and the DNA was partially sequenced starting from the flanking pUC sequences and reaching into the CAT gene. No changes were found as compared to the expected sequences with the exception of a silent G to A transition in the CAT gene at position 106 relative to the start of the IVACAT1 RNA.  
     7.1.3. T7 RNA Transcription  
      Plasmid pIVACAT1 was digested with HgaI ( FIG. 1I ), to allow run-off transcription. The 5 nt overhang generated by this enzyme was filled in with Klenow enzyme (BRL) and the DNA was purified over a spin column (Boehringer). The T7 polymerase reaction was performed using standard procedures in the presence of Rnasin (Promega). Template DNA was removed from Rnase free Dnase I (Promega). The RNA was purified over Qiagen tip-5 columns (Qiagen, Inc.) and quantitated using 4% polyacrylamide gels which were silver stained. NS RNA was prepared from plasmid pHgaNS in the same way.  
     7.1.4. Purification of Influenza A Virus Polymerase and In Vitro Transcription  
      The RNA polymerase complex was purified from influenza A/PR/8/34 as described in Section 6, supra. In vitro transcriptions of cold IVACAT1 or HgaNS RNA template were carried out using the conditions which have been described in Section 6, supra. Radiolabeled transcripts were analysed on 4% acrylamide gels.  
     7.1.5. RNP-Transfection of MDCK and 293 Cells  
      35 mm dishes containing approximately 10 6  cells were treated with 1 ml of a solution of 300 μg/ml DEAE-dextrin, 0.5% DMSO in PBS/gelatine (0.1 mg/ml gelatine) for 30 minutes at room temperature. After removal of this solution, 200 μg of μl PBS/gelatine containing 1 μg IVACAT1 RNA (1-2 μl), 20 μl of the purified polymerase preparation and 4 μl of Rnasin was added to the cells and incubated for 1 hour at 37° C. This was followed by the addition of gradient purified influenza A/WSN/33 virus (moi 2-10). After incubation for one hour at 37° C., 2.5 ml of either DMEM+10% FCS media (293 cells) or MEM media (MDCK cells) was added. In some experiments MDCK cells were first infected and subsequently RNP-transfected. Harvesting of cells was done in NET buffer or in media, using a rubber policemen (MDCK cells), or by gentle suspension (293 cells). Cells were spun down and the pellets were resuspended in 100 μl of 0.25 M Tris buffer, pH 7.5. The samples were subsequently freeze-thawed three-times and the cell debris was pelleted. The supernatant was used for CAT assays.  
     7.1.6. Passaging of Virus from RNP-Transfected Cells  
      MDCK cells were infected with helper virus and RNP-transfected 2 hours later as described above. After 1 hour cells and media were collected and cells were spun down. 100 μl of the supernatant media, containing virus, was added to 35 mm dishes with MDCK cells. After 12 hours these cells and media were collected and assayed for CAT activity. Virus contained in this supernatant media was used for subsequent rounds of infection of MDCK cells in 35 mm dishes.  
     7.1.7. CAT Assays  
      CAT assays were done according to standard procedures, adapted from Gorman et al., 1982, Mol. Cell. Biol. 2: 1044-1051. The assays contained 10 μl of  14 C chloramphenicol (0.5 μCi; 8.3 nM; NEN), 20 μl of 40 mM acetyl CoA (Boehringer) and 50 μl of cell extracts in 0.25 M Tris buffer (pH 7.5). Incubation times were 16-18 hours.  
     7.2. Results  
      rRNA templates were prepared from HgaI digested, end filled linearized pCATcNS using the bacteriophage T7 RNA polymerase as described in Section 6. The rRNA templates were combined with the viral RNA polymerase complex prepared as described in Section 6.1.1., and the resulting rRNPs were used to transfect MDCK and 293 cells lines which were superinfected with influenza A/WSN33. In each cell line transfected with the rRNPs, high levels of expression of CAT was obtained 6 hours post-infection. In addition, virus stocks obtained 24 hours post-infection synthesized high levels of CAT enzyme after subsequent passage in MDCK cells. The CAT-RNP was packaged into virus particles.  
     7.2.1. Synthesis of IVACAT1 Template RNA  
      In order to study the transcription and replication signals of influenza A virus RNAs in vivo, we constructed plasmid pIVACAT1 ( FIG. 1I ) which directs the synthesis of an NS RNA-like transcript. This RNA shares the 22 5′ terminal and the 26′ 3′ terminal nucleotides with the NS RNA of influenza A/PR/8/34 virus and contain—instead of the coding sequences for the NS1 and NS2 proteins—those for a full-length CAT protein. For cloning purposes it also contains eight additional nucleotides including a BglII site between the stop codon of the CAT gene and the stretch of U&#39;s in the 5′ noncoding region. The T7 promoter adjacent to the 5′ noncoding sequences and the HgaI site downstream of the 3′ end allow for the exact tailoring of the 5′ and 3′ ends. Run-off transcription using T7 polymerase generates a 716 nt long RNA:  FIG. 12 , lanes 2-4 show that this RNA is of discrete length and shorter than the 890 nt long marker NS RNA, which was synthesized by T7 transcription of pHgaNS (lane 1).  
     7.2.2. The IVACAT1 RNA is Transcribed In Vitro by the Influenza A Virus RNA Polymerase  
      In the examples described in Section 6, it was demonstrated that synthetic RNAs containing at the 3′ end the 15 3′ terminal nucleotides of influenza virus RNA segment 8 can be transcribed in vitro using purified influenza A virus RNA polymerase. We tested whether unlabeled IVACAT1 RNA could be transcribed in a similar way.  FIG. 12  lane 5 shows that the in vitro transcription reaction generated an RNA of discrete length and similar size to the product of the T7 transcription reaction suggesting synthesis of a full length product.  
     7.2.3. RNP-Transfection and CAT Activity  
      Since the recombinant CAT RNA could be transcribed in vitro, a system was designed to test whether this RNA can be recognized and replicated in vivo ( FIG. 13 ). Recombinant RNA was mixed with the purified polymerase to allow formation of viral RNP-like particles. To facilitate the association, the RNA/polymerase mixture was incubated in transcription buffer without nucleotides for 30 minutes at 30° C. prior to RNP-transfection. In some experiments, this preincubation step was omitted. RNP-transfections were either preceeded or followed by infection with influenza A/WSN/33 virus, since the production of viral polymerase protein was expected to be necessary for efficient amplification of the gene. The cells used were either MDCK cells, which are readily susceptible to influenza A/WSN/33 virus infection, or human 293 cells, which support infection at a slower rate.  
      In order to determine whether the minus sense IVACAT1 RNA could be amplified and transcribed in vivo, an experiment was performed in 293 cells. Cells were transfected with RNP, virus infected one hour later and harvested at various times post-infection.  FIG. 14A  shows that at early times post infection only background levels of CAT activity were detected (lanes 5,7 and 9). However, significant levels of CAT activity appeared seven hours after virus infection (lane 11). A similar level of CAT activity was detected two hours later (lane 13). There were background levels of CAT activity in the mock transfected cells at any time point (lanes 6, 8, 10, 12 and 14), and in control cells not infected with A/WSN/33 virus (lanes 1-4).  
      Preincubation of RNA and polymerase complex was not necessary for successful RNP-transfection. As can be seen in  FIG. 14B , lanes 2 and 3, preincubation might actually cause a decrease in CAT activity, presumably due to RNA degradation during preincubation. In another control experiment, infection by helper virus of RNP-transfected cells was omitted ( FIG. 14B , lanes 4 and 5). Since these lanes show no CAT activity we conclude that the IVACAT1 RNA is amplified specifically by the protein machinery supplied by the helper virus. In an additional control experiment, naked RNA was transfected into cells which were subsequently helper-infected or mock-infected. Again, no CAT activity was detected in these samples ( FIG. 14B , lanes 6-9). Finally, virus-infected cells which were not transfected with recombinant CAT-RNP also did not exhibit endogneous acetylation activity ( FIG. 14B , lane 10). It thus appears that addition of the purified polymerase to the recombinant RNA as well as infection of cells by helper virus is important for successful expression of the CAT enzyme.  
      Experiments were also performed using MDCK cells, the usual tissue culture host cell for influenza virus ( FIG. 14C ). When the reconstituted recombinant CAT-RNP complex was transfected 1 hour before virus infection, little CAT activity was observed at 7 hours post virus infection ( FIG. 14C , lane 1). When RNP-transfection was accomplished 2 hours after virus infection, expression of CAT was greatly enhanced at 7 hours post-virus infection ( FIG. 14C , lane 3). Therefore, MDCK cells are also viable host cells for these experiments.  
     7.2.4. The CAT-RNP is Pacakaged into Virus Particles  
      Since the recombinant CAT RNA can be replicated in vivo via helper virus functions, we examined whether virus produced in RNP-transfected and helper virus infected cells contained the CAT gene. MDCK cells were used in the experiment because they yield higher titers of infectious virus than 293 cells. MDCK cells were infected with A/WSN/33 virus, RNP-transfected 2 hours later and allowed to incubate overnight. At 14 hours post infection, media was harvested and cells were pelleted. Virus supernatant was then used to infect new MDCK cell monolayers. The inoculum was removed after 1 hour and cells were harvested at 12 hours post infection and assayed for CAT activity.  FIG. 15  reveals that the virus preparation induces a level of CAT activity (lanes 2 and 3) which is significantly above control (lane 1). In this case, the addition of helper virus to the inoculum did not increase CAT activity (lane 4). Further passaging of supernatant virus on fresh MDCK cells did not result in measurable induction of CAT activity. This is not surprising as there is no selective pressure for retaining the CAT gene in these viral preparations. We excluded the possibility that we were transferring the original RNA/polymerase complex by pretreating the inocula with RNase. This treatment destroys viral RNPs of influenza virus (Pons et al. 1969 Virology 39: 250-259; Scholtissek and Becht, 1971 J. Gen. Virol. 10: 11-16).  
     8. Rescue of Infectious Influenza Viruses Using RNA Derived from Specific Recombinant DNA  
      The experiments described in the subsections below demonstrate the rescue of infectious influenza viruses using RNA which is derived from specific recombinant DNAs. RNAs corresponding to the neuraminidase (NA) gene of influenza A/WSN/33 virus (WSN virus) were transcribed in vitro from appropriate plasmid DNAs and—following the addition of purified influenza virus polymerase complex (as described in Section 6.1.1. supra)—were transfected into MDBK cells as described in Section 7, supra. Superinfection with helper virus, lacking the WSN NA gene, resulted in the release of viruses containing the WSN NA gene. Thus, this technology allows the engineering of infectious influenza viruses using cDNA clones and site-specific mutagenesis of their genomes. Furthermore, this technology may allow for the construction of infectious chimeric influenza viruses which can be used as efficient vectors for gene expression in tissue culture, animals or man.  
      The experiments described in Sections 6 and 7 supra, demonstrate that the 15 3′ terminal nucleotides of negative strand influenza virus RNAs are sufficient to allow transcription in vitro using purified influenza virus polymerase proteins. In addition, the studies using the reporter gene chloramphenicol acetyltransferase (CAT) show that the 22 5′ terminal and the 26 3′ terminal nucleotides of the viral RNAs contain all the signals necessary for transcription, replication and packaging of influenza virus RNAs. As an extension of these results, a plasmid, pT3NAv, was constructed which contained the complete NA gene of influenza A/WSN/33 virus downstream of a truncated T3 promoter ( FIG. 16 ). Therefore, runoff transcription of this plasmid, cut at the Ksp632I site, yields an RNA which is identical to the true genomic NA gene of the WSN virus ( FIG. 17 , lane 3). This RNA was then incubated with purified polymerase (purified as described in Section 6.1.1) and used in a ribonucleoprotein (RNP) transfection experiment to allow the rescue of infectious virus using helper virus which did not contain the WSN virus NA. The choice of WSN-HK helper virus was based on the need for a strong selection system by which to isolate a rescued virus. Previously, it was shown that the WSN-HK virus can only form plaques in MDBK cells when protease is added to the medium. This is in marked contrast to WSN virus (isogenic to WSN-HK virus except for the neuraminidase gene), which in the absence of protease readily replicates in MDBK cells and forms large, easily visible plaques (Schulman et al., 1977, J. Virol. 24:170-176).  
     8.1. Materials and Methods  
     8.1.1. Viruses and Cells  
      Influenza A/WSN/33 virus and A/WSN-HK virus were grown in Madin-Darby canine kidney (MDCK) cells and embryonated eggs, respectively (Sugiura et al., 1972, J. Virol. 10:639-647; Schulman et al., 1977, J. Virol. 24:170-176. Influenza A/PR/8/34 virus was also grown in embryonated eggs. Madin-Darby bovine kidney (MDBK) cells were used for the transfection experiments and for selection of rescued virus (Sugiura et al., 1972, J. Virol. 10:639-647).  
     8.1.2. Construction of Plasmids  
      The pT3NAv, pT3NAv mut 1 and pT3NAv mut 2 plasmids were constructed by PCR-directed mutagenesis using a cloned copy of the WSN NA gene, which was obtained following standard procedures (Buonagurio et al., 1986, Science 232:980-982). To construct pT3NAv, the following primers were used: 5′-CGGAATTCTCTTCGAGCGAAAGCAGGAGTT-3′ (SEQ ID NO.: 28) and 5′-CCAAGCTTATTAACCCTCACTAAAAGTAGAAACAAGGAGTTT-3′ (SEQ ID NO.: 63). After 35 cycles in a thermal cycler (Coy Lab products, MI), the PCR product was digested with EcoRI and HindIII and cloned into pUC19. Plasmid pT3NAv mut 1 was constructed in a similar fashion except that the sequence of the primer was altered ( FIG. 16 ). Plasmid pT3NAv mut 2 was constructed by cassette mutagenesis through the digestion of pT3NAv with PstI and NcoI and religation in the presence of the synthetic oligonucleotides—5′-CATGGGTGAGTTTCGACCAAAATCTAGATTATAAAATAGGATACATATGCA-3′ (SEQ ID NO.: 29) and 5′-AATGTATCCTATTTTATAATCTAGATTTTGGTCGAAACTCACC-3′. (SEQ ID NO.: 31) Oligonucleotides were synthesized on an applied Biosystems DNA synthesizer. The final clones pT3NAv, pT3NAv mut 1 and pT3NAv mut 2 were grown up and the DNAs were partially sequenced starting from the flanking pUC19 sequences and reaching into the coding sequences of the NA gene. The mutations in pT3NAv mut 2 were also confirmed by sequencing.  
     8.1.3. Purification of Influenza A Virus Polymerase and RNP Transfection in MDBK Cells  
      The RNA polymerase complex was purified from influenza A/PR/8/34 virus as described in Section 6.1.1, supra, and was then used for RNP transfection in MDBK cells using the protocol described in Section 7, supra, except that WSN-HK virus was used as helper virus at an moi of 1. RNAs used for RNP transfection were obtained by phenol extraction of purified virus or by transcription (using T3 polymerase) of pT3NAv, pT3NAv mut 1 and pT3NAv mut 2. All plasmids were digested with Ksp632I, end-filled by Klenow enzyme (BRL) and then transcribed in a runoff reaction as described in Section 7, supra.  
     8.2. Results  
     8.2.1. Rescue of Infectious Influenza Virus in MDBK Cells Using RNA Derived from Recombinant Plasmid DNA  
      A plasmid, pT3NAv, was constructed to contain the complete NA gene of influenza WSN virus downstream of a truncated T3 promoter ( FIG. 16 ). Runoff transcription of the plasmid, cut at the Ksp632I site, yields an RNA which is identical in length to the true genomic NA gene of the WSN virus ( FIG. 17 , lane 3). This RNA was then incubated with purified polymerase and used in a ribonucleoprotein (RNP) transfection experiment to allow the rescue of infectious virus using helper virus. The choice of WSN-HK virus as helper virus was based on the need for a strong selection system by which to isolate a rescued virus. Previously, it was shown that the WSN-HN virus can only form plaques in MDBK cells when protease is added to the medium (Schulman et al., 1977, J. Virol. 24:170-176). This is in marked contrast to WSN virus (isogenic to WSN-HK helper virus except for the neuraminidase gene), which is the absence of protease readily replicates in MDBK cells and forms large, easily visible plaques (Sugiura et al., 1972, J. Virol. 10:639-647). MDBK cells were first infected with the WSN-HK helper virus and RNP-transfected one hour after virus infection. Following overnight incubation in the presence of 20 μg/ml plasminogen, supernatant from these cells was then amplified and plaqued in MDBK cells in the absence of protease in the medium. The appearance of plaques in MDBK cells (Schulman et al., 197, J. Virol. 10:639-647) indicated the presence of virus which contained the WSN virus NA gene, since supernatant from control experiments of cells infected only with the WSN-HK virus did not produce plaques. In a typical experiment involving the use of a 35 mm dish for the RNP-transfection, 2.5×10 2  plaques were observed.  
      In another control experiment, synthetic NA RNA was used which was derived from plasmid pT3NAv mut 1 ( FIG. 16 ). This RNA differs from the wild type NA RNA derived from pT3NAv by a single nucleotide deletion in the nontranslated region of the 5′ end ( FIG. 16 ). RNP-transfection of MDBK cells with this RNA and superinfection with WSN-HK virus did not result in the formation of rescued virus. This negative result is readily explained since we have shown in Sections 6 and 7, supra, that the essential sequences for the recognition of viral RNA by viral polymerases as well as the packaging signals are located within the 3′ and 5′ terminal sequences of the viral RNAs. However, we cannot exclude the possibility that rescue of virus using this mutated RNA does occur, albeit at an undetected frequency.  
     8.2.2. RNA Analysis of Rescued Virus  
      Virus obtained in the rescue experiment was plaque purified, amplified in MDBK cells and RNA was extracted from this preparation. The RNA was then analyzed by electrophoresis on a polyacrylamide gel.  FIG. 17  shows the RNA of the helper virus WSN-HK (lane 1) and the synthetic NA RNA (lane 3), which was transcribed by T3 polymerase from plasmid pT3NAv. The migration pattern of the RNAs of the rescued virus (lane 2) is identical to that of control WSN virus (lane 4). Also, the NA RNAs in lanes 2 and 4 migrate at the same position as the NA RNA derived from cDNA (lane 3) and faster than the HK virus NA band in the helper WSN-HK virus (lane 1). These experiments support the conclusion that as a result of the RNP-transfection, infectious virus was formed containing WSN virus NA RNA derived from cDNA.  
     8.2.3. Rescue of Infectious Influenza Virus Using Virion RNA  
      In another transfection experiment, RNA extracted from purified WSN virus was employed. When this naked RNA is transfected together with the polymerase proteins into helper virus infected cells, rescue of WSN virus capable of replicating in MDBK cells is observed. RNA isolated from an amplified plaque in this experiment is analyzed in lane 5 of  FIG. 17  and shows a pattern indistinguishable from that of the control of WSN virus in lane 4.  
     8.2.4. Introduction of Site-Specific Mutations into the Viral Genome  
      The experiments described so far involved the rescue of influenza WSN virus. Since the synthetic RNA used in these experiments is identical to the authentic WSN NA gene, the unlikely possibility of contamination by wild type WSN virus could not be rigorously ruled out. Therefore, we introduced five silent point mutations into the coding region of the NA gene in plasmid pT3NAv. These mutations were introduced by cassette mutagenesis through replacement of the short NcoI/PstI fragment present in the NA gene. The five mutations in the cDNA included a C to T change at position 901 and a C to A change at position 925, creating a new XbaI site and destroying the original PstI site, respectively. In addition, the entire serine codon at position 887-889 of the cDNA clone was replaced with an alternate serine triplet ( FIG. 17 ). RMP-transfection of this mutagenized RNA (pT3NAv mut 2) and helper virus infection of MDBK cells again resulted in the rescue of a WSN-like virus which grew in MDBK cells in the absence of added protease. When the RNA of this virus was examined by sequence analysis, all five point mutations present in the plasmid DNA ( FIG. 16 ) were observed in the viral RNA ( FIG. 18 ). Since it is extremely unlikely that these mutations evolved in the wild type influenza WSN virus, we conclude that successful rescue of infectious influenza virus containing five site-specific mutations was achieved via RNP-transfection of engineered RNA.  
     9. EXAMPLE  
     Synthetic Replication System  
      In the experiments described below, a cDNA clone which can produce an influenza virus-like vRNA molecule coding for a reporter gene was used. This resultant RNA is an NS-like vRNA which contains the antisense of the coding region of the chloramphenicol acetyltransferase gene (CAT) in place of the antisense coding regions for the nonstructural proteins, NS1 and NS2 (Section 7, supra). This recombinant RNA (IVACAT-1) was incubated with purified influenza virus RNP proteins and used in an attempt to develop a non-influenza virus dependent replication system. Mouse fibroblast C127 cells were infected with mixtures of recombinant vaccinia viruses (Smith et al., 1987, Virology, 160: 336-345) and transfected one hour later with the IVACAT-1 RNP. Mixtures of vectors expressing the three polymerases (PB2, PB1 and PA) and the nucleoprotein were used. Replication and transcription of the synthetic RNP was assayed by analyzing cells for CAT activity after overnight incubation.  FIG. 19  examines the CAT activity present in cells initially infected with many of the possible mixtures of the 4 recombinant vaccinia viruses.  FIG. 19 , lane 4 is a positive control in which the influenza A/WSN/33 virus was used in lieu of the recombinant vaccinia viruses. CAT activity is present in this sample as well as in cells infected with all four vaccinia vectors ( FIG. 19 , lanes 8 and 10). Cells expressing any of the subsets of these four proteins did not produce detectable CAT protein ( FIG. 19 , lanes 5-7, 9, 11). In addition, transfected RNA not incubated with the purified polymerase was also negative for CAT expression (Section 7, supra). Thus, the presence of the PB2, PB1, PA and NP proteins are all that is necessary and sufficient for RNP expression and replication in this system. The levels of CAT activity obtained in vaccinia vector-infected cells are reproducibly higher than in cells infected with influenza as helper virus. The most probable explanation for this is that in influenza virus-infected cells, the CAT-RNP competes with the endogenous viral RNP&#39;s for active polymerase whereas in the vaccinia driven system that CAT-RNP is the only viral-like molecule present.  
      A number of other cell lines were then tested as hosts for this vaccinia virus driven system.  FIG. 20A  shows the results using MDBK, Hela, 293 and L cells. In each case, no CAT activity was observed when cells were infected with vectors that express only the 3 polymerase proteins but significant CAT activity was obtained if the additional vaccinia-vector inducing NP expression was also added.  
      Previously, a cell line (designated 3PNP-4) was constructed which constitutively expresses low levels of the PB2, PB1 and PA proteins and high levels of the NP protein. These cells can complement the growth of ts mutants mapping either to the PB2 or NP gene segments (Krystal et al., 1986, Proc. Natl. Acad. Sci. USA 83:2709-2713; Li et al., 1989, Virus Research 12:97-112). Since replication through recombinant vaccinia virus vectors is dependent only on these proteins, it was conceivable that this cell line may be able to amplify and express the synthetic CAT-RNP in the absence of any virus infection. However, when this experiment was attempted, no detectable CAT activity was obtained. In order to investigate the reasons why this cell line did not support replication, mixtures of recombinant vaccinia viruses were used to infect 3PNP-4 cells. As we expected from the results described in Section 7, supra, the addition of the four polymerase proteins supported the expression of CAT ( FIG. 20B , lane 2).  FIG. 20B , lane 3 shows that the minimum mixture of vectors needed to induce CAT activity in 3PNP-4 cells are those expressing only the PB1 and PA proteins. Therefore, the steady state levels of PB2 and NP proteins in 3PNP-4 cells are sufficient but the levels of PB1 and PA are below threshold for CAT expression in the absence of helper virus. This correlates with the complementation phenotype exhibited by these cells, since only the growth of PB2 and NP mutants and not PB1 and PA mutants can be recovered at non-permissive temperature (Desselberger et al., 1980, Gene 8:315-328).  
      Since the synthetic IVACAT-1 RNA is of negative polarity, CAT can only be synthesized via transcription off the RNP molecule. Theoretically, detectable levels of CAT can be produced either through transcription off the transfected input RNP (equivalent to primary transcription) or first through amplification of RNP and subsequent transcription (necessitating RNP replication) or a combination of both. However, previous experiments described in Section 7, supra, using influenza virus infection to drive the expression of the CAT protein showed that detectable expression occurred only if the input CAT-RNP was replicated (Section 7, supra). This was shown by the use of a second CAT-RNA, IVACAT-2, which contains 3 mutations within the 12 bases at the 5′ end of the viral RNA (Section 7, supra). This 12 base region is conserved among all eight gene segments in all influenza A viruses (Desselberger, at al., 1980, Gene 8:315-328). This synthetic IVACAT-2 RNP is competent for transcription by the influenza virus polymerase but it is not replicated and when transfected into influenza virus-infected cells CAT activity remained undetected (Section 7, supra). Therefore, primary transcription off the input RNA does not produce detectable levels of protein in influenza virus infected cells. Accordingly, we used this mutant RNA to examine whether the vaccinia vector-expressed influenza proteins induces CAT activity solely through primary transcription of input RNP or can allow for amplification through replication and subsequent transcription. C127 cells were infected with the recombinant vaccinia viruses and then transfected with either IVACAT-1 and IVACAT-2 generated RNPs.  FIG. 20C  shows that low levels of CAT activity can be detected in cells transfected with IVACAT-2 RNP (lane 2). When quantitated, 0.5-1% of the chloramphenical is converted to an acetylated form, compared to 0.2-0.4% in mock transfected lanes. However, much greater levels of activity are present in cells transfected with CAT-1 RNP (lane 1; routinely 15-50% conversion of chloramphenicol), indicating that amplification is occurring in these cells. Therefore, this recombinant vaccinia virus-driven system is sequence-specific and the RNP&#39;s are undergoing replication.  
      In the experiments described, neither the NS1 nor NS2 proteins were required for RNP replication. Although their function is not known it has been speculated that these proteins may play a major role in replication because both proteins are synthesized in large amounts and are present in the nucleus (Krug et al., 1989, The Influenza Viruses, Krug, R. Ed., Plenum Press, NY, 1989, pp. 89-152; Young et al., 1983, Proc. Natl. Acad. Sci. USA 80:6105-6109; Greenspan et al., 1985, 1985, J. Virol. 54:833-843; Lamb et al., 1984, Virology 135:139-147). Based on the data presented, these proteins are not absolutely required for genome replication. It may be speculated that these proteins may actually have ancillary roles with regard to the replication of RNP, such as interaction with host factors, regulation of the expression of viral genes or some function involved with packaging of the RNP into infectious virions. However, it can not be ruled out that a function of these NS proteins may be complemented by a vaccinia virus protein, although upon inspection, no obvious similarities were found between either the NS1 or NS2 proteins and known vaccinia virus proteins. The contrasting properties of these two viruses also argues against a complimenting vaccinia virus protein, as vaccinia is a large double-stranded DNA virus replicating exclusively in the cytoplasm while influenza virus is a negative sense RNA virus replicating exclusively in the nucleus. In addition, the replication of the synthetic RNPs occurred even in the presence of cytosine arabinoside (ara-C), an inhibitor of late gene expression in vaccinia virus (Oda et al., 1967, J. Mol. Biol. 27:395-419; Kaverin et al., 1975, Virology 65:112-119; Cooper et al., 1979, Virology 96:368-380).  
      This recombinant vaccinia vector dependent scheme possesses a number of advantages over the use of influenza virus infection to drive the replication of synthetic RNA. For one, since the expression of the viral proteins is completely artificial it will allow for a precise dissection of the processes involved in replication. Replication first involves the synthesis of positive sense template from the negative sense genomic RNA. This positive sense cRNA is then copied in order to amplify genomic sense RNP, which is then used for protein expression and packaging (Krug et al., 1989, supra). The system described herein demonstrate that only the influenza viral PB2, PB1, PA and NP proteins are required for the detection of expressed protein and for replication of RNP. Another advantage of this vaccinia vector driven replication scheme is that since the influenza polymerase proteins are expressed from cDNA integrated into the vaccinia virus, the mutagenesis of the polymerase proteins becomes a feasible and powerful method to further analyze structure-function relationships of the viral polymerase proteins.  
     10. EXAMPLE  
     Use of Bicistronic Influenza Vectors for the Expression of a Foreign Protein by a Transfectant Influenza Virus  
      In the example presented herein, the coding capacity of an influenza virus was successfully increased by the construction of a bicistronic influenza virus RNA segment. The use of the bicistronic approach allowed this increase in the coding capacity with no alteration in the structure of the viral proteins.  
      Specifically, two influenza A viruses containing bicistronic neuraminidase (NA) genes were constructed. The mRNA molecules derived from the bicistronic NA genes have two different open reading frames (ORFs), with the first encoding a foreign polypeptide and the second encoding the NA protein. The second (NA) polypeptide&#39;s translation is achieved via an internal ribosome entry site (IRES) which is derived from the 5′ noncoding region of the human immunoglobulin heavy-chain binding protein (BiP) mRNA.  
     10.1. Material and Methods  
     Viruses and Cells  
      The influenza A virus strain X-31, which is a reassortant of influenza A/HK/68 and A/PR/8/34 viruses, was grown in the allantoic fluid of embryonated chicken eggs, purified by sucrose density gradient centrifugation, and supplied by Evans Biological Ltd, Liverpool, UK. Influenza A/WSN/33 (WSN) virus was grown in Madin-Darby bovine kidney (MDBK) cells in reinforced minimal essential medium (REM). WSN-HK virus, a reassortant influenza virus which derives the NA gene from influenza A/HK/8/68 virus and the seven remaining RNA segments from WSN virus, was grown in the allantoic fluid of 10-day-old embryonated chicken eggs (Section 8, supra). MDBK cells were used in RNP transfection experiments and for the selection and plaque purification of transfectant viruses. Madin-Darby canine kidney (MDCK) cells were infected with transfectant viruses for use in immunostaining experiments.  
      Construction of plasmids. Plasmids were constructed by standard techniques (Maniatis, T., 1982, Molecular Cloning: A Press Laboratory Manual, Cold Spring Harbor Laboratory, Cold Spring Harbor, N.Y.). pT3NACAT(wt) contains the CAT gene in negative polarity flanked by the 3′ and 5′ noncoding regions of the WSN NA gene, under the transcriptional control of a truncated T3 promoter. pT3NA/BIP was constructed as follows: first, a PCR product was obtained using the oligonucleotides 5′-GGCCACTAGTAGGTCGACGCCGGC-3′ (SEQ ID NO.: 32) and 5′-GCGCTGGCCATCTTGCCAGCCA-3′ (SEQ ID NO.: 33) as primers, and a plasmid containing the 5′ noncoding region of the BiP gene as template. This PCR product was digested with restriction enzymes Msci and Spel and cloned into Msci and Xba1 digested pT3NA/EMC. The resulting plasmid, pT3NA/BIP contains the ORF of the NA followed by the IRES sequences of the BiP gene (nucleotide positions 372 to 592 of the GenBank data base entry HUMGRP78). pT3NA/BIP-CAT contains, in addition, the CAT ORF following the BiP-IRES-derived sequences. pT3NA/BIP-CAT was constructed by inserting into Msci digested pT3NA/BIP the PCR product which was obtained by using the primers 5′-AGAAAAAAATCACTGGG-3′ (SEQ ID NO.: 34) and 5′-TTACGCCCCGCCCTGCC-3′ (SEQ ID NO.: 35) and template pIVACAT1/S (Piccone, M. E. et al., 1993, Virus Res. 28:99-112). A fragment of approximately 920 nt derived from the NA ORF was deleted from pT3NA/BIP-CAT by digestion with PpuM1 and Spe1, trimming and religation of the plasmid. The resulting plasmid was called pT3delNA/BIP-CAT. To construct pT3BIP-NA, the PCR product obtained by using the primers 5′-GCGCATCGATAGGTCGACGCCGG-3′ (SEQ ID NO.: 36) and 5′-GGCCATCGATCCAATGGTTATTATTTTCTGGTTTGGATTCATCTTGCCAGTTGGG-3′ (SEQ ID NO.: 37) and a plasmid containing the 5′ noncoding region of the BiP gene as template was digested with Cla1 and inserted into Cla1 digested pT3NAM1. pT3NAM1 contains the NA gene of WSN virus into which a Cla1 site has been inserted at nucleotide positions 52-57 by two silent changes. The resulting plasmid, pT3BIP-NA, has the BIP-IRES-derived sequences in front of the NA ORF. To construct pT3GP2/BIP-NA, a PCR product was obtained using oligonucleotides 5′-ATGACTGGATCCGCTAGCATGGCCATCATTTATCTCATTCTCCTGTTCACAGCAGTGAGAGGGGACCAGATAGAAGAATCGCAAAACCAGC-3′ (L primer) (SEQ ID NO.: 38) and 5′-ATGACAGAATTCGTCGACTTATCTATTCACTACAGAAAG-3′ (M primer) (SEQ ID NO.: 39) as primers and a plasmid containing the DNA copy of the genome of the HIV-1 isolate BH 10 (GenBank data base entry HIVBH102) as template. The PCR product was digested with BamH1 and EcoR1 and cloned into BamH1/EcoR1 digested pGEX-2T (Pharmacia). This clone was used as template for the generation of a PCR product with the primers “M” and 5′-GCGCGAAGACGCAGCAAAAGCAGGAGTTTAAGCTAGCATGGCCATCATTTATC-3′ (SEQ ID NO.: 40). The resulting PCR product was digested with Bbs1 and Sal1, and ligated into Bbs1/Sal1 digested pT3BIP-NA. The resulting plasmid, pT3GP2/BIP-NA, has an ORF in front of the BIP-IRES sequences which codes for a gp41-derived polypeptide, containing 38 aa of the ectodomain of gp41, the 22 aa of the transmembrane domain and 2 aa of the cytoplasmic tail of gp41. This sequence is preceded by the signal peptide (15 aa) and the first 2 aa of the HA of influenza A/Japan/305/57. For the construction of pT3HGP2/BIP-NA, a PCR product containing the sequences encoding the transmembrane and cytoplasmic tail of the HA of WSN virus was obtained using the primers 5′-CGATGGATCCGCTAGCTTGGAATCGATGGGGGTGTATC-3′ (SEQ ID NO.: 41) and 5′ ATCGATGAATTCGTCGACTCAGATGCATATTCTGCAC-3′ (SEQ ID NO.: 42) and pT3/WSN-HA (Enami, M. and Palese, P., 1991, J. Virol. 65:2711-2713) as template. This PCR product was digested with restriction enzymes BamH1 and Sal1 and subcloned into BamH1/Sal1 digested pGEX-2T. A second PCR product was inserted into this subclone between the BamH1 and Cla1 restriction sites. The second PCR product was obtained using oligonucleotides “L” and 5′-ATGACTGTCGACCCATGGAAGTCAATCGATGTTATGTTAAACCAATTCCAC-3′ (SEQ ID NO.: 43) as primers and the plasmid containing the DNA copy of the HIV-1 genome as template. From this plasmid an Nhe1-Sal1 fragment was recloned into Nhe1/Sa1 digested pT3GP2/BIP-NA, and the resulting plasmid was called pT3HGP2/BIP-NA. The first ORF of pT3HGP2/BIP-NA codes for a polypeptide which has an ectodomain (39 aa, of which 31 aa are derived from the gp41 protein ectodomain), followed by the transmembrane and cytoplasmic domains of the HA of influenza WSN virus (37 aa). Oligonucleotides were synthesized using an Applied Biosystems DNA synthesizer. The presence of the appropriate sequences in the plasmid DNAs was confirmed by sequencing with a DNA sequencing kit (United States Biochemical Corporation).  
      Ribonucleoprotein (RNP) transfections. Influenza virus RNA polymerase was isolated from influenza X-31 virus as previously described (Section 7, supra). RNP transfections were performed in influenza virus-infected MDBK cells according to Enami and Palese (Enami, M. and Palese, P., 1991, J. Virol. 65:2711-2713). Briefly, plasmids used in transfections were digested with Ksp6321 or Bbs1 restriction enzymes. 500 ng of linearized plasmid was transcribed with T3 RNA polymerase (Stratagene) in the presence of purified influenza virus polymerase. The resultant RNP complexes were DEAE-transfected into WSN or WSN-HK infected MDBK cells.  
      CAT assays. After RNP-transfection of 10 6  WSN-infected MDBK cells in a 35-mm-diameter dish, cells were further incubated at 37° C. for 16 h in the presence of 1.5 ml of REM medium and after that period cells were harvested by using a rubber policeman.  
      Following low speed centrifugation, cell pellets were resuspended in 100 μl of 0.25 M Tris-HCl buffer (pH 75) and determinations of CAT activity in these samples were performed as previously described (Li, X. and Palese, 1992, J. Virol., 66:4331-4338).  
      Selection of Influenza virus transfectants. RNP transfections were performed in the same way as described before for CAT expression, except that MDBK cells were infected with WSN-HK helper virus (Enami, M. and Palese, P., 1991, J. Virol. 65:2711-2713). Medium from transfected cells was harvested 18 h after transfection and used for infection of a subconfluent monolayer of MDBK cells in an 80-cm 2  flask. Infected cells were incubated 4 days at 37° C. in REM and transfectant viruses released to the medium were plaque purified three times in MDBK cells covered with agar overlay media.  
      Virus purification. WSN virus and transfectant viruses were grown in MDBK cells after infection at an MOI of 0.01. Media from infected cells was harvested 2 days after infection and clarified by two 30 minute centrifugations at 3,000 rpm and 10,000 rpm, respectively. Viruses were purified from supernatants by pelleting in a Beckman SW27 rotor at 25,000 rpm (90 min) through a 10 ml 30% sucrose cushion in NTE buffer (NaCl 100 mM, Tris-HCl 10 mM, EDTA 1 mM, pH 8.0). Virus pellets were resuspended in NTE and the protein concentration in purified virus samples was determined by the Bio-Rad protein assay (Bio-Rad). In order to test the purity of the samples, 100 ng of protein were subjected to SDS-PAGE in a 10% polyacrylamide gel according to Laemmli (Laemmli, U. K., 1970, Nature 227:680-685) and protein bands were visualized by silver staining (Merril, C. R. et al., 1981, Science 211:1437-1438). In all cases, the viral proteins in the nt of protein. Samples represented more than 90% of the total amount of protein.  
      RNA extraction, electrophoresis and sequencing. RNAs were extracted from purified viruses as previously described (Luo, G. et al., 1992, J. Virol. 66:4679-4685). Approximately 100 ng of virion RNAs were electrophoresed on a 2.8% polyacrylamide gel containing 7.7 M urea at 150 V for 110 min. The RNA segments were visualized by silver staining (Section 8, supra). The NA-RNA segment of transfectant viruses was sequenced as follows: first, 100 ng of viron RNAs were used for a reverse transcription reaction using the primer 5′-GCGCGAATTCTCTTCGAGCAAAAGCAGG-3′ (SEQ ID NO.: 44) (EKFLU, annealing to the last 12 nt at the 3′ end of the influenza A virus RNAs), and SuperScript reverse transcriptase (GibcoBRL). The obtained cDNAs were PCR-amplified using the primers EKFLU and 5′-AGAGATGAATTGCCGGTT-3′ (SEQ ID NO.: 45) (corresponding to nt positions 243-226 of the NA gene). PCR products were cloned into pUC19 (New England Biolabs), and sequenced with a DNA sequencing kit (United States Biochemical Corporation).  
      Immunostaining of Infected cells. Confluent MDCK monolayers in a 96-well plate were infected with transfectant or wild-type influenza viruses at an MOI≧2. Nine hours postinfection, cells were washed with PBS and fixed with 25 μl of 1% paraformaldehyde in PBS. Then, cells were incubated with 100 μl of PBS containing 0.1% BSA for 1 hour, washed with PBS three times, and incubated 1 h with 50 μl of PBS, 0.1% BSA containing 2 μg/ml of the human monoclonal antibody 2F5. This antibody recognizes the amino acid sequence Glu-Leu-Asp-Lys-Trp-Ala (ELDKWA) (SEQ ID NO.: 46) which is present in the ectodomain of gp41 of HIV-1 (Muster, T. et al., 1993, J. Virol. 67:6642-6647). After three PBS washings, 2F5-treated cells were incubated with 50 μl of PBS, 0.1% BSA, containing a 1:100 dilution of a peroxidase-conjugated goat antibody directed against human immunoglobulins (Boehringer Mannheim). Finally, cells were PBS-washed three times, and stained with a peroxidase substrate (AEC chromogen, Dako Corporation).  
      Western Immunoblot analysis. Confluent monolayers of MDBK cells in 35-mm dishes were infected with transfectant or wild-type influenza viruses at an MOI≧2. Eight hours postinfection, infected cells were lysed in 100 μl of 50 mM Tris-HC 1  buffer pH 8.0 containing 150 mM NaCl, 1% NP-40 and 1 mM Pefabloc (Boehringer Mannheim). 5 μl of these cell extracts or 2 μg of purified viruses were subjected to SDS-PAGE on a 5-20% polyacrylamide gradient gel, according to Fairbanks et al. (Fairbanks, G. et al., 1991, Biochemistry 10:2606-2617). Proteins were subsequently transferred to a nitrocellulose membrane, and the monoclonal antibody 2F5 was used to detect the gp41-derived polypeptides. The Western blot was developed with an alkaline phosphatase-coupled goat antibody against human immunoglobulins (Boehringer Mannheim).  
     10.2. Results  
      In this report, it is shown that the IRES element derived from the 5′ noncoding region of the human immunoglobulin heavy-chain binding protein (BiP) mRNA (Macejak, D. G. and Sarnow, P., 1991, Nature 353:90-94) is able to promote translation of a downstream ORF in influenza virus-infected cells. Specifically, transfectant influenza viruses were constructed containing bicistronic NA segments which express a foreign polypeptide on the surface of infected cells in addition to the NA protein. The foreign polypeptide is translated in infected cells from the bicistronic mRNA via cap-dependent initiation of translation. The NA is translated via internal binding of the ribosome to the bicistronic mRNA, which contains the BIP-IRES element. Furthermore, it is shown that the foreign polypeptide is incorporated into the virus particle.  
     10.2.1. A Synthetic Influenza Virus-Like Gene is Expressed Under the Translational Control of the BIP-IRES Element in Influenza Virus-Infected Cells  
      pT3delNA/BIP-CAT ( FIG. 21A ) was constructed in order to study an influenza virus gene whose second ORF is preceded by the BIP-IRES element. This plasmid contains the CAT ORF in negative polarity downstream of the BIP-IRES sequence. Since the BIP-IRES derived sequences do not contain an initiation codon, a short ORF (110 aa) derived from the NA gene (delNA) was inserted upstream of the IRES. The delNA ORF was used instead of the full-length NA in order to reduce the size of the synthetic gene, since the RNP-transaction efficiency decreases with the length of the transfected gene. Recognition of the delNA/BIP-CAT gene by the influenza virus transcription machinery was achieved by flanking the gene with the 3′ and 5′ noncoding regions of the influenza virus NA gene. pT3delNA/BIP-CAT was linearized with Ksp6321 to allow runoff transcripbon in vitro using T3 RNA polymerase. The resulting delNA/BIP-CAT RNA was incubated with purified influenza virus polymerases to form RNP complexes which were transfected into WSN infected MDBK cells.  
      Cells were collected 16 hours posttransfection and extracts were assayed for CAT enzyme ( FIG. 22 ). For comparison, an RNP transfection was performed using NACAT(wt) RNA, which contains the CAT gene flanked by the noncoding sequences of the NA gene of WSN. As shown in  FIG. 22 , the delNA/BIP-CAT RNA was transcribed in influenza virus-infected cells and the resulting mRNA was translated into the CAT protein. It is thus likely that CAT expression from the delNA/BIP-CAT mRNA started by internal binding of the ribosomes to the BIP-IRES sequences and that initiation of translation began at the ATG of the CAT ORF. This could be due to 1) different transfection efficiencies of the RNAS, delNA/BIP-CAT RNA being longer than NACAT(wt) RNA, or 2) a lower translation efficiency of the CAT ORF in the context of the delNA/BIP-CAT construct.  
     10.2.2. Rescue of a Transfectant Influenza Virus in Which the BIP-IRES Element was Inserted Upstream of the NA ORF  
      CAT expression from delNA/BIP-CAT RNA in influenza virus-infected cells suggested that the BIP-IRES could be used for the construction of transfectant influenza viruses containing a bicistronic gene. Such a gene could direct from the same mRNA the synthesis of both a foreign protein and an essential virus protein. Thus, the rescue of a transfectant virus whose NA-specific ORF was preceded by the BIP-IRES element was attempted. Plasmid pT3BIP-NA was engineered ( FIG. 21B ) to direct the synthesis of an RNA which contains the NA ORF of WSN virus downstream of a BIP-IRES element, both in negative polarity, and flanked by the 3′ and 5′ noncoding regions of the NA RNA segment of WSN virus. As in delNA/BIP-CAT RNA, a short ORF (42 aa) derived also from the NA gene was introduced upstream of the IRES sequences. Thus, initiation of translation of the NA protein by ribosomal scanning of the 5′ end of the BIP-NA derived mRNA is unlikely. RNP-transfection of BIP-NA into WSN-HK infected MDBK cells resulted in the rescue of infectious virus.  
      In order to confirm the presence of the transfected gene in the rescued BIP-NA virus, viral RNA was extracted from purified virions and analyzed by polyacrylamide gel electrophoresis ( FIG. 23 ). The RNA preparation of the transfectant virus did not show an RNA band at the position of the wild type NA gene, and contained a new RNA segment whose length was identical to that of the in vitro synthesized pT3BIP-NA RNA. Confirmation of the presence of the transfected gene in the BIP-NA virus was obtained by PCR-amplification and sequencing of the 3′ end of the NA gene, as described in Materials and Methods, in Section 10.1., above. The sequence was found to be identical to that of the corresponding plasmid.  
     10.2.3. Rescue of Transfectant Influenza Viruses Whose Bicistronic NA Genes Contain a Foreign ORF  
      Since in was now possible to rescue the transfectant BIP-NA virus, the next step was to attempt to insert a foreign ORF upstream of the BIP-IRES element. pT3GP2/BIP-NA and pT3HGP2/BIP-NA were constructed ( FIG. 21C ). The GP2/BP-NA RNA contains two ORFS: The first ORF, GP2, codes for a polypeptide containing 38 aa of the ectodomain of the gp41 protein of HIV-1, the complete transmembrane domain and the first two amino acids of the cytoplasmic tail of gp41.  
      In order to target the GP2 polypeptide to the endoplasmic reticulum (ER), the leader peptide-coding sequence of the HA protein of influenza A/Japan/305/57 virus was fused in frame to the GP2 ORF. The second ORF of the GP2/BIP-NA RNA codes for the WSN NA protein. This second ORF is under the translational control of the BIP-IRES element. The HGP2/BIP-NA RNA is identical to the GP2/BIP-NA RNA except that 7 aa of the ectodomain and the transmembrane and cytoplasmic domains of the encoded GP2 polypeptide were substituted by 6 aa of the ectodomain and the transmembrane and cytoplasmic domains of the HA protein of the influenza WSN virus.  
      RNP-transfection of GP2/BIP-NA and HGP2/BIP-NA RNAs into WSN-HK infected MDBK cells resulted in the rescue of these RNAs into transfectant viruses. The presence of the transfected gene in GP2/BIP-NA and HGP2/BIP-NA transfectant viruses was confirmed by polyacrylamide gel electrophoresis of viral RNAs isolated from purified virions ( FIG. 24 ), and by PCR-amplification and sequencing of the 3′ end of the NA RNA segments of the transfectants. GP2/BIP-NA and HGP2/BIP-NA transfectant viruses grew to half a log and one and a half log lower titers, respectively, than wild-type transfectant virus.  
     10.2.4. GP2 and HGP2 Expression in Transfectant Virus-Infected Cells  
      Since the NA is required for viral infectivity in MDBK cells, transfectant viruses GP2/BIP-NA and HGP2/BIP-NA were able to express the NA protein in infected cells from their bicistronic NA genes. In order to study the expression of the GP2 and HGP2 polypeptides in transfectant virus-infected cells, infected MDCK cells were immunostained using the human monoclonal antibody 2F5. This antibody is specific for the amino acid sequence ELDKWA (SEQ ID NO.: 46) of gp41, which is present in the polypeptides GP2 and HGP2. The results are shown in  FIG. 25 . Wild-type WSN virus-infected cells did not stain with the gp41-specific antibody 2F5. In contrast, both GP2/BIP-NA and HGP2/BIP-NA virus-infected cells showed positive staining with the 2F5 antibody. These results indicate that the first cistron (GP2 or HGP2) of the NA RNA segment of these two transfectant viruses is expressed in infected cells.  
      The pattern of immunostaining was different for GP2/BIP-NA and HGP2/BIP-NA virus-infected cells. Most of the staining in HGP2/BIP-NA infected cells was localized on the cellular surface, specifically at the junction between cells. In contrast, cytoplasmic structures, possibly corresponding to the ER or the Golgi, are strongly stained in GP2/BIP-NA infected cells. This finding might indicate that the GP2 protein is transported to the membrane at a slower rate than HGP2 protein. Alternatively, the GP2 protein may be retained in the ER or the Golgi. Although a conventional reagent was not utilized for cell permeabilization in these experiments, it is assumed that cell fixation with 1% paraformaldehyde permeabilizes the cells to some extent, allowing cytoplasmic staining.  
     10.2.5. HGP2 Polypeptide is Incorporated into Virus Particles  
      The presence of polypeptides GP2 and HGP2 in GP2/BIP-NA and HGP2/BIP-NA virus-infected MDBK cells and in purified virions was analyzed by Western immunoblotting, using the 2F5 antibody. As shown in  FIG. 26A , a low molecular weight polypeptide was detected in GP2/BIP-NA and HGP2/BIP-NA virus-infected cells ( FIG. 26A , lanes 2 and 3) and not in wild-type WSN virus-infected cells ( FIG. 26A , lane 1). The presence of an additional protein band in HGP2/BIP-NA infected cells ( FIG. 26A , lane 3) might be attributed to different levels of glycosylation of the HGP2 protein. A putative glycosylation site, Asn-X-Thr, is present in both the GP2 and the HGP2 polypeptides. When the infected cell extracts were incubated with PNGase (NEB) prior to electrophoresis only one band was detected. In addition, a gp41-derived polypeptide was also detectable in purified HGP2/BIP-NA (and not in in GP2/BIP-NA or WSN) virions ( FIG. 26B ). These results indicate that both polypeptides GP2 and HGP2 are expressed in cells infected with the corresponding transfectant viruses. However, only the HGP2 polypeptide, which contains the transmembrane and the cytoplasmic tail of the WSN HA protein, is incorporated into virus particles.  
      In conclusion, it is demonstrated here that transfectant influenza viruses containing bicistronic NA genes have been constructed. These bicistronic genes are maintained in the virus population after passaging since the gene is required for the expression of the essential viral NA protein. In addition, this transfectant virus directs the synthesis of a foreign polypeptide.  
      For the construction of a bicistronic influenza virus gene a mammalian IRES sequence was utilized. IRES sequences were first discovered in the nontranslated regions of picornaviral mRNAs (Jang, S. K. et al., 1989, J. Virol. 63:1651-1660; Jang, S. K. et al., 1988, J. Virol. 62:2636-2643; Pelletier, J. and Sonenberg, 1988, Nature 334:320-325) derived from EMCV, poliovirus, rhinovirus, or foot-and-mouth disease virus. Although, the EMCV IRES has been used for the construction of bicistronic genes in chimeric retroviruses and polioviruses (Adam, M. A. et al., 1991, J. Virol. 65:4985-4990; Alexander, L. et al., 1994, Proc. Natl. Acad. Sci. USA 91:1406-1410; Molla, A. et al., 1992, Nature 356:255-257) expression of a reporter gene engineered downstream of an EMCV IRES in a synthetic influenza virus gene was undetectable. Nonviral IRES elements were therefore considered. In this example, the generation of functional bicistronic genes using the IRES element derived from the human BiP mRNA is described.  
      Other strategies to express foreign polypeptides involve the use of a fusion protein containing a protease signal, but these approaches result in the expression of altered proteins due to the presence of the specific protease signal in the polyproteins.  
      The BIP-IRES element, which allows the expression of a second independently translated cistron, was attractive for two reasons. First, it only has about 220 nucleotides and is thus shorter than the functionally equivalent picornaviral sequences. This is a desired characteristic since we have previously found packaging limitations with respect to the length of the influenza virus RNAS. Secondly, the BIP-IRES element shares no sequence or structural homology with the picornaviral elements, which is desirable since the latter appears to have only low activity in influenza virus-infected cells. The hepatitis C virus IRES element (Tsukiyama-Kohara, K. et al., 1992, J. Virol. 66:1476-1483) would be another attractive candidate for the construction of bicistronic influenza virus genes for the same reasons.  
      It was first attempted to determine if the BIP-IRES element was active in influenza virus-infected cells after RNP transfection using the CAT reporter system. Experiments demonstrated that the BIP-IRES element (in T3 delNA/BIP-CAT RNA) can initiate translation of the CAT gene in influenza virus-infected cells ( FIG. 22 ). Similar CAT constructs which contained the EMCV IRES or the rhinovirus 14 IRES instead of the BiP-derived sequence, did not express CAT protein in influenza virus-infected cells (see above).  
      Next, the investigation was extended by constructing an influenza virus (BIP-NA) whose NA protein was translated from an internal ORF preceded by the BIP-IRES element. The fact that this virus was viable indicates that the NA expression achieved using the IRES element was sufficient for the generation of an infectious influenza virus.  
      Finally, two transfectant influenza viruses, GP2/BIP-NA and HGP2/BIP-NA, were generated whose NA mRNAs directed translation of a foreign cistron (GP2 or HGP2, containing sequences derived from gp41 of HIV-1) by a conventional cap-dependent scanning mechanism, and of a second cistron (NA) by internal initiation from the BIP-IRES. The results are in agreement with the reported IRES activity of the nontranslated region of the BiP mRNA (Macejak, D. G. and Sarnow, 1991, Nature 353:90-94).  
      Although the proteins were not quantified, it is assumed that the levels of expression of the GP2 and HGP2 proteins in infected cells are similar to those of the NA in wild-type virus-infected cells, since the foreign recombinant ORFs are in the same background as the ORF of the wild-type NA. It is likely that the levels of expression of the foreign protein could be increased by using other influenza virus genes, such as the HA gene, which on a molar level appears to express 5-10 times more protein than the NA gene. Although the expression of the NA protein in transfectant virus-infected cells was not quantified, one might expect lower levels than in wild type virus-infected cells, since the transfectant viruses are attenuated in tissue culture. Also, RNP-transfection experiments using the construct delNA/BiP-CAT showed lower CAT expression levels than the control involving the wild-type construct NACAT(wt) ( FIG. 22 ).  
      Surface expression of a foreign protein may be desirable for the induction of a humoral immune response against the protein (Both, G. W. et al., 1992, Immunol. and Cell Biol. 70:73-78). Surface expression of both gp41-derived polypeptides was attempted by fusing inframe the coding sequence for the leader peptide of the HA of influenza A/Japan/305/57 with the coding sequence of the foreign protein. Surface expression was detected by immunostaining of infected cells using the antibody 2F5, which is specific for a linear epitope in the ectodomain of gp41. In addition, HGP2 polypeptide was successfully packaged into virus particles by including the transmembrane and cytoplasmic domains of the influenza virus HA. (It should be noted, however, that initial experiments following infection of mice with HGP2/BIP-NA virus did not show a vigorous immune response directed against the gp41 derived sequences.) In contrast to the finding with HGP2, the GP2 polypeptide, which contains the transmembrane domain derived from the HIV-1 gp41, was not packaged into virus particles. Recently, Naim and Roth (Naim, H. Y. and Roth, M. G., 1993, J. Virol. 67:4831-4841), reported that an HA-specific transmembrane domain is required for incorporation of the HA into influenza virions. The results discussed here are in agreement with this finding and they also demonstrate that the transmembrane and cytoplasmic domains of the HA protein contain all the signals required for incorporation of a protein into influenza virus envelopes.  
      In summary, the use of the BIP-IRES element for the construction of bicistronic influenza virus vectors may represent a new methodology for expressing foreign genes which should have practical applications in molecular and medical virology.  
     11. Deposit of Microorganisms  
      An  E. coli  cell line containing the plasmid pIVACAT is being deposited with the Agricultural Research Culture Collection (NRRL), Peoria, Ill.; and has the following accession number  
                                       Strain   Plasmid   Accession Number                    E. coli  (DH5a)   pIVACAT   NRRL                  
 
      The present invention is not to be limited in scope by the specific embodiments described which are intended as single illustrations of individual aspects of the invention, and any constructs, viruses or enzymes which are functionally equivalent are within the scope of this invention. Indeed, various modifications of the invention in addition to those shown and described herein will become apparent to those skilled in the art from the foregoing description and accompanying drawings. Such modifications are intended to fall within the scope of the appended claims.