ENHANCED SYSTEM FOR CONSTRUCTION OF ADENOVIRUS VECTORS

Abstract: In the present invention, viruses, plasmids or both are constructed which contain viral DNA, either at least one head-to-head ITR junction, recombinase recognition sites positioned such that site-specific recombination between recombinase recognition sites in separate plasmids results in generation of infectious viral DNA at high-efficiency in cotransfected host cells that have been engineered to express a site-specific recombinase, or both. Because of the high-efficiency and specificity of the Cre enzyme, the FLP enzyme, or both, suitably engineered plasmids can be readily recombined to produce infectious virus at high-efficiency in cotransfected 293 cells, without, at the same time, producing wild-type adenovirus, with the attendant problems for removal thereof. Use of recombinases besides Cre or FLP, and recombinase recognition sites besides lox or frt sites, and use of cells other than 293 cells are also disclosed and enabled, as are kits incorporating the site-specific vector system, as well as compositions and methods for using such compositions as vaccines or in gene therapeutic applications. Enhancements in the efficiency of both site-specific and homologous recombination are provided by inclusion of at least one head-to-head ITR junction.

Description: TITLE OF THE INVENTION ENHANCED SYSTEM FOR CONSTRUCTION OF A-DENOVIRUS VECTORS CROSS REFERENCE TO RELATED APPLICATIONS This application is a continuation-in-part of copending parent application serial no. 09/263,650, filed on March 5, 1999, and of copending parent application serial no. 09/415,899, filed on October 8, 1999. FIELD OF THE INVENTION The present invention relates to methods for efficient and reliable construction of adenovirus vectors that contain and express foreign DNA and are useful for gene transfer into mammalian cells, for vaccines and for gene therapy. The vector system described herein is an improvement and modification of the two plasmid pBHG system, described in copending patent application Serial No.08/250,885, a foreign equivalent of which published as WO95/00655, hereby incorporated by reference, whereby a replication defective genomic adenoviral vector and a shuttle plasmid were recombined via homologous recombination in a cell in which they were cotransfected. This invention further represents an improvement over the 09/263,650 system whereby, through insertion of a head-to-head ITR junction in the shuttle plasmid, enhanced vector rescue efficiency is achieved, whether through homologous or site-specific recombination mechanisms. BACKGROUND OF THE INVENTION As taught in WO95/00655, adeno viruses (Ads) can be used as mammalian cell expression vectors, with excellent potential as live recombinant viral vaccines, as transducing vectors for gene therapy, for research, and for production of proteins in mammalian cells. In the human Ad genome, early region 1 (El), E3, and a site upstream of E4 have been utilized as sites for introducing foreign DNA sequences to generate adenovirus recombinants. In the absence of compensating deletions in El or E3, a maximum of about 2 kb can be inserted into the Ad genome to generate viable virus progeny. The El region is not required for viral replication in complementing 293 cells, or other cells known to complement El, and up to approximately 3.2 kb can be deleted in this region to generate conditional helper independent vectors with a capacity of 5.0-5.2 kb. In the E3 region, which is not required for viral replication in cultured cells, deletions of various sizes have been utilized to generate nonconditional helper independent vectors with a capacity of up to 4.5-4.7 kb. The combination of deletions in El and E3 permits the construction and propagation of adenovirus vectors with a capacity for insertions of up to approximately 8 kb of foreign DNA. The construction of Adenovirus vectors can be performed in many ways. One approach is to cotransfect permissive cells, usually, but not limited to, 293 cells, with a shuttle plasmid containing a portion of the left end of the Ad genome and, most commonly, having the El sequences replaced by a foreign DNA, and with DNA isolated from virions cleaved near the left end by a suitable restriction enzyme. Homologous recombination between overlapping viral DNA sequences of the shuttle plasmid and the virion DNA results in production of recombinant viruses containing the foreign DNA. A disadvantage of this method is the need to prepare purified viral DNA. In addition, such methods typically result in the presence of contaminating parental virus in the resulting vector preparations, such as when 100% of the viral DNA is not cleaved, or when the two viral DNA fragments produced by restriction cleavage are rejoined. -Another method has recently been described (Hardy S, Kitamura M, Harris-Stansil T, Dai Y, Phipps ML, "Construction of adenovirus vectors through Cre-lox recombination." J Virol 1997 Mar;71(3):1842-1849; see also PCT publication WO97/32481 relating to use of site-specific recombination of virus and helper dependent vectors) which involves infection of 293Cre cells (293 cells engineered to express Cre recombinase) with an Adenovirus containing a floxed packaging signal (Ψ) and transfection with a shuttle plasmid containing an ITR, a packaging signal and an expression cassette followed by a lox site, or cotransfection of 293Cre cells with purified deproteinized Adenoviral DNA and a shuttle plasmid. According to that method, Cre-mediated excision of the packaging signal from virus followed by site-specific recombination with the lox site in the shuttle plasmid produces a recombinant vector containing the expression cassette. However, as Cre action is not 100% efficient, the resulting virus preparations remain contaminated with parental virus, and must be passaged in 293Cre cells to eliminate the contaminating parental virus. A further disadvantage of this method is that it requires use of an infectious virus or DNA extracted from a virus as one of the starting materials, and is thus less attractive for commercial distribution than kits containing only bacterial plasmid DNA. Furthermore, the parental virus can recombine with Ad El sequences present in 293 cells, resulting in a virus containing a wild-type packaging signal and a wild-type El region. Such recombinant virus has the propensity to overgrow the original vector, leading to contamination of subsequent vector preparations with non-attenuated El expressing Ads. One of the most frequently used and most popular methods for construction of adenovirus vectors is based on "the two plasmid method" (see Bett, A. J., Haddara, W., Prevec, L. and Graham, F.L An efficient and flexible system for construction of adenovirus vectors with insertions or deletions in early regions 1 and 3. Proc. Natl. Acad. Sci. US 91 : 8802-8806, 1994), whereby suitable host cells (typically 293 cells) are cotransfected with two plasmids that separately are incapable of generating infectious virus, but which, when recombined within the transfected cell by homologous recombination, can generate replicating virus. The most widely used plasmids of this type are described in patent application S/N Serial No.08/250,885, and in PCT publication number WO95/00655, hereby incoφorated by reference. That system has advantages over other methods using viruses or viral DNA as components since only easily-prepared plasmid DNAs are needed, and there is no background of parental virus that could contaminate the final vector isolates. Furthermore, the plasmids are not only easy and inexpensive to produce by those skilled in the art, but can be easily stored and transported, making them convenient for commercial distribution, (i.e. particularly when precipitated with ethanol or when lyophilized, these vectors do not require a cold chain for distribution). However, although this currently available system has proven utility and is widely used, the efficiency of virus production by homologous recombination can be low and variable, and the system cannot always be used easily by those not skilled in the art. As demonstrated in the art (Anton, M. and Graham, F. L. "Site-specific recombination mediated by an adenovirus vector expressing the Cre recombinase protein: a molecular switch for control of gene expression," J. Virol. 69:4600-4606, 1995), and as described also in application serial number 08/486,549 ( "Adenoviruses for control of gene expression", hereby incoφorated by reference), provision of Cre recombinase in Ad- infected cells can catalyse excision or rearrangement of viral DNA sequences that contain the target sites (lox P) for Cre-mediated site-specific recombination. Such techniques are combined in the present invention with a head-to-head ITR containing shuttle plasmid to provide a long-needed advancement in the art of adenoviral vector production by enhancing the efficiency of vector production, whether through homologous recombination or site-specific recombination events. In addition, the present patent disclosure demonstrates that the FLP recombinase, which recognizes a nucleic acid site known as frt (O'Gorman, S., Fox, D. T. and Wahl, G. M. Recombinase-mediated gene activation and site specific integration in mammalian cell. Science 251 : 1351-1355, 1991; Senecoff, J. F., Rossmeissl, P. J. and Cox, M. M., DNA recognition by the FLP recombinase of the yeast 2μ plasmid, J. Mol. Biol. 201 : 405-421, 1988) may be used in similar fashion to Cre recombinase and with similar efficiency for construction of Ad vectors by recombination between two plasmids cotransfected into host cells "two plasmid method". It will be appreciated by those skilled in the art based on this disclosure that the method is not limited to the use of Cre recombinase and its recognition sites and FLP and its recognition sites, as other site specific recombinases that act in similar fashion to Cre and FLP could be substituted for Cre or FLP recombinases, or used in combination with such enzymes. It will also be appreciated that although the examples disclosed herein describe methods to rescue foreign DNA into the El region of "first generation" Ad vectors these examples are not meant to be limiting as the target sites for site specific recombination can be readily inserted into other regions of the viral genome and site specific recombination can consequently be utilized for manipulation of other regions of the viral genome besides El . SUMMARY OF THE INVENTION In the present invention, viruses, plasmids or both are constructed which contain head-to- head ITR junctions, and optionally, wherein said viral DNA may also contain lox P sites positioned such that site-specific recombination between lox P sites in separate plasmids results in generation of infectious viral DNA at high-efficiency in cotransfected host cells that have been engineered to express the Cre recombinase. Such cells (293 Cre cells) have been described by Parks, R. J., Chen, L., Anton, M., Sankar, U., Rudnicki, M. A. and Graham, F. L. "A new helper-dependent adenovirus vector system: removal of helper virus by Cre-mediated excision of the viral packaging signal," Proc. Natl. Acad. Sci. U.S. 93: 13565-13570, 1996, by Chen, L., Anton, M. and Graham, F. L., " Production and characterization of human 293 cell lines expressing the site-specific recombinase Cre," Somat. Cell and Molec. Genet. 22: 477-488, 1996, in US patent application serial number 08/473 , 168, and in PCT publication WO96/40955, hereby incoφorated by reference for this puφose. Because of the high-efficiency and specificity of the Cre enzyme, suitably engineered plasmids can be readily recombined to produce infectious virus at high- efficiency in cotransfected 293 cells, without, at the same time, producing a contaminating parental adenovirus, with the attendant problems for removal thereof. Due to the enhancement of recombination efficiency achieved by the instant invention, whereby a head-to-head ITR junction is included in the shuttle plasmid, even in the absence of site-specific recombination, enhanced efficiency is achieved in production of recombinant virus vectors. In one embodiment of this invention a head-to-head ITR junction is included in the shuttle plasmid, whereby enhanced efficiency of production of recombinant virus vectors is achieved whether by homologous recombination or by site-specific recombination. In a further embodiment of this invention, homologous recombination via cellular enzymes is replaced by site-specific recombination, via a recombinase such as Cre, FLP or the like, to join two DNAs that separately are noninfectious to form an infectious DNA molecule, wherein the efficiency of the site-specific recombination is enhanced by inclusion of head- to-head ITR junctions in the shuttle plasmids of a two-plasmid system. One application of the techniques disclosed herein is the isolation of "first generation" vectors with insertions of foreign DNA in El. Such applications utilize a series of plasmids such as pBHGloxΔE 1 ,3 (see Figure 1 , and variations and equivalents thereof), and various shuttle plasmids containing a head-to-head ITR junction, a packaging signal, an expression cassette, and a lox or other recombinase recognition site. -Another application is in a sense the mirror image. Using a plasmid such as pFG 173 lox, sequences are rescued into the right end of the viral DNA, into E3 or into sites rightward of E3. The most important applications of this latter technology will likely be rescue of mutations into the fibre gene located immediately rightward of E3 (Figure 9) (fibre is important because it is a major ligand for binding to cellular receptors) but one can also rescue mutations, deletions, insertions and other modifications in E4 genes (located between fibre and the right ITR) or the method is used to rescue inserts of foreign DNA into E3 (cotransfection of a plasmid such as that depicted in figure 11 with pFGl 73 lox). Note that the plasmid pFGl 73 lox has a deletion of fibre, but E4 sequences could just as well be deleted as well as or instead of fibre. Note also that lox sites could be inserted at other locations in the Ad genome to enable the rescue of mutations engineered in other viral genes besides those of fibre or E4, or rescue of DNA inserts into other sites. In a further embodiment of this invention, DNA-TP complexes are utilized to combine the high efficiency of head-to-head ITR mediated recombination, with or without site-specific recombination, such as Cre-lox recombination, with the high infectivity of DNA-TP. While the rescue of infectious virus via head-to-head ITR mediated homologous recombination, or head-to-head ITR mediated recombination with Cre-mediated recombination is suφrisingly efficient compared to homologous recombination alone, and is more than adequate to produce viral vectors and to introduce mutations into the viral genome for most applications, there may be certain applications for which even higher efficiencies are desirable or necessary. It is known by those skilled in the art that the infectivity of adenovirus DNA is up to 100 fold higher if the virion DNA is extracted and purified by methods that leave intact the terminal protein (TP) that is normally linked to the 5' end of each strand of the duplex Ad DNA molecule (Shaφ PA, Moore C, Haverty JL, "The infectivity of adenovirus 5 DNA-protein complex," Virology 1976 Dec;75(2):442-456, Chinnadurai G, Chmnadurai S, Green M, "Enhanced infectivity of adenovirus type 2 DNA and a DNA-protein complex." J Virol 1978 Apr;26(l):195-199). For rescue of cassettes, the two plasmid system is more than sufficiently efficient, especially with the approximately 10-fold enhancement in efficiency demonstrated herein for head-to-head ITR mediated recombination, or the approximately 100-fold enhancement in efficiency demonstrated herein for head-to-head ITR mediated recombination when coupled with Cre-lox mediated recombination (over homologous recombination alone), and consequently would be preferred for most puφoses. However, there may be times when even higher efficiencies are required, as when, for example, one wishes to develop a library of fibre mutations (a large number of different viruses - the more the better). Then the chore of preparing DNA-TP might be worthwhile and could be accomplished by those skilled in the art. Thus, an aspect of the present invention includes the combination of the enhanced recombination mediated by head-to-head ITR junctions in shuttle plasmids, with or without Cre-lox recombination, with the high specific infectivity of adenoviral DNA-TP complexes. In a further embodiment of this invention, combinations of Cre-lox with FLP-frt are made. For example, based on the present disclosure, one skilled in the art could use the FLP-frt system to rescue cassettes comprising molecular switches regulated by Cre-lox or vice versa. One could use the FLP-frt system to rescue a Cre expression cassette whereas it would otherwise be difficult or not feasible to do this using the Cre-lox system for vector rescue. As 293 cells that express Cre are available per this invention, 293 cells that express FLP are available per this invention, and 293 cells that express Cre and FLP are available per this invention, this mvention provides many options to those skilled in the art for manipulation of the viral genome by site specific recombination. Thus the availablility of an additional site specific recombinase increases the flexibility and expands the number of available options in the design and construction of Ad vectors. Furthermore, those skilled in the art will appreciate that the use of site specific recombination for manipulation of DNA is not limited to manipulation of adenovirus DNA as one skilled in the art will readily appreciate that other genomes can be similarly recombined by cotransfection into host cells expressing a site specific recombinase. Therefore, it is an object of the present invention to provide a highly efficient, reliable, and simple method for isolation of viral vectors in which rescue of viruses is enhanced by provision of head-to-head ITR junctions whether recombination is based on homologous recombination, or on site-specific recombination catalysed by a site-specific recombinase, such as but not limited to the Cre recombinase. It is a further object of this invention to use head- to-head ITR constructs, with or without Cre-/o -mediated recombination, FLP-frt mediated recombination, or both, and known two plasmid vector production systems to provide a simple method for introducing mutations or other modifications of viral genes into any desired location in the viral genome. It is a further object of this invention to provide a simple and useful system by which adenovirus cloning vectors may be developed. It is a further obj ect of this invention to provide a kit for efficient production of adenoviral vectors for vaccine and gene-therapeutic applications which relies on homologous or site- specific recombination, and which does not require a cold-chain for distribution. A further object of this invention is to provide a system whereby the high-efficiency of the head-to-head ITR mediated recombination, with or without Cre-lox mediated recombination, is combined with enhanced infectivity achieved when adenovirus-TP complexes are utilized. Further objects of this invention will become apparent from a review of the complete disclosure and the claims appended hereto. BRIEF DESCRIPTION OF DRAWINGS Figure 1 is a diagrammatic representation showing a method for isolation of an Ad vector containing an expression cassette in El using the Cre/lox recombination system. pBHGloxΔE 1 ,3 comprises a circularized form of the Ad genome with a deletion of the El region including the packaging signal and a bacterial plasmid origin of replication and an ampicillin resistance gene. The plasmid has a loxP site near the 5' end of the pIX gene of the Ad genome and a deletion of E3 sequences. The "shuttle plasmid" contains an ITR of the virus genome and a packaging signal, a polycloning site for insertion of a foreign DNA (eg bacterial β-galactosidase (LacZ)) and a loxP site inserted in the same relative orientation as the loxP site in pBHGloxΔE 1,3. Cotransfection of these two plasmids into 293 Cre cells that express Cre results in Cre-mediated recombination and formation of joint molecules that generate infectious viruses containing the foreign DNA insert. According to this invention, the ITR of the shuttle plasmid is replaced with a head-to-head ITR junction, thereby enhancing the efficiency of the site-specific recombination method. Figure 2 illustrates a cotransfection experiment similar to that depicted in Figure 1 except that the shuttle plasmid contains Ad sequences 3' of the lox site that overlap (are homologous) with viral sequences in pBHGloxΔEl ,3 to the right of the lox site. Therefore an Ad vector containing an expression cassette in El can be generated by Cre/lox recombination when the two plasmids are cotransfected into 293Cre cells or alternatively by homologous recombination between overlapping sequences. The shuttle plasmid in the illustration permits a comparison of the efficiency obtainable from the two recombination modes. According to this invention, the ITR of the shuttle plasmid is replaced with a head- to-head ITR junction, thereby enhancing the efficiency of the site-specific recombination method. Figure 2a. Construction of shuttle plasmids. The plasmid pCA351ox was constructed by replacing theNbαl/Sα/I fragment containing the LacZ expression cassette under the control of the short MCMV promotor in pCA361ox with the Xbal Sall fragment containing the LacZ expression cassette under the control of the long MCMV promotor from pCA351oxΔCreITR. The plasmid pCA351oxΔlTR was constructed by replacing the Asel fragment bearing the single left end ITR in pCA361oxΔ with the Asel fragment bearing an ITR junction from pCA351oxΔCreITR. The plasmid pCA351oxITR was constructed by replacing the Asel fragment bearing the single left end ITR in pCA361ox with the --4-seI fragment bearing an ITR junction from pCA351oxΔCreITR. The plasmid pCA36ITR was constructed by replacing the Scal/Xbal fragment bearing the single left end ITR in pCA36 with the Scal/Xbal fragment bearing an ITR junction from pCA351oxITR. The plasmid pCA35ITR was generated by replacing the Xbal Sail fragment containing the LacZ expression cassette under the control of the short MCMN promotor in pCA36ITR with the XballSaR fragment containing the LacZ expression cassette under the control of the long MCMV promotor from pCA351oxITR. Thin black lines represent bacterial plasmid sequences and thick black lines represent Ad sequences. The position and orientation of loxP sites and Ad ITR are indicated by white triangles and small horizontal arrows, respectively. Plasmids are not drawn to scale and only the relevant restriction enzyme sites are shown. Figure 3 illustrates four sets (pairs) of oligonucleotides used in various cloning procedures. The oligos are annealed prior to use to produce the double stranded DΝA segments illustrated. Three of the oligonucleotide pairs contain loxP, the recognition site for Cre recombinase as well as one or more restriction endonuclease sites used for diagnostic puφoses or for subsequent cloning steps. One of the oligonucleotide pairs contains several restriction endonuclease sites and was used to introduce a polycloning site into various shuttle plasmids. Figure 4 illustrates the construction of a plasmid, derived from pBHGlO (Bett, A. J., Haddara, W., Prevec, L. and Graham, F.L "An efficient and flexible system for construction of adenovirus vectors with insertions or deletions in early regions 1 and 3." Proc. Natl. Acad. Sci. US 91: 8802-8806, 1994., available from Microbix Biosystems), wherein a loxP site is inserted at the 3' end of an El deletion and 5' (upstream) of the pIX gene. pBHGloxΔEl ,3 was constructed by replacing the 4604 bp R-stl 1071 fragment from pBHGlO with the 2326 bp EcoRV/R-stl 1071 fragment from pΔΕlspl Alox (see Figures 5 and 5 a). Foreign DNA sequences can be inserted into the unique Rαcl site of pBHGloxΔΕl,3 for rescue of genes in E3. Figure 4a illustrates the construction of aplasmid, pBHGdXIPlox, containing a modified E3 deletion (taken from pFG23dXl) and a lox site 5' of the pIX gene. The plasmid pFG23dXlP was constructed by inserting an oligonucleotide containing a Pad site (AB14566; 5'- CTAGCTTAATTAAG -3'; this oligo self anneals to produce a double stranded DNA with 5' overhangs that hybridize to overhangs generated by Xbal cleavage) into the Xbal site of pFG23dXl. The resulting plasmid, pFG23dXlP, is identical to pFG23dXl except that the unique Xbal site at nt 11392 is changed to a unique Pac I site. The plasmid pNG17 was constructed by cloning the 6724 bp Spel/Clal fragment from pBHGloxΔEl ,3 into pBluescript. The plasmid pNGl 7dXlP was constructed by replacing the 1354 bp Spel/Ndel fragment from pNG17 with the 2143 bp Spe /Ndel fragment from pFG23dXlP. Finally, the plasmid pBHGdXIPlox was constructed by replacing the 6724 bp Spel/Clal fragment from pBHGloxΔEl, 3 with the 7513 bp Spel/Clal fragment from pNG17dXlP. pBHGdXIPlox thus contains a modified E3 region such that the deletion of E3 sequences is that of the parental plasmid pFG23dXl ( a deletion of 1878 bp) rather than the larger deletion of the other parental plasmid pBHGloxΔEl, 3. Figure 4b illustrates the construction of pBHGE31ox, a plasmid derived from pBHGE3 and pBHGloxΔE 1,3 constructed by replacing the 6724 bp Spel/Clal fragment from pBHGloxΔEl, 3 with the 9377 bp Spel/Clal fragment from pBHGE3. PBHGE31ox contains a complete E3 region for isolation of viral vectors that retain a wild-type E3. Figure 4c illustrates the construction of Ad genomic plasmids encoding Cre. The plasmid pBHGloxpolyl was constructed by insertion of an oligonucleotide pair containing R-stB I, Xba I and Swa I sites and Cla I compatible ends into the Cla I site of pBHGloxΔEl, 3. The Cre expression cassettes, taken from the pBSKS-Crel or pBSKS-Cre2 as a Sma I- Spe I fragment fragment, were inserted into Swa I, Xba I digested pBHGloxpolyl as illustrated to generate pBHGloxΔE 1,3 Cre and pBHGloxΔEl, 3CreR. Figure 5 illustrates the construction of shuttle plasmids derived from pΔElSPlA and pΔElSPlB wherein a loxP site is introduced 3' of the packaging signal. The plasmids pΔE 1 sp 1 Alox and pΔE 1 sp 1 Blox were constructed by inserting an oligonucleotide bearing a lox? site (comprised of annealed oligos AB3233 and AB 3234) into the Bglll site of pΔElsplA. Subsequent digestion with Nru I and partial Sea I digestion followed by ligation generated pΔElSPl AloxΔ and pΔElSPlBloxΔ. Figure 5a. Construction of shuttle plasmids. The plasmids pΔElSPlAlox and pΔEl SPlBlox were constructed by inserting a loxP linker (AB3233/AB3234) into the Bgl π site of pΔElSPlA and pΔElSPlB. The plasmids pΔElSPlAloxITR and pΔEl SPlBloxITR were produced by replacing the Pvu I! Xba I fragment of pΔEl SP 1 Alox and pΔEl SPlBlox with a PvuVXba I fragment from pCA351oxITR. The resulting plasmids now contain an ITR junction instead of just a single left end ITR. The plasmids pΔElSPlAloxITR(MCS) and pΔElBloxITR(MCS) were generated by inserting oligonucleotides (AB 16853/AB 16854) and (AB 16855/AB 16856) into the unique Xba VSal I sites of pΔElSPlAloxITR and-^bα I/RαmH I sites of pΔElSPlBloxITR respectively. Figure 5b. Construction of shuttle plasmids. The plamids pDCl l l and DC 112 were produced by digesting pΔE 1 SP 1 AloxITR(MCS) and pΔE 1 SP 1 BloxITR(MCS) with Ehe I followed by a partial digest with Sea I followed by ligation. The resulting plasmids now lack essential left end Ad sequences required for rescue of Ad vectors by homologous recombination. These shuttle plasmids will only produce plaques in cotransfection with Ad genomic plasmids if there is recombination between loxP sites. The shuttle plasmid pDC 113 was generated by replacing the Pvu VSal I site of pDC 111 with a similar fragment from pΔElSPlA(del Bgl II). The plasmid pDCl 14 was produced by replacing the Pvu 1/BamR I of pDCl 12 with a similiar fragment from pΔEl SPlB(del Bgl H). The plasmids, pDC113 and pDC114, are shuttle plasmids which contain an ITR junction and Ad sequences for homologous recombination, but they lack the loxP site found in previous plasmids. Figure 5c. Synthetic oligonucleotides for use in construction of pΔEl SP1 AloxITR(MCS) and pΔElBloxITR(MCS) of Figure 5a. Figure 5d. Construction of additional shuttle plasmids. The plasmids pDC115 and pDC 116 were generated by replacing the Pvu II Xba I fragment of pDCMH41ox( Addel) and pDCMH41ox(Addel)linker with a similar Pvu l/Xba I ITR junction containing fragment from pCA351oxITR. The resulting plasmids contain an ITR junction instead of a single left end ITR. The plasmids pDCl 17 and pDCl 18 were produced by inserting a Pvu VSal I fragment from pC Al 4(del Bgl II) into pDC 115 and pDC 116 digested with Pvu VSal I. The plasmids pDCl 17 and pDCl 18 have no loxP site, but they do contain Ad sequences for virus rescue by homologous recombination. Figure 6 illustrates the construction of pMH41ox, pMH41oxΔ (also referred to herein as pDCMH41ox(Ad del) and pMH41oxΔlink (also referred to herein as pDCMH41ox(Ad del) Linker), plasmids that contain lox sites and a promoter and polyadenylation signal and polycloning sites for insertion of foreign DNA to produce expression cassettes in which transcription is driven by the murine cytomegalovirus immediate early gene promoter. Plasmid pVDB3 (see Figure 6a) is derived from pMH4 but contains a pUC based origin of replication rather than a pBR322 origin. It contains Ad5 sequences from m.u. 0-15.8 with El sequences deleted between m.u. 1 and 9.8 and substituted with an expression cassette: a 0.5kbp (-491 to +36) fragment of the MCMV IE promoter, unique restriction enzyme sites for cloning (Eco RI, Nhe I, Bam HI and Sal I) followed by an SV40 polyadenylation signal. To make pMH41ox, a loxP linker (AB3233/3234) was introduced into the BglR site of pVDB3. Ad5 sequences m.u. 9.8-15.8 were deleted from pMH41ox by digesting with Hind IH, treating with the Klenow fragment of E. coli DNA polymerase then partially digesting with Sea I followed by self- ligation. The resulting shuttle plasmid, pMH41oxΔ, (also referred to herein as pDCMH41ox(Ad del)), can be used with pBHGloxΔE 1,3 to produce Ad vectors via Cre/lox mediated recombination. To make pMH41oxΔ a more flexible plasmid for cloning puφoses, a linker (AB 14626/14627) containing a different multiple cloning region was introduced between the Eco RI and Sal I sites resulting in pMH41oxΔlink, (also referred to herein as pDCMH41ox( Ad del)Linker) . Figure 6a illustrates the construction of plasmid pNDB3 derived from pMH4 but containing a pUC based origin of replication rather than a pBR322 origin. A Pvul to Bst 11071 fragment from pMH4 (Microbix Biosystems) was ligated to a Bst 11071 to Pvu I fragment from pD47El containing a pUC based (pΝEB 193, New England Biolabs) origin of plasmid DNA replication to generate pNDB3. Figure 7 illustrates construction of HCMN loxP plasmids, pCA131oxΔ and pCA141oxΔ, in which transcription of foreign genes is regulated by the human cytomegalovirus immediate early gene promoter. The plasmids pCA13(ΔRg7II) and pCA14(ΔRg-/II) were generated by digesting pCA13 and pCA14 partially with BglK, Klenowing and self- ligating. A synthetic loxP oligonucleotide (AB3233/3234) was introduced into the unique BglΕ sites of pCA13(ΔRg/π) and pCA14(ΔRg π) producing pCA131ox and pCA141ox respectively. Ad5 sequences, m.u. 9.8-15.8, were removed frompCA131ox andpCA141ox by cutting each plasmid with Nrwl and partially digesting each with Seal followed by self ligation. Figure 8 is a diagrammatic representation of a method for constructing pCA361oxΔ a shuttle plasmid containing the leftmost approximately 340 nt of Ad5, an expression cassette encoding β-galactosidase, and a lox P site for rescue of the LacZ gene into adenovirus vectors. A synthetic loxP site (AB3233/3234) was introduced into the Bgl II site of pCA36 resulting in pCA361ox. This plasmid was then digested with Nru I and partially digested with Sea I, a 7646bp fragment was gel purified and self ligated yielding pCA361oxΔ. Figure 8a is a diagrammatic representation of a means to isolate adenoviral vectors containing an expression cassette by cotransfection of 293Cre cells with (a) AdLC8c DNA-TP complex having covalently bound terminal protein (TP) linked to the 5' ends of Adenoviral DNA and (b) a shuttle plasmid containing an expression cassette and a lox P site. Cre-mediated excision of the floxed packaging signal of AdLC8c renders the AdLC8c genome defective for packaging. A second Cre-mediated recombination event between the lox sites in the shuttle plasmid and the AdLC8c genome results in a vector with a packaging signal, the foreign DNA insert, and a single lox site. According to this invention, the ITR of the shuttle plasmid is replaced with a head-to-head ITR junction, thereby enhancing the efficiency of the site-specific recombination method. Figure 8b is a diagrammatic representation of a means to isolate adenoviral vectors containing an expression cassette by cotransfection of 293Cre cells with restricted AdLC8c DNA-TP and a shuttle plasmid containing an expression cassette and a lox P site. AdLC8c DNA-TP is cleaved with an endonuclease such as Asu II or Swa I that recognize unique restriction enzyme sites between the lox sites flanking ψ. Cleavage of viral DNA with restriction enzymes prior to cotransfection reduces the infectivity of parental virus DNA and when combined with the high-efficiency of Cre-mediated recombination results in high-efficiency of vector isolation in cotransfected 293 Cre cells as illustrated. Rejoining of parental DNA fragments and generation of infectious parental virus rather than the desired vector is avoided because of the action of Cre on the floxed packaging signal in AdLC8c. However, when the viral DNA-TP complex is cut with a restriction enzyme as illustrated, the level of Cre-mediated recombination is sufficiently high that most, if not all, progeny viruses result from recombination between the shuttle plasmid and the large DNA-TP fragment. Therefore, the left-most lox site of AdLC8c and equivalent vectors is not essential. According to this invention, the ITR of the shuttle plasmid can be replaced with a head-to-head ITR junction, thereby enhancing the efficiency of the site- specific recombination method. Figure 8c is a diagrammatic representation of a method for constructing shuttle plasmids expressing Cre. The Cre expression cassette was obtained from the plasmid pLC2 (Chen, L., Anton, M. and Graham, F.L., "Production and characterization of human 293 cell lines expressing the site-specific recombinase Cre," Somat. Cell and Molec. Genet.22:477-488, 1996), as a 2175 bp Bgl fragment which was end-modified with Klenow DNA polymerase and inserted into the Ehel site of pC A36loxt to generate pC A36/o ΔCreR and pCA36/oxΔCreT. The plasmid pCA35/o ΔCreITR was constructed by replacing the 1402 bp ScaVKpnl fragment in pCA36/o ΔCreT with the 2753 bp ScaVKpnl fragment from the plasmid pRP 1029. Plasmid pCA35/o ΔCreITR contains ITR junctions which are known to be functionally capable of generating replicating linear Ad DNA molecules (Graham, F.L., "Covalently closed circles of human adenovirus DNA are infections," The EMBO J. 3, 2917-2922, 1984). Figure 8d provides a schematic representation of a cotransfection experiment wherein an Ad genomic plasmid bearing a lox site such as pBHGloxΔEl, 3 and a "Lox" shuttle plasmid expressing Cre are introduced into 293 cells in order to generate Ad expression vectors, without having to use cells which stably express Cre. This technique is applicable to any cell type suitable for Ad vector generation, including but not limited to 293 cells, and PER-C6 cells (Fallaux et al., Hum. Gene Ther. 1998, Sep. 1;9(13):1909-17), 911 cells (Fallaux et al., Hum. Gene Ther. 1996 Jan. 20;7(2):215-222), or other cells. According to this invention, the ITR of the shuttle plasmid is optionally replaced with a head-to-head ITR junction, thereby enhancing the efficiency of the site-specific recombination method. Thus a shuttle plasmid such as pCA35 loxΔCrelTRof figure 8c is also suitable for generation of an adenovirus vector. Figure 8e. Demonstrates the construction of an Ad genomic plasmid encoding Cre. The plasmid pBHGloxΔ was constructed by collapsing pBHGloxΔE 1,3 with Spel and PshAl. The Cre expression cassette, taken from the plasmid pLC2 as a 2175 bp BgU fragment, was inserted into the BamHI site of pBHGloxΔ to generate pBHGloxΔCreR and pBHGloxΔCreL. The 1238 bp R-stl l07I/RαcI fragment from pBHGloxΔCreR and pBHGloxΔCreL was replaced with the 22380 bp R-stl l07I/Rα fragment from pBHGloxΔE 1,3 to generate pBHGloxCreR and pBHGloxCreL, respectively. Figure 9 is a diagrammatic representation of a method for rescuing fibre mutations into infectious virus using Cre-lox recombination. Plasmid pFGl 73 lox is derived from pFGl 73 which is a bacterial plasmid containing most of the Ad5 genome but from which sequences have been deleted (represented by "deletion" in the diagram) that render the plasmid noninfectious. The sequences are substituted with bacterial DNA containing an antibiotic resistance gene and a bacterial plasmid origin of DNA replication. A lox site upstream (leftward in the conventional map of the Ad genome) of the deletion/substitution is inserted in the plasmid for Cre-mediated recombination with a similar lox site in a shuttle plasmid containing the right region of the viral genome from approximately 85 mu to approximately 100 mu and including most or all of the right ITR. Recombination as illustrated generates an infectious virus containing sequences representing the left approximately 78 mu of the Ad genome derived from pFG1731ox and sequences from approximately 85- 100 mu derived from the shuttle plasmid. According to this invention, the ITR of the shuttle plasmid is optionally replaced with a. head-to-head ITR junction, thereby enhancing the efficiency of the site-specific recombination method. Figure 9a is a diagrammatic representation of a method for constructing a plasmid containing a lox site and ampicillin resistance gene substituting for the fibre gene. Starting with p AB 141ox whose construction is described in Figure 14, the DNA sequences between the Cla I site and the Blp I site containing fibre are substituted with a DNA segment containing the ampicillin resistance gene and a plasmid origin of DNA replication. The Ndel to Ssp I DNA fragment from pCA14 (Microbix Biosystems) containing ampicillin resistance gene and plasmid origin of DNA replication is treated with Klenow DNA polymerase and ligated with a similarly treated Blp I to Clal fragment of pAB141ox to generate the ampicillin and kanamycin doubly resistant, fibre gene deleted, pAB141oxΔ. Figure 9b is a diagrammatic representation of a method for combining the plasmid of Fig. 9a with pFG173 to produce pFG1731ox for rescuing fibre or E4 mutations into infectious virus using Cre-lox recombination. The plasmid pAB141oxΔ is treated with restriction enzymes that cut in and around the kanamycin resistance gene and pFG173 is similarly digested with Eco RI as illustrated. Transformation of E. coli with the fragmented DNA from the two plasmids results in formation of a replicating plasmid in which the sequences in and around the shaded portion indicated in pFG173 are substituted with corresponding sequences from pAB141oxΔ by homologous recombination (Chartier C, Degryse E, Gantzer M, Dieterle A, Pavirani A, Mehtali M. Efficient generation of recombinant adenovirus vectors by homologous recombination in Escherichia coli. J Virol 1996 Jul;70(7):4805-4810 ). Figure 10 is a diagrammatic representation of method for constructing a plasmid containing the right approximately 40% of the virus genome wherein a lox P site has been inserted near the 5' end of the fibre gene. The plasmid pFG23dXl contains the right 40% of the Ad5 genome cloned into the bacterial plasmid pBR322, and has a deletion of an Xbal fragment from nt 28,589 (79.6 mu) of the wt Ad5 sequence to nt 30470 (mu 84.8) leaving a unique Xbal site suitable for insertion of a loxP site. A loxP site comprised of two synthetic oligonucleotides (AB6920/-AB6921 , Figure 3) was ligated into the Xba I site of pFG23dXl to generate pFG23dXllox which contains a loxP site upstream of the sequences encoding fibre. Finally, pFG23dXl lox was further modified by deletion of viral sequences between a unique Bstl 1071 site and a BsiWl site immediately 5' of the lox P site to generate pFG23dXlloxc. Figure 11 illustrates a pFG23dXllox plasmid with an expression cassette encoding bacterial β-galactosidase inserted into the Cla I site between the lox P site and the fibre gene. Figure 11 a is a diagrammatic representation of a method for combining two noninfectious plasmids by site specific recombination between lox sites inserted 5' of the Ad E3 region in each plasmid to generate a replicating packageable recombinant viral genome. Both plasmids comprise ITR junctions that allow for DNA replication in transfected host cells that express viral functions required in trans for Ad DNA replication. The combined plasmid DNAs encode all viral functions required in trans and in cis for viral DNA replication and packaging into virions. In the illustrated example, not meant to be limiting, the method is used for rescuing fibre mutations into infectious virus using Cre-lox recombination. Plasmid pFGl 73 lox is derived from pFGl 73 which is a bacterial plasmid containing most of the Ad5 genome but from which sequences have been deleted (represented by "deletion" in the diagram) that render the plasmid noninfectious. The sequences are substituted with bacterial DNA containing an antibiotic resistance gene and a bacterial plasmid origin of DNA replication. Said sequences could be additionally substituted with an expression cassette encoding Cre recombinase. A lox site upstream (leftward in the conventional map of the Ad genome) of the deletion/substitution is inserted in the plasmid for Cre-mediated recombination with a similar lox site in a shuttle plasmid containing the right region of the viral genome from approximately 85 mu to approximately 100 mu and including the right ITR joined "head to head" with the right ITR. Linearization of the two plasmids DNAs and site specific recombination between the lox sites will generate an infectious virus containing sequences representing the left approximately 78 mu of the Ad genome derived from pFG1731ox and sequences from approximately 85- 100 mu derived from the shuttle plasmid. Figure 12 is a diagrammatic representation showing rescue of a fibre mutation into a virus genome by cotransfection of 293Cre cells with DNA-TP of an Adfloxed fibre and a plasmid containing a lox P site 5' of a (optionally mutated) fibre gene. Viral DNA-TP complex extracted from virus preparations of Adfloxfibre (Figure 15) and plasmid DNA (pFGdXl lox) optionally carrying a mutated fibre gene are cotransfected into 293Cre cells to produce a recombinant virus expressing the optionally mutated fibre. If desired, viral DNA can be prepared so that the terminal protein remains linked to the ends of the virion DNA as indicated. As will be seen from the present disclosure, a shuttle plasmid with a head to head ITR junction will provide for enhanced efficiency of virus rescue. Figure 13 is a diagrammatic representation showing rescue of a foreign DNA sequence into a virus genome by cotransfection of 293 Cre cells with DNA-TP of an Adfloxed fibre and a plasmid containing a lox P site, and a foreign DNA inserted 5' of the fibre gene. Cotransfection of cells with Adfloxfibre DNA-TP and pFG23dXlLacZlox results in production of a vector carrying the foreign (e.g. lacZ) gene inserted upstream of fibre. As noted above in the description of figure 8B, the right-most lox site depicted in the Adfloxed fibre genome can be omitted if the DNA-TP is digested with one or more restriction enzymes which cut rightward of the lox site located 5' of fibre. As will be seen from the present disclosure, a shuttle plasmid with a head to head ITR junction will provide for enhanced efficiency of virus rescue. Figure 14 is a diagrammatic representation showing construction of a plasmid containing a fibre gene with flanking lox P sites. Plasmid pAB14 (described in: Bett, A. J., Prevec, L., and Graham, F. L. Packaging capacity and stability of human adenovirus type 5 vectors. J. Virol. 67: 5911- 5921, 1993.) contains Ad sequences from approximately mu 0 to 1.0, 10.6 to 16.1, 69.0 to 78.3, and 85.8 to 100. The plasmid has unique Xbal and Blpl restriction sites suitable for insertion of synthetic oligonucleotides containing lox P sites as illustrated. PAB14flox was constructed by first inserting a lox site into the Xbal site that is upstream of fibre to produce pAB 141ox. Subsequently a second lox site was inserted into the unique Blp I site in pAB14 which is located between the 3' terminus of the fibre gene and the coding regions of E4 genes (pAB14flox: fibre flanked by lox sites). Figure 15 is a diagrammatic representation showing isolation of a virus genome containing lox P sites flanking the fibre gene (floxed fibre). Cotransfection of pAB14flox with pFG173 (described in Hanke, T., Graham, F.L., V. Lulitanond and D.C. Johnson. Heφes simplex virus IgG Fc receptors induced using recombinant adenovirus vectors expressing glycoproteins E and I. Virology 177: 437-444, 1990. PFGl 73 is available from Microbix Biosystems) generates a virus containing a floxed fibre gene, Adfloxfibre. Figure 16A illustrates the essential features of the FLP recombinase and its target site "fit". The core recognition site of the enzyme is only approximately 34 bp in size and is thus readily synthesized in the form of synthetic oligonucleotides for insertion into restriction sites by standard recombinant DNA techniques. Figure 16B illustrates the construction of a plasmid, derived from pBHGlO (Bett, A. J., Haddara, W., Prevec, L. and Graham, F.L "An efficient and flexible system for construction of adenovirus vectors with insertions or deletions in early regions 1 and 3." Proc. Natl. Acad. Sci. US 91 : 8802-8806, 1994., available from Microbix Biosystems), wherein a frt site is inserted at the 3' end of an El deletion and 5' (upstream) of the pIX gene. pBHGfrtΔEl,3 was constructed by replacing the 4604 bp R-stl 1071 fragment from pBHGlO with the 2316 bp EcoRV/Bstl 1071 fragment from pΔElsplAfrt. pΔElsplAfrt was constructed by inserting a synthetic DNA comprising oligonucleotides ABI 0352 and AB10353 containing an fit site into the unique Bgl II site of pΔElsplA (Bett, A. J., Haddara, W., Prevec, L. and Graham, F.L "An efficient and flexible system for construction of adenovirus vectors with insertions or deletions in early regions 1 and 3." Proc. Natl. Acad. Sci. US 91 : 8802-8806, 1994., available from Microbix Biosystems). Foreign DNA sequences can be inserted into the unique Rαcl site of pBHGfrtΔEl,3 for rescue of genes in E3. Figure 16C provides the sequences of oligonucleotides used in various cloning procedures. The oligos are annealed prior to use to produce the double stranded DNA segments for insertion into restriction enzyme digested plasmid DNAs as indicated. Three of the oligonucleotide pairs (AB10352 & AB10353; AB19509 & AB19510; AB19816 & ABI 9817) contain fit, the recognition site for FLP recombinase as well as one or more restriction endonuclease sites used for diagnostic puφoses or for subsequent cloning steps. Figure 17 A. Construction of a shuttle plasmid containing a LacZ expression cassette and fit site for rescue into an Ad vector by FLP-frt recombination. The plasmid pCA35ITR is described in copending application AdVec 10IA, USSN 09/415,899. It contains the left end of the Ad5 genome with the El region substituted with a LacZ expression cassette under the control of the short MCMV promotor. An fit site was introduced into this plasmid by replacing the Bg VEhel fragment containing Ad sequences rightward of El with a synthetic oligonucleotide containing an fit site. With the removal of all Ad sequences save the ITRs and packaging signal this plasmid can only rescue the LacZ cassette into a recombinant Ad vector by FLP mediated recombination with a genomic plasmid containing a fit site since sequences for overlap homologous recombination are deleted. Figure 17B is a diagrammatic representation showing a method for isolation of an Ad vector containing an expression cassette in El using the FLP/frt recombination system. pBHGfrtΔEl ,3 comprises a circularized form of the Ad genome with a deletion of the El region including the packaging signal and a bacterial plasmid origin of replication and an ampicillin resistance gene. The plasmid has a fit site near the 5' end of the pIX gene of the Ad genome and a deletion of E3 sequences. The "shuttle plasmid", pCA35frtΔITR (Figurel7A) contains an ITR junction of the virus genome and a packaging signal, a foreign DNA (in this example bacterial β-galactosidase (LacZ)) and a fit site inserted in the same relative orientation as the fit site in pBHGfrtΔE 1,3 (Figure 16B). Cotransfection of these two plasmids into 293FLP cells that express FLP results in FLP -mediated recombination and formation of joint molecules that generate infectious viruses containing the foreign DNA insert. Figure 18 illustrates the construction of an Ad genomic plasmid encoding FLP. The plasmid pBHGfrtΔEl, 3poly2 was constructed by insertion of an oligonucleotide pair containing BstB I, Xba I and Swa I sites and Cla I compatible ends into the Cla I site of pBHGfrtΔEl,3. The FLP expression cassette, taken frompdelElCMVFLP as aHpa I- Sal I fragment, was inserted into Swa I digested pBHGfrtΔE 1 ,3poly2 as illustrated to generate pBHGfrtΔEl,3FLP. Figure 19A illustrates the construction of shuttle plasmids pDC511 and pDC512 wherein a fit site is introduced 3' of the packaging signal and all Ad sequences are deleted save for the ITR junctions and packaging signal. The plasmids pDC511 and pDC512 are designed for insertion of expression cassettes into any of several cloning sites located between the packaging signal and the fit site. Figure 19B. illustrates the construction of shuttle plasmid pDC515 from pDC512 and pMH4 by insertion of the Xba I- Bgl II fragment from pMH4 (Addison, C. L., Hitt, M., Kunsken, D. and Graham, F. L. Comparison of the human versus murine cytomegalovirus immediate early gene promoters for transgene expression in adenoviral vectors. J. Gen. Virol.78: 1653-1661, 1997.) that contains the MCMV promoter and SV40 polyadenylation signal into the Xbal - BamHI site in the polycloning region of pDC512. Figure 19C. illustrates the construction of shuttle plasmid pDC516 from pDC512, pDC316 and pMH4. The plasmid pDC316 was disclosed previously (see Figure 5d wherein this plasmid was designated as pDC 116). PDC516 was generated by combining the Xbal-Sall fragment from pDC316 containing the MCMV promoter and a polycloning region with the Sail - Bgiπ fragment from pMH4 containing the SV40 polyadenylation signal and inserting these into the Xbal-BamHI site of pDC512. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the present invention provides a bacterial plasmid comprising an antibiotic resistance gene and origin of replication for replication of said plasmid in host cells and further comprising a circularized modified human adenovirus genome that optionally contains sequences that can be recognized and acted upon by the site-specific recombinase such as Cre, FLP or like recombinases. Said bacterial plasmid is designed to be unable to generate infectious adenovirus by virtue of a deletion of viral DNA sequences, such as the packaging signal, which is normally located at the left end of wild- type Ad DNA, and which is essential for virus replication. Alternatively, formation of infectious virus may be prevented by the insertion of DNA ("stuffer DNA") such that the overall size of the resulting virus DNA exceeds the upper packaging limit for Ad virions (approximately 38 kb). Deletion of the pIX sequences from the Ad sequences makes the size-limitation of the packaging limitation more stringent, unless complementing cells which express the pDC gene product are used. Optionally, certain additional viral DNA sequences maybe deleted, such as sequences from E3, which can in any event be omitted from the viral genome without preventing a viral genome from replicating in such cells as may be permissive for replication of said viral genome in the form of infectious virus. -Another embodiment of the invention provides a second bacterial plasmid, known as a "shuttle" plasmid, comprising minimally approximately 340 base pairs from the left end of the Ad5 genome, a head-to-head terminal repeat sequence (referred to in abbreviated form herein as "hthlTR"), and the packaging signal sequences thereof, optionally a promoter, optionally a foreign DNA encoding a protein, optionally a polyadenylation signal, and optionally a lox site (various lox sites are known in the art, including, but not limited to loxP, lox511, lox514, loxPsym, and mention of any one of these sites incoφorates the mention of the other lox sites), or like site-specific recombinase recognition sites, such as FRT, recognized by the FLP recombinase. The promoter, foreign gene and poly adenylation signal are referred to herein collectively as an "expression cassette". Cotransfecting 293Cre cells with said shuttle plasmid and the plasmid of the first embodiment of the mvention results in recombination between said plasmids and rescue of said expression cassette into an infectious viral vector by homologous recombination or by action of said Cre recombinase. The hthlTR present in the shuttle plasmid has, suφrisingly, been found to significantly enhance the level or recombination and rescue of recombinant virus, whether through homologous recombination or through site-specific, recombinase directed recombination, or both. It will be appreciated that the term "bacterial plasmid" is not meant to be limiting, since one skilled in the art would recognize that other types of DNA could be recombined with equal efficiency, by homologous or site-specific recombination, such as through use of the Cre recombinase. For example, the Cre recombinase could be expressed in yeast cells to allow for high-efficiency recombination between yeast artificial chromosomes (YAC's) harboring an Ad genome, or, similarly, in bacteria, to allow for Cre-mediated recombination between cosmids or bacteriophage genomes harboring Ad sequences. Similarly, expression of Cre in mammalian cells could be used to allow for efficient recombination between two or more infectious Ad vectors, between an Ad vector and a bacterial plasmid, between an adenoviral genome and a linear DNA fragment and the like. A third embodiment of the invention provides a mammalian cell line, such as a human cell line, that provides the Cre recombinase enzyme. Alternatively, Cre may be provided by an Ad5 derived vector that expresses the Cre protein in suitable cells or Cre may be provided by a third plasmid encoding Cre or optionally Cre could be expressed from an expression cassette inserted into one of the two plasmids including a shuttle plasmid comprising said hthlTR for use in the two plasmid rescue system. Alternatively, Cre could be expressed in other species, for example bacteria or yeast, to allow for recombination and generation of recombinant Ad genomes in said species. Alternatively, Cre could be provided as a pure or crude protein extract from expression in a variety of species for recombination of said bacterial plasmids in vitro. One skilled in the art would recognize that other recombinase systems are available which could catalyse similar recombination events in place of Cre, for example, not meant to be limiting, the yeast FLP recombinase recognizes and recombines FRT target sites and is therefore expected to provide functions similar to those described herein with reference to Cre and its loxP target sites. A fourth embodiment of the invention provides an adenovirus or a plasmid containing adenovirus DNA wherein a segment of the viral DNA such as, but not limited to, the region encoding fibre is flanked by lox P sites. A fifth embodiment of the invention provides an adenovirus or a plasmid containing adenovirus DNA wherein a segment of the viral DNA such as, but not limited to, the region encoding fibre is deleted and substituted by a lox P site. A sixth embodiment of the invention provides a plasmid containing a portion of the viral genome including a segment of viral DNA comprising, for example, fibre coding sequences wherein a single lox P site is embedded upstream of fibre coding sequences such that Cre-mediated recombination between said plasmid DNA and the plasmid of the fifth embodiment results in production of an infectious viral genome. Optionally the fibre gene in said plasmid may be modified by mutation, insertion or deletion of portions of the fibre coding sequences. Similar plasmids can be constructed that have lox P sites at other locations, depending on the viral DNA segment that is to be manipulated by site-specific recombination. For example, a site exists in the Ad genome between the coding sequences of fibre and the coding sequences of E4 that is suitable for insertion of DNA. In this embodiment of the invention, the efficiency of recombination is enhanced several fold by inclusion of an hthlTR junction in both of the recombining plasmids. In a seventh embodiment of the invention, plasmids containing adenovirus sequences and lox sites are recombined in the presence of Cre recombinase to generate novel adenovirus mutants containing modifications of the fibre gene or modifications of other viral genes. In this embodiment of the invention, the efficiency of recombination is enhanced through inclusion of a hthlTR junction in both of the recombining plasmids. In a preferred embodiment of the present invention, a system is described for the construction of novel Ad vectors, or alteration of existing Ad vectors, by the use of a site- specific recombinase wherein hthlTR junctions are included to enhance the efficiency of Ad vector production. In a further embodiment of the invention, an infectious viral DNA-TP complex is engineered to take advantage of recombinase-mediated site-specific recombination and the enhanced level of infectivity achieved through presence of the terminal protein. It will be appreciated by those skilled in the art that the present invention disclosure provides significant advances over techniques known in the art for generation of adenoviral vectors. First, the efficiency by which recombinants are produced is enhanced through use of an hthlTR junction in combination with homologous recombination or in combination with site-specific recombination, rather than relying exclusively on homologous recombination. This invention further advances the art in that it facilitates use of vectors which are themselves non-infectious and stable. Further, by use of the methods disclosed herein, rapid production of recombinant virus is facilitated wherein every virus produced is a recombinant virus, as opposed to known methods wherein a starting virus is used in a site-specific recombination wherein substantial levels of non- recombinant starting virus remain in the preparation which has to then be serially passaged to remove the contaminating starter virus. As a result of this enhanced efficiency, while it may in many instances be desirable to colony or plaque-purify the results of a given cotransfection, because all viruses produced according to this embodiment of the instant technique are recombinants, plaque purification is not absolutely required. Accordingly, the instant method provides the option of rapid production of recombinants and screening of products, in a "shot-gun" approach, which will provide significant labor and time savings to those skilled in the art. In reviewing the detailed disclosure which follows, it should be borne in mind that any publications referenced herein are hereby incoφorated by reference in this application in order to more fully describe the state of the art to which the present invention pertains. It is important to an understanding of the present invention to note that all technical and scientific terms used herein, unless otherwise defined, are intended to have the same meaning as commonly understood by one of ordinary skill in the art. The techniques employed herein are also those that are known to one of ordinary skill in the art, unless stated otherwise. Reference to particular buffers, media, reagents, cells, culture conditions and the like, or to some subclass of same, is not intended to be limiting, but should be read to include all such related materials that one of ordinary skill in the art would recognize as being of interest or value in the particular context in which that discussion is presented. For example, it is often possible to substitute one buffer system or culture medium for another, such that a different but known way is used to achieve the same goals as those to which the use of a suggested method, material or composition is directed. The terms used herein are not intended to be limiting of the invention. For example, the term "gene" includes cDNAs, RNA, or other polynucleotides that encode gene products. "Foreign gene" denotes a gene that has been obtained from an organism or cell type other than the organism or cell type in which it is expressed; it also refers to a gene from the same organism that has been translocated from its normal situs in the genome. In using the terms "nucleic acid", "RNA", "DNA", etc., we do not mean to limit the chemical structures that can be used in particular steps. For example, it is well known to those skilled in the art that RNA can generally be substituted for DNA, and as such, the use of the term "DNA" should be read to include this substitution. In addition, it is known that a variety of nucleic acid analogues and derivatives is also within the scope of the present invention. "Expression" of a gene or nucleic acid encompasses not only cellular gene expression, but also the transcription and translation of nucleic acid(s) in cloning systems and in any other context. The term "recombinase" encompasses enzymes that induce, mediate or facilitate recombination, and other nucleic acid modifying enzymes that cause, mediate or facilitate the rearrangement of a nucleic acid sequence, or the excision or insertion of a first nucleic acid sequence from or into a second nucleic acid sequence. The "target site" of a recombinase is the nucleic acid sequence or region that is recognized (e.g., specifically binds to) and/or acted upon (excised, cut or induced to recombine) by the recombinase. The term "gene product" refers primarily to proteins and polypeptides encoded by other nucleic acids (e.g., non-coding and regulatory RNAs such as tRNA, sRNPs). The term "regulation of expression" refers to events or molecules that increase or decrease the synthesis, degradation, availability or activity of a given gene product. The present invention is also not limited to the use of the cell types and cell lines used herein. Cells from different tissues (breast epithelium, colon, lymphocytes, etc.) or different species (human, mouse, etc.) are also useful in the present invention. It is important in this invention to detect the generation and expression of recombinant nucleic acids and their encoded gene products . The detection methods used herein include, for example, cloning and sequencing, ligation of oligonucleotides, use of the polymerase chain reaction and variations thereof (e.g., a PCR that uses 7-deaza GTP), use of single nucleotide primer-guided extension assays, hybridization techniques using target-specific oligonucleotides that can be shown to preferentially bind to complementary sequences under given stringency conditions, and sandwich hybridization methods. Sequencing may be carried out with commercially available automated sequencers utilizing labeled primers or terminators, or using sequencing gel-based methods. Sequence analysis is also carried out by methods based on ligation of oligonucleotide sequences which anneal immediately adjacent to each other on a target DNA or RNA molecule (Wu and Wallace. Genomics 4: 560-569 (19S9y. l m.drea etal.. Proc. Natl. Acad. Sci. 87: 8923- 8927 (1990); Barany, F., Proc. Natl. Acad. Sci. 88: 189-193 (1991)). Ligase-mediated covalent attachment occurs only when the oligonucleotides are correctly base-paired. The Ligase Chain Reaction (LCR), which utilizes the thermostable Taq ligase for target amplification, is particularly useful for interrogating late onset diabetes mutation loci. The elevated reaction temperatures permits the ligation reaction to be conducted with high stringency (Barany, F., PCR Methods and Applications 1 : 5-16 (1991)). The hybridization reactions may be carried out in a filter-based format, in which the target nucleic acids are immobilized on nitrocellulose or nylon membranes and probed with oligonucleotide probes. Any of the known hybridization formats may be used, including Southern blots, slot blots, "reverse" dot blots, solution hybridization, solid support based sandwich hybridization, bead-based, silicon chip-based and microtiter well-based hybridization formats. The detection oligonucleotide probes range in size between 10-1,000 bases. In order to obtain the required target discrimination using the detection oligonucleotide probes, the hybridization reactions are generally run between 20°-60°C, and most preferably between 30°-50° C. As known to those skilled in the art, optimal discrimination between perfect and mismatched duplexes is obtained by manipulating the temperature and/or salt concentrations or inclusion of formamide in the stringency washes. The cloning and expression vectors described herein are introduced into cells or tissues by any one of a variety of known methods within the art. Such methods are described for example in Sambrook et al., Molecular Cloning: A Laboratory Manual, Cold Spring Harbor Laboratory, New York (1992), which is hereby incoφorated by reference. See, also, Ausubel et al., Current Protocols in Molecular Biology-, John Wiley and Sons, Baltimore, MD (1989); Hitt et al, "Construction and propagation of human adenovirus vectors," in Cell Biology: A Laboratory Handbook-, Ed. J.E. Celis., Academic Press. 2 nd Edition, Volume 1, pp: 500-512, 1998; Hitt et al, "Techniques for human adenovirus vector construction and characterization," in Methods in Molecular Genetics, Ed. K.W. Adolph, Academic Press, Orlando, Florida, Volume 7B, pp: 12-30, 1995; Hitt, et al., "Construction and propagation of human adenovirus vectors," in Cell Biology: A Laboratory Handbook." Ed. J. E. Celis. Academic Press. pp:479-490, 1994, also hereby incoφorated by reference. The methods include, for example, stable or transient transfection, lipofection, electroporation and infection with recombinant viral vectors. The protein products of recombined andunrecombined coding sequences maybe analyzed using immune techniques. For example, a protein, or a fragment thereof is injected into a host animal along with an adjuvant so as to generate an immune response. Immunoglobulins which bind the recombinant fragment are harvested as an antiserum, and are optionally further purified by affinity chromatography or other means. Additionally, spleen cells may be harvested from an immunized mouse host and fused to myeloma cells to produce a bank of antibody-secreting hybridoma cells. The bank of hybridomas is screened for clones that secrete immunoglobulins which bind to the variant polypeptides but poorly or not at all to wild-type polypeptides are selected, either by pre-absoφtion with wild-type proteins or by screening of hybridoma cell lines for specific idiotypes that bind the variant, but not wild-type, polypeptides. Nucleic acid sequences capable of ultimately expressing the desired variant polypeptides are formed from a variety of different polynucleotides (genomic or cDNA, RNA, synthetic olignucleotides, etc.) as well as by a variety of different techniques. The DNA sequences are expressed in hosts after the sequences have been operably linked to (i.e., positioned to ensure the functioning of) an expression control sequence. These expression vectors are typically replicable in the host organisms either as episomes or as an integral part of the host chromosomal DNA. Commonly, expression vectors contain selection markers (e.g., markers based on tetracycline resistance or hygromycin resistance) to permit detection and/or selection of those cells transformed with the desired DNA sequences. Further details can be found in U.S. Patent No. 4,704,362. Polynucleotides encoding a variant polypeptide include sequences that facilitate transcription (expression sequences) and translation of the coding sequences such that the encoded polypeptide product is produced. Construction of such polynucleotides is well known in the art. For example, such polynucleotides include a promoter, a transcription termination site (polyadenylation site in eukaryotic expression hosts), a ribosome binding site, and, optionally, an enhancer for use in eukaryotic expression hosts, and optionally, sequences necessary for replication of a vector. E. Coli is one prokaryotic host useful particularly for cloning DNA sequences of the present invention. Other microbial hosts suitable for use include bacilli, such as Bacillus subtilus, and other enterobacteriaceae, such as Salmonella, Serratia, and various Pseudomonas species. Expression vectors are made in these prokaryotic hosts which will typically contain expression control sequences compatible with the host cell (e.g., an origin of replication). In addition, any number of a variety of well-known promoters are used, such as the lactose promoter system, a tryptophan (Tφ) promoter system, a beta-lactamase promoter system, or a promoter system from phage lambda. The promoters typically control expression, optionally with an operator sequence, and have ribosome binding site sequences, for example, for initiating and completing transcription and translation. Other microbes, such as yeast, are used for expression. Saccharomyces is a suitable host, with suitable vectors having expression control sequences, such a promoters, including 3- phosphoglycerate kinase or other glycolytic enzymes, and an origin of replication, termination sequences, etc. as desired. In addition to microorganisms, mammalian tissue cell culture is used to express and produce the polypeptides of the present invention. Eukaryotic cells are preferred, because a number of suitable host cell lines capable of secreting intact human proteins have been developed in the art, and include the CHO cell lines, various COS cell lines, HeLa cells, myeloma cell lines, Jurkat cells, and so forth. Expression vectors for these cells include expression control sequences, such as an origin of replication, a promoter, an enhancer, and necessary information processing sites, such as ribosome binding sites, RNA splice sites, polyadenylation sites, and transcriptional terminator sequences. Preferred expression control sequences are promoters derived from immunoglobin genes, S V40, Adenovims, Bovine Papilloma Vims, Heφes Vims, and so forth. The vectors containing the DNA segments of interest (e.g., polypeptides encoding a variant polypeptide) are transferred into the host cell by well-known methods, which vary depending on the type of cellular host. For example, calcium chloride transfection is commonly utilized for prokaryotic cells, whereas calcium phosphate treatment or electroporation is useful for other cellular hosts. The method lends itself readily to the formulation of test kits for use in diagnosis. Such a kit comprises a carrier compartmentalized to receive in close confinement one or more containers wherein a first container contains reagents useful in the localization of the labeled probes, such as enzyme substrates. Still other containers contain restriction enzymes, buffers etc., together with instmctions for use. The methods provided herein for production of recombinant Ad vectors are a significant improvement over and are significantly different from previously described methods that rely on homologous recombination catalysed by recombinases in host cells or that rely on in vitro ligation of viral DNA fragments to produce infectious viral DNA. For viral DNA replication and packaging of viral DNA into virion particles, only three regions of the viral DNA are known to be required in cis. These are the left inverted terminal repeat , or ITR, (bp 1 to approximately 103) the packaging signals (approximately 194 to 358 bp) (Hearing and Shenk, 1983, Cell 33: 695-703; Grable and Hearing 1992, J. Virol. 64: 2047-2056) and the right ITR. Among the regions of the viral genome that encode proteins that function in trans, two have been most important in the design and development of adenovims vectors. These are early region 3 (E3) located between approximately 76 and 86 mu (mu = % distance from the left end of the conventionally oriented genome) and early region 1 (El) located between approximately 1 and 11 mu. E3 sequences have long been known to be nonessential for vims replication in cultured cells and many viral vectors have deletions of E3 sequences so that the capacity of the resulting vector backbone for insertion of foreign DNA is thereby increased significantly over that allowable by the wild- type vims (Bett, A. J., Prevec, L., and Graham, F. L. Packaging capacity and stability of human adenovims type 5 vectors. J. Virol. 67: 5911- 5921, 1993.). El encodes essential functions. However, El can also be deleted, providing that the resulting vims is propagated in host cells, such as the 293 cell line, PER-C6 cells, 911 cells, and the like, which contain and express El genes and can complement the deficiency of El (-) vimses. Vimses with foreign DNA inserted in place of E 1 sequences, and optionally also carrying deletions of E3 sequences are conventionally known as "first generation" adenovims vectors. First generation vectors are of proven utility for many applications. They can be used as research tools for high-efficiency transfer and expression of foreign genes in mammalian cells derived from many tissues and from many species. First generation vectors can be used in development of recombinant viral vaccines when the vectors contain and express antigens derived from pathogenic organisms. The vectors can be used for gene therapy, because of their ability to efficiently transfer and express foreign genes in vivo, and due to their ability to transduce both replicating and nonreplicating cells in many different tissues. Adenovims vectors are widely used in these applications. There are many known ways to constmct adenovims vectors. As discussed above, one of the most commonly employed methods is the so called "two plasmid" technique. In that procedure, two noninfectious bacterial plasmids are constmcted with the following properties: each plasmid alone is incapable of generating infectious vims. However, in combination, the plasmids potentially can generate infectious vims, provided the viral sequences contained therein are homologously recombined to constitute a complete infectious vims DNA. According to that method, typically one plasmid is large (approximately 30,000-35,000 nt) and contains most of the viral genome, save for some DNA segment (such as that comprising the packaging signal, or encoding an essential gene) whose deletion renders the plasmid incapable of producing infectious vims or said plasmid contains an insertion such that said viral genome would be too large to be packaged into virions. The second plasmid is typically smaller (eg 5000-10,000 nt), as small size aids in the manipulation of the plasmid DNA by recombinant DNA techniques. Said second plasmid contains viral DNA sequences that partially overlap with sequences present in the larger plasmid. Together with the viral sequences of the larger plasmid, the sequences of the second plasmid can potentially constitute an infectious viral DNA. Cotransfection of a host cell with the two plasmids produces an infectious vims as a result of homologous recombination between the overlapping viral DNA sequences common to the two plasmids. One particular system in general use by those skilled in the art is based on a series oflarge plasmids known as pBHG10, pBHG 11 andpBHGE3 described by Bett, A. J., Haddara, W., Prevec, L. and Graham, F.L: "An efficient and flexible system for constmction of adenovims vectors with insertions or deletions in early regions 1 and 3," Proc. Natl. Acad. Sci. US 91 : 8802-8806, 1994 and in US patent application S/N 08/250,885, and published as WO95/00655 (hereby incoφorated by reference). Those plasmids contain most of the viral genome and are capable of producing infectious vims but for the deletion of the packaging signal located at the left end of the wild-type viral genome. The second component of that system comprises a series of "shuttle" plasmids that contain the left approximately 340 nt of the Ad genome including the packaging signal, optionally a polycloning site, or optionally an expression cassette, followed by viral sequences from near the right end of El to approximately 15 mu or optionally to a point further rightward in the genome. The viral sequences rightward of El overlap with sequences in the pBHG plasmids and, via homologous recombination in cotransfected host cells, produce infectious vims. The resulting vimses contain the packaging signal derived from the shuttle plasmid, as well as any sequences, such as a foreign DNA inserted into the polycloning site or expression cassette located in the shuttle plasmid between the packaging signal and the overlap sequences. Because neither plasmid alone has the capability to produce replicating vims, infectious viral vector progeny can only arise as a result of recombination within the cotransfected host cell. However, as has been noted above, such homologous recombination processes can be inefficient, resulting in variable success in the isolation of vectors and occasional failure, particularly in the hands of those who are not specifically skilled in the art of virology, and more particularly, in the art of adenovirology. Site-specific recombination catalysed by an efficient recombinase, such as the Cre or FLP recombinase, can be many fold more efficient than homologous recombination. This invention disclosure provides methods and nucleic acid constmcts which significantly enhance the ease of production of viral vectors by the two plasmid method by enabling site-specific recombination between individual nucleic acids constmcts, neither of which alone is able to replicate and produce infectious adenovims. The methodology described herein furthermore utilizes Cre-loxP and other known recombination systems for efficient introduction of mutations of viral genes into the viral genome. Furthermore, the instant methodology is also applicable to insertion of foreign DNA sequences into various regions of the viral DNA, in addition to the El region classically used for that puφose. In additional embodiments of this invention, site-specific recombination is utilized in combination with infectious viral DNA having covalently bound terminal protein (DNA- TP complex), at either or both 5' strands of the DNA. Additional embodiments and applications of the site-specific recombination methodology will also become apparent based on the instant disclosure. In addition, as disclosed herein, we have unexpectedly found that inclusion of a head-to-head ITR junction in the shuttle plasmid significantly enhances the efficiency of recombination, whether through homologous recombination or site-specific recombination mechanisms. Having generally described the puφoses, advantages, applications and methodology of this invention, the following specific examples are provided to describe in a detailed fashion, various embodiments of this invention. However, it should be appreciated that the invention described herein is not limited to the specifics of the following examples, which are provided merely as a guide for those wishing to practice this invention. The scope of the invention is to be evaluated with reference to the complete disclosure and the claims appended hereto. It should further be recognized that the following examples using the human adenovims serotype 5 are not meant to be limiting. One skilled in the art would realize that similar plasmids, vimses and techniques could be utilized with a different human adenovims serotype, for example Ad2. Similarly, the use of human Ads is not meant to be limiting since similar plasmids, vimses and techniques could be utilized for different non-human adenovimses, for example bovine. Similarly, the use of adenoviruses is not meant to be limiting since similar plasmids, vimses and techniques could be utilized with other vimses, both human and non-human, for example baculovims. Use of Cre recombinase in these and other examples is not meant to be limiting as a person skilled in the art will readily appreciate that other enzymes capable of catalysing site- specific recombination between DNA sequences recognized by said enzymes could equally be employed in place of the Cre recombinase. An example, not meant to be limiting, of such an enzyme that could be substituted for Cre is the "FLP" recombinase of yeast in combination with its target site FRT (O'Gorman et al. Science 251, 1351, 1991). A component of the invention is the use of human cells, such as 293 cells or other cells that may be deemed suitable in that they support the replication of the viral components of the invention, that express Cre recombinase and that can be transfected with the plasmids described herein and in the examples which follow, to generate a vims containing the desired modifications such as an insertion of foreign DNA or a modified fibre gene. It will be appreciated by those skilled in the art that the requisite cell lines can be generated by transfecting 293 cells or other cells, with a plasmid comprising the coding sequences for Cre under the control of suitable regulatory sequences, including a promoter and polyadenylation signal and containing, in addition, a selectable gene encoding, for example, resistance to G418 or histidinol. A person skilled in the art can readily obtain dmg resistant cells that express the Cre recombinase in addition to the dmg resistance gene used for selection. It will also be appreciated by one skilled in the art, based on the present disclosure, that host cells can also be induced to transiently express a recombinase by transfection with a plasmid comprising an expression cassette containing said recombinase gene or by infection with a viral vector that expresses the recombinase. Thus the example of 293 Cre cells or other permanently transformed recombinase expressing cell lines is not meant to be limiting. EXAMPLE 1 TWO-PLASME). SITE-SPECIFIC ADENOVIRAL RECOMBINATION Figure 1 provides a graphic representation of the use of a plasmid, pBHGloxΔE 1 ,3 , which comprises a circularized form of the Ad genome in which part or all of the El region, including the packaging signal, is substituted by sequences comprising a bacterial plasmid origin of replication and an antibiotic resistance gene, such as that encoding ampicillin resistance. The plasmid further comprises a loxP site near the 5' end of the pIX gene of the Ad genome. The plasmid may also, optionally, have a deletion of E3 sequences (as shown in this illustration by the symbol ΔE3) which may optionally be substituted with one or more unique cloning sites for insertion of foreign DNA in the ΔE3 region. A second component of the invention comprises a "shuttle plasmid" containing an ITR of the vims genome and a packaging signal, a polycloning site into which may be inserted a foreign DNA such as that encoding for bacterial β-galactosidase (LacZ) or any other gene, expression of which is desired either in a gene therapeutic or vaccine context, and a loxP site inserted in the same relative orientation as the loxP site in pBHGloxΔE 1,3. To obtain high-efficiency rescue of the foreign DNA into an infectious viral vector, the two plasmids are cotransfected into human cells, such as 293Cre cells, PER-C6 cells, 911 cells, and the like, engineered to express Cre and which, in addition, express the El region of the Ad genome. It should be appreciated that the manner of provision of the recombinase is not critical. The recombinase may be constirutively expressed by the cell into which the two plasmids are introduced. The recombinase may be provided in trans, via a third plasmid, or in cis, by inclusion of a recombinase expression cassette in one of the introduced plasmids. In addition, it will be appreciated that any recombinase which efficiently induces site-specific recombination between sequences present on the two plasmids may be employed according to this methodology. Thus, the FLP recombinase, which recognizes the sequences known as FRT, may be used in place of the Cre/loxP combination, and thus, wherever Cre or loxP are mentioned herein, such mention should be read to include any other site-specific recombination system now known or henceforth discovered, when applied to the specific techniques disclosed and claimed herein. Cre-mediated recombination results in formation of joint molecules that generate infectious vimses containing the foreign DNA insert. Because pBHGloxΔE 1,3 lacks the viral packaging signal, the only vimses that can form are those containing the packaging signal and foreign DNA of the shuttle plasmid. These are generated in large numbers because of the high-efficiency and specificity of Cre recombinase, and there is no background of non-recombinant vims in contrast to a method such as that of Hardy et al., J. Virol. 71(3): 1842-1849, (1997), which, even after three sequential passages in Cre expressing cells, results in a vector preparation still contaminated by starting (non- recombinant) vims. EXAMPLE 2 COMPARISON OF HOMOLOGOUS AND SITE-SPECIFIC RECOMBINATION Figure 2 illustrates use of a modified shuttle plasmid wherein Ad sequences from about lOmu to about 15mu are present to the right of the lox site. These sequences permit homologous recombination to occur in the absence or presence of Cre. A shuttle plasmid such as that shown in this figure is generally used only for comparison puφoses to assess the relative efficiency of homologous versus Cre-mediate recombination. As will be seen in the subsequent description of the invention, in the presence of Cre, overlapping sequences are unnecessary and can be omitted, although this disclosure does not require the absence of such sequences. EXAMPLE 3 SEQUENCES USEFUL IN THE PRODUCTION OF PLASMIDS WHICH MAY BE RECOMBINED IN A SITE-SPECIFIC MANNER TO PRODUCE ADENOVIRAL VECTORS Figure 3 illustrates sets of oligonucleotides used in various cloning procedures. The double stranded oligonucleotide (AB3233/3234) contains a loxP site with restriction sites for Seal and EcoRI at one end of the oligo outside of the loxP region. When annealed, the oligonucleotides have BamHVBgl II overhangs which are designed for cloning into and concomitant destruction of the BgUL site. The internal Seal site found in (AB3233/3234) was designed to facilitate determination of the orientation of the linker and also for subsequent deletion of Ad5 sequences from m.u. 9.8-15.8. The second linker (AB 14626/14627) has EcoRI and Sail overhangs and a multiple cloning region containing Smαl, BglR, Hind∑π. and Seal restriction sites. EXAMPLE 4 CONSTRUCTION OF BACTERIAL PLASMIDS CONTAINING CIRCULARIZED FORMS OF THE ADENOVIRUS GENOME SUITABLE FOR RESCUE OF VIRAL VECTORS USING SITE-SPECIFIC RECOMBINATION ACCORDING TO THE GENERAL SCHEME ILLUSTRATED ACCORDING TO FIGURE 1 Figure 4 illustrates production of a plasmid, pBHGloxΔEl, 3, a derivative of pBHGlO, modified to contain a lox? site at the 3' end of the El deletion. As can be seen with reference to the figure, this plasmid was constmcted by replacing the 4604 bp Bstl 1071 fragment from pBHG 10 with the 2326 bp EcoRWlBstl 1071 fragment from pΔΕ 1 sp 1 Alox . The plasmid pΔΕlsplAlox (Figure 5) was constmcted by inserting an oligonucleotide bearing a lox? site (comprised of annealed oligos AB3233 and AB 3234) into the Bglll site of pΔΕlsplA. Foreign sequences can be inserted into the unique Rαcl site of pBHGloxΔΕl,3 for rescue of genes in E3. The plasmid illustrated in Figure 4 can be selected from the series pBHGlO (as illustrated), pBHGl 1, pBHGE3, or like plasmid, for modification to contain a lox ? site near the 3' end of El ie. near the 5' end of the pIX gene at approximately nt 3520 in the conventional sequence of Ad5. Optionally El sequences from approximately nt 188 to approximately 3520 maybe deleted from said plasmid. Like the parental plasmids (such as pBHGlO, pBHGl l or pBHGE3) the modified pBHG derivative (eg. pBHGloxΔE 1,3, pBHGdXIPlox, pBHGE31ox, or like plasmid) lacks the packaging signal (ψ), and is consequently incapable of producing infectious vims in transfected host cells. Figure 4a illustrates the constmction of a plasmid, pBHGdXIPlox, containing a modified E3 deletion (taken from pFG23dXlP)and a lox site 5' of the pIX gene. The plasmid pFG23dXlP was constmcted by inserting an oligonucleotide containing a Rαcl site (AB14566; 5'- CTAGCTTAATTAAG -3', SEQ ID NO.:9) into the Nbαl site of pFG23dXl. The plasmid pNG17 was constructed by cloning the 6724 bp SpeVClal fragment from pBHGloxΔEl,3 into pBluescript. The plasmid pNG17dXlP was constmcted by replacing the 1354 bp SpeVNdel fragment from pNG17 with the 2129 bp SpeVNdel fragment from pFG23dXlP. The plasmid pBHGdXIP was constmcted by replacing the 6724 bp SpeVClal fragment from pBHGloxΔEl ,3 with the 7495 bp SpeVClal fragment from pNGl 7dXlP. Figure 4b illustrates the constmction of a plasmid containing a wild-type E3 region and a loxP site 5' of the pIX gene.The plasmid pBHGE31ox was constmcted by replacing the 6724 bp SpeVClal fragment from pBHGloxΔEl, 3 with the 9377 bp SpeVClal fragment from pBHGE3. EXAMPLE 5 CONSTRUCTION OF SHUTTLE PLASMIDS FOR RECOMBINATION WITH ADENOVIRAL RESCUE PLASMID. CONSTRUCTED ACCORDING TO EXAMPLE 4 As described above, a second embodiment of the invention comprises a shuttle plasmid selected from a series of plasmids containing, minimally: the left end of the viral genome including all or most of the left Inverted Terminal Repeat (ITR nts 1- 103 of the Ad 5 DNA) and the packaging sequence, and optionally a polycloning site or optionally an expression cassette. With reference to figures 5-8, such shuttle plasmids are modified to contain a lox ? site in the same orientation as the loxP site in the pBHG derivative, (see Example 4, referred to herein as the "rescue plasmid"), said lox? site being positioned in said shuttle plasmid to the right of said polycloning site or said expression cassette. Figure 5 illustrates the constmction of shuttle plasmids derived from pΔElSPlA and pΔEl SP1B wherein loxP sites are introduced 5' of the pIX gene. The plasmids, pΔEl sp 1 A and pΔElSPlB are left end shuttle plasmids containing Ad5 sequences from m.u. 0-15.8 with El sequences deleted between m.u.land 9.8. They are identical except that the restriction sites in the multiple cloning region are reversed. A synthetic loxP linker (AB3233/3234) was introduced into the Bglll site of each plasmid generating pΔElSPl Alox and pΔElSPlBlox. Ad5 sequences from m.u. 9.8-15.8 were removed by digesting the plasmids with Nrul, partially cutting with Seal followed by self-ligation. The plasmids thus generated are called pΔElSPlAloxΔ and pΔElSPlBloxΔ. Figure 6 illustrates the constmction of pMH41ox and pMH41oxΔ plasmids that contain a promoter and polyadenylation signal and polycloning sites for insertion of foreign DNA to produce expression cassettes in which transcription is driven by the murine cytomegalovirus immediate early gene promoter. Plasmid pVDB3 is derived from pMH4 but contains a pUC -based origin of replication, rather than a pBR322 origin. It contains Ad5 sequences from m.u. 0-15.8 with El sequences deleted between m.u. 1 and 9.8 and subsituted with an expression cassette: a 0.5kbp (-491 to +36) fragment of the MCMV IE promoter, unique restriction enzyme sites for cloning (Eco RI, Nhe I, Bam HI and Sal I) followed by an SV40 polyadenylation signal. To make pMH41ox, a loxP linker (AB3233/3234)wasintroducedintotheRg/πsiteofpNDB3. Ad5 sequences m.u.9.8-15.8 were deleted from pMH41ox by digesting with Hmd in, treating with the Klenow fragment of E. coli DΝA polymerase then partially digesting with Sea I followed by self-ligation. The resulting shuttle plasmid, pMΗ41oxΔ, can be used with pBHGloxΔE 1,3 to produce Ad vectors via Cre/lox mediated recombination. To make pMH41oxΔ a more flexible plasmid for cloning puφoses, a linker (AB14626/14627), containing a different multiple cloning region, was introduced between the Eco RI and Sal I sites resulting in pMH41oxΔlink. Figure 6a illustrates the constmction of plasmid pVDB3. A Pvul to Bst 11071 fragment from pMH4 (Microbix Biosystems) was ligated to a Bst 11071 to Pvu I fragment from pD47El containing a pUC-based (pΝEB193, New England Biolabs) origin of plasmid DNA replication to generate pVDB3. Figure 7 illustrates constmction of HCMV loxP plasmids in which transcription of foreign genes is regulated by the human cytomegalovirus immediate early gene promoter. The plasmids pCA13 and pCAl 4 contain the Ad5 genomic sequences from m.u. O to 15.8 with El sequences between m.u. 1 and 9.8 replaced by the HCMV EE promoter (-299 to +72, relative to the transcription start), a polycloning region and an SV40 polyadenylation signal. (Plasmids pCA13 and pCA14 are available from Microbix Biosystems). The expression cassette in each case is oriented parallel to the direction of El transcription (rightwards). The only difference between pCA13 and pCA14 is in the orientation of the multiple clomng region. The plasmids pCA 13(ΔBgRT) and pCAl4(ABglE) were generated by digesting pCA13 and pCA14 partially with BglE, Klenowing and self-ligating. A synthetic loxP oligonucleotide (AB3233/3234) was introduced into the unique BglU sites ofpCA13(ΔRg/π) and pCA14(ΔRg/II) producing pCA131ox and pCA141ox respectively. Ad5 sequences, m.u. 9.8-15.8, were removed from pCA131ox and pCA141ox by cutting each plasmid with Nrwl and partially digesting each with Seal followed by self ligation. The resulting plasmids, pCA131oxΔ and pCA141oxΔ are useful shuttle plasmids for the rescue of first generation Ad vectors by Cre/lox recombination. Figure 8 illustrates the constmction of a plasmid, pCA361oxΔ, for rescue of the β- galactosidase gene into adenovims vectors. Naturally, the rescued gene maybe any foreign gene, and is not restricted to the use of a marker gene, such as the marker beta-gal gene, which is used herein for illustrative puφoses. The plasmid pCA36 contains the β-gal cDNA under control of the short MCMV IE promoter (-491 to +36) followed by an SV40 polyadenylation signal. Plasmid pCA36 was made by inserting the LacZ gene into pMH4 (available from Microbix Biosystems) and is described by Addison, C. L., Hitt, M., Kunsken, D. and Graham, F. L., in "Comparison of the human versus murine cytomegalovirus immediate early gene promoters for transgene expression in adenoviral vectors," J. Gen. Virol. 78: 1653-1661, 1997." A synthetic loxP site (AB3233/3234) was introduced into the Bgl II site of pCA36 resulting in pCA361ox. This plasmid was then digested with Nru I and partially digested with Sea I, a 7646bp fragment was gel purified and self ligated yielding pCA361oxΔ. This plasmid contains Ad sequences from m.u. 0-1 , and not only has the deletion of El sequences present in the parental plasmids pCA36 and pCA361ox, but additionally is deleted of Ad5 sequences from m.u.9.8-15.8. EXAMPLE 6 DEMONSTRATION OF ENHANCED EFFICIENCY OF SITE-SPECIFIC RECOMBINATION IN COMPARISON WITH HOMOLOGOUS RECOMBINATION In a third embodiment of the invention, two plasmids containing loxP or other recombinase recognition sites are cotransfected into 293Cre or other appropriate cells (expressing an appropriate recombinase, Cre for puφoses of this example). The Cre enzyme catalyses site-specific recombination between said lox ? sites present in each vector. As illustrated in Figure 1 , it will be readily seen by one skilled in the art that Cre- mediated recombination between said lox ? sites generates a viable vims by j oining pBHG sequences to a DNA segment containing ψ and ITR sequences. Furthermore, by virtue of the design and constmction of the pBHG derivative and the shuttle plasmid, the resulting viral vector contains the expression cassette located to the left of the lox ? site in said shuttle plasmid, thereby providing a simple and efficient means for isolating viral vectors containing foreign DNA insertions and expression cassettes for synthesis of proteins from foreign genes. To test and demonstrate the validity of the approaches outlined above and to determine the degree of improvement in efficiency of vector isolation compared to known methods, a number of experiments were conducted in which a vector carrying a LacZ expression cassette inserted near the left end of the Ad genome was constmcted. The efficiency of Cre/lox mediated recombination was compared with that of homologous recombination, by measuring the numbers of vims plaques obtained from cotransfections of 293 cells versus the numbers obtained following cotransfections of 293Cre4 cells (see, for example, US patent application serial number 08/473, 168, filed June 7, 1995; see also WO96/40955, hereby incoφorated by reference). The results shown in Table 1 indicate that Cre/lox mediated recombination (cotransfections of 293Cre4 cells with plasmids that both contain lox sites) was approximately 35-fold more efficient than homologous recombination (cotransfections of 293 cells or cotransfections of 293Cre4 cells with plasmids that do not both contain lox sites). A 35-fold increase represents a very significant and unexpectedly high improvement over efficiencies of vector rescue when vims isolation is dependent on homologous recombination. Coupled with the fact that the only infectious vims present in the transfected cell preparation are recombinants, rather than contaminating starting vims, the efficiency, cleanliness and convenience of this method in comparison to known methods represent significant advances in the art. Thus, with this new method it will be possible to reduce the amount of plasmid DNA used in cotransfections and reduce the number of dishes of 293 (293 Cre) cells needed in cotransfections for rescue of viral vectors. It will also aid in the rescue of constmcts which, for unknown reasons, might be otherwise difficult to rescue (e.g. rescue of vectors containing large foreign DNA inserts in El is often inefficient for reasons that are not known). To confirm that the enhanced efficiency of plaque formation following cotransfection of 293Cre cells with pCA36 + pBHGloxΔEl ,3 was due to Cre-lox dependent recombination (versus, for example, enhanced efficiency of homologous recombination) we constmcted a derivative of pCA361ox, named pCA361oxΔ (see Figure 8), from which overlapping Ad sequences to the right of the lox site had been removed, thus virtually eliminating any possibility of homologous recombination. This new shuttle plasmid was then tested for ability to generate vectors in a second experiment in which 293 or 293Cre cells were cotransfected with this plasmid or with pCA36 or pCA361ox for comparison along with pBHGloxΔEl, 3. It can be seen from the results shown in Table 2 that pCA361oxΔ only generated viral plaques following cotransfection of293Cre cells with pBHGloxΔEl, 3. In contrast pCA36 or pCA361ox were able to generate small numbers of plaques on 293 cells. However, again, the efficiency was markedly enhanced if 293Cre cells were cotransfected with pCA361ox and pBHGloxΔE 1,3. Thus the use of Cre-lox recombination results in a suφrisingly efficient system for rescue of foreign DNA into Adenovims vectors. To confirm that transfection of 293Cre cells with pCA361ox (a lacZ-containing shuttle plasmid with a loxP site located between the expression cassette and the pIX coding sequence as illustrated in Fig. 8) and pBHGloxΔE 1,3 resulted in vimses containing the desired insert of foreign DNA, 26 recombinant plaques were isolated, expanded and analyzed for expression of LacZ. All 26 (100%) were positive for β-galactosidase expression. Furthermore, analysis of the structure of the vimses confirmed that all 26 had the expected DNA stmcture illustrated in Figure 1. Further confirmation of the efficiency and specificity of the Cre/lox system for rescue of expression cassettes was obtained through analysis of 6 plaque isolates obtained by cotransfection of 293Cre cells with pCA361oxΔ and pBHGloxΔEl, 3 (Table 2). All 6 plaque isolates expressed β- galactosidase and all 6 had the expected DNA stmcture illustrated in Figure 1. Because 100%) of recombinant vimses produced by cotransfection of 293Cre cells with plasmids containing appropriately engineered lox sites have the correct stmcture and express the transgene, ( β-galactosidase in these examples), it will be appreciated by those skilled in the art that one could readily produce recombinant vimses carrying other foreign DNA inserts by constructing shuttle plasmids derived from the plasmids shown in Figures 5, 6 and 7 or similar plasmids, and cotransfecting said modified shuttle plasmids into 293 Cre or like cells, along with pBHGloxΔEl, 3 or similar pBHG plasmids containing a lox site near the end of El. It will be further appreciated by those skilled in the art that because of the high-efficiency of rescue with this approach, only small numbers of 293 Cre cultures and small amounts of DNA need be used to obtain the desired recombinant vimses. Furthermore, because only the desired recombinant vimses are obtained from said cotransfections, it would not be essential to plaque purify and analyze viral progeny obtained according to the method of this invention. In addition, after the initial isolation of the recombinant vimses from 293 Cre cells, said vimses can be propagated in host cells such as 293, 911 or PERC-6 cells or the like which do not express recombinase. EXAMPLE 7 SITE-SPECIFIC SHUTTLE PLASMID- VIRUS RECOMBINATION Hardy et al., J. Virol. 1997, Mar:71(3): 1842- 1849, and see also WO97/32481 disclosed a method whereby an infectious DNA vector was used in combination with a plasmid in combination with lox-Cre recombination to generate recombinant adeno vimses. However, according to that method, residual infectious starter vims remains in the recombinant vims preparation, requiring repeated passage of the preparation in a Cre expressing cell to eliminate this background. An advancement to such techniques is provided herein by combination of Cre-lox recombination and use of adenoviral DNA bound to the adenoviral terminal protein (TP). The result of this combination is high-efficiency infection combined with site-specific recombination. The use of a two plasmid system for isolation of viral vectors or modified vimses is not meant to be limiting. From the instant disclosure, it will be appreciated by those skilled in the art that one could use, as one component of the system, viral DNA from a modified vims whose genome contains lox ? sites at useful positions. An excellent example, not meant to be limiting, is use of AdLC8, AdLC8c or AdLCδcluc described by Parks, R. J., Chen, L., Anton, M., Sankar, U., Rudnicki, M. A. and Graham, F. L., in "A new helper- dependent adenovims vector system: removal of helper vims by Cre-mediated excision of the viral packaging signal," Proc. Natl. Acad. Sci. U.S. 93: 13565-13570, 1996. These vimses contain a "floxed" packaging signal, which is excised following vims infection of 293Cre cells. Therefore, cotransfection of 293Cre cells with viral DNA extracted from AdLC8, AdLC8c or AdLC8cluc in such a way as to retain the covalent linkage to TP, according to methods taught by Shaφ et al., "The infectivity of adenovims 5 DNA-protein complex," Virology, 1976 Dec:75(2):442-456; Chinnadurai, et al., "Enhanced infectivity of adenovims type 2 DNA and a DNA-protein complex," J. Virol. 1978 Apr:26(l):195- 199, and a shuttle plasmid such as that illustrated in Figures 5, 6, 7 or 8 results in Cre- mediated recombination to generate a new vector containing the sequences derived from the shuttle plasmid, spanning the region from the ITR and packaging signal of the shuttle across the optional polycloning site or optional expression cassette to the lox ? site of said shuttle plasmid. For example, not meant to be limiting, as illustrated in Figure 8a, using a lacZ-encoding plasmid, similar to that shown in Figure 8, and AdLC8c DNA-TP, one skilled in the art could readily isolate the desired recombinant vims containing lacZ or other foreign genes by cotransfection of 293Cre cells with DNA extracted from AdLC8c- TP and said Lac Z-encoding plasmid. Optionally, as illustrated in Figure 8b, one could cotransfect 293Cre cells with restriction endonuclease treated AdLC8c DNA-TP and a shuttle plasmid selected from the set of plasmids illustrated in Figures 5, 6, 7 and 8 to produce infectious vims by Cre-mediated recombination. The viral DNA extracted from AdLC8c according to this method retains the terminal protein which has been shown to increase the efficiency of transduction of recipient cells with said DNA (Shaφ PA, Moore C, Haverty JL, "The infectivity of adenovims 5 DNA-protein complex," Virology 1976 Dec;75(2):442-456). It will be apparent to those skilled in the art that the left most lox site is not needed and may optionally be deleted if AdLC8cDNA-TP is to be cut with restriction enzymes prior cotransfection. Furthermore, optionally, after restriction enzyme digestion, the large right end fragment of AdLC8cDNA-TP could be purified prior to cotransfection. Figure 8c is a diagrammatic representation of a method for constmcting shuttle plasmids expressing Cre. The Cre expression cassette was obtained from the plasmid pLC2 (Chen, L., -Anton, M. and Graham, F.L., "Production and characterization of human 293 cell lines expressing the site-specific recombinase Cre," Somat. Cell andMolec. Genet.22:477-488, 1996), as a 2175 bp Bglll fragment which was end-modified with Klenow DNA polymerase and inserted into the Ehel site of pCA36/oxΔ to generate pCA36/θΛ:ΔCreR and pCA36/o.Λ:ΔCreT. Theplasmid pCA35/αxΔCreITR was constmcted by replacing the 1402 bp ScaVKpnl fragment in pCA36/oxΔCreT with the 2753 bp ScaVKpnl fragment from the plasmid pRP 1029. Plasmid pCA35/θΛ;ΔCreITR contains ITR junctions which are known to be functionally capable of generating replicating linear Ad DNA molecules (Graham, F.L., "Covalently closed circles of human adenovims DNA are infectious," The EMBO J. 3, 2917-2922, 1984). Figure 8d provides a schematic representation of a cotransfection experiment wherein a pBHGloxΔEl, 3 plasmid and a "Lox" shuttle plasmid expressing Cre are introduced into 293 cells in order to generate Ad expression vectors, without having to use cells which stably express Cre. This technique is applicable to any cell type suitable for Ad vector generation, including but not limited to 293 cells, and PER-C6 cells (Fallaux et al., Hum. Gene Ther. 1998, Sep. 1;9(13):1909-17), 911 cells (Fallaux et al., Hum. Gene Ther. 1996 Jan. 20;7(2):215-222), or other cells. A shuttle plasmid such as pCA351oxΔCreITR of figure 8c is also suitable for generation of an Ad vector. The efficiency of Ad vector rescue by cotransfection with pBHGloxΔE 1 ,3 and various shuttle plasmids is summarized in Tables 3 and 4. It can be seen from the results in Table 4 that inclusion of an ITR junction in the shuttle plasmid increases the efficiency of rescue significantly. Thus, provision of an ITR junction is a preferred embodiment. Insertion of an expression cassette encoding Cre recombinase in the shuttle plasmid is not meant to be limiting as one skilled in the art will appreciate that one could also insert a Cre cassette in the larger plasmid, pBHGloxΔE 1,3. An example, not meant to be limiting, is diagrammed in Figure 8e, which illustrates the construction of such a plasmid. It will be appreciated that the Cre expression cassette could be carried by either of the two plasmids used in the cotransfections such as that illustrated in Figure 1, or by both of them, so that Cre is supplied at adequate levels in cotransfected 293 cells to catalyse efficient recombination between the lox sites of the cotransfected plasmids. Thus mention of the use of 293 Cre cells or like cells expressing Cre recombinase is not meant to be limiting. Figure 8e demonstrates the constmction of an Ad genomic plasmid encoding Cre. The plasmid pBHGloxΔ was constmcted by collapsing pBHGloxΔEl ,3 with Spel and PshAl. The Cre expression cassette, taken from the plasmid pLC2 as a 2175 bp Bglll fragment, was inserted into the BamHI site of pBHGloxΔ to generate pBHGloxΔCreR and pBHGloxΔCreL. The 1238 bp R-stl l07I/RαcI fragment from pBHGloxΔCreR and pBHGloxΔCreL was replaced with the 22380 bp R-?tl l07I/RαcI fragment from pBHGloxΔE 1,3 to generate pBHGloxCreR and pBHGloxCreL, respectively. EXAMPLE 8 RESCUE OF FOREIGN DNA AND MUTATIONS INTO ANY DESIRED LOCATION IN THE ADENOVIRAL GENOME The above examples illustrating rescue of foreign DNA into the El region of Ad vectors are not meant to be limiting. It will be appreciated by those skilled in the art that one could equally follow the instmctions outlined above to constmct similar plasmids for the rescue of insertions or mutations or deletions into El or other regions of the viral genome. For example, not meant to be limiting, one could constmct a series of analogous plasmids suitable for rescue of fibre mutations into the viral genome or for rescue of foreign DNA inserts in the E3 region of the viral genome into infectious vims. An example, not meant to be limiting, is provided in Figure 9, which is a diagrammatic representation of a method for rescuing fibre mutations into infectious vims using Cre-loxP recombination. Cotransfection of 293Cre cells with pFG1731ox and a shuttle plasmid containing a loxP site 5' of the fibre gene results in site-specific recombination between the lox sites and rescue into infectious vims of the adenoviral sequences of the shuttle, which sequences may optionally contain a mutated fibre gene. Figure 9a is a diagrammatic representation of a method for constmcting a plasmid containing a lox site and ampicillin resistance gene substituting for the fibre gene. Starting with a plasmid such as pAB141ox, constmction of which is described in Figure 14, the DNA sequences between the Cla I site and the Blp I site containing fibre are substituted with a DNA segment containing the ampicillin resistance gene and a plasmid origin of DNA replication (which may optionally be obtained by restriction endonuclease digestion of an ampicillin resistant plasmid such as pCA14 (Microbix Biosystems)). Figure 9b is a diagrammatic representation of a method for combining the plasmid of Fig. 9a with pFGl 73 to produce pFGl 73 lox for rescuing fibre mutations into infectious vims using Cre-lox recombination. The plasmid pAB 141oxΔ illustrated in Figure 9a comprises Ad sequences 3' of fibre to mu 100. The plasmid additionally contains viral DNA sequences 5' of fibre, but has all of the fibre coding sequences deleted and substituted with a plasmid origin of DNA replication and an antibiotic resistance gene, such as for ampicillin resistance. Sequences from pAB141oxΔ can be recombined with pFG173 (Microbix Biosystems) by homologous recombination in E. coli (Chartier C, Degryse E, Gantzer M, Dieterle A, Pavirani A, Mehtali M., "Efficient generation of recombinant adenovims vectors by homologous recombination in Escherichia coli," J Virol 1996 Jul;70(7):4805-4810 ). The resulting plasmid, pFG1731ox, has a deletion of sequences comprising all of the fibre gene or optionally part of the fibre gene or optionally all or part of E4 or optionally a deletion of all or part of both fibre and E4, and is consequently unable to produce infectious vims following transfection of cells. However, on recombination with a plasmid such as pFG23dXl lox or a similar plasmid, infectious vims can be readily generated, as illustrated in Figure 9. Said recombination can be efficiently catalysed by Cre recombinase, if pFG1731ox and pFG23dXllox are cotransfected into 293Cre cells or similar host cells expressing Cre recombinase. Constmction of plasmids suitable for rescue of fibre or E4 gene mutations or deletions or substitutions can be readily accomplished by one skilled in the art based on the present disclosure. -An example, not meant to be limiting, of the constmction of one such plasmid is illustrated in Figure 10, which is a diagrammatic representation of aplasmid containing the right approximately 40%> of the vims genome, wherein a lox ? site has been inserted near the 5' end of the fibre gene. PFG23dXl contains the right approximately 40% of the Ad 5 genome from nt 21563 (mu 60) to approximately the right end of the genome (mu 100) cloned into the BamH I site of pBR322 and additionally has a deletion of Ad5 sequences from 28593 to 30471, comprising most of E3 (Haj-A-hmad, Y. and Graham, F.L., "Development of a helper independent human adenovims vector and its use in the transfer of the Heφes Simplex Vims thymidine kinase gene," J. Virol. 57, 267-274, 1986). PFG23dXl was digested with Xbal and a synthetic oligonucleotide (AB6920/AB6921, Figure 3) containing a loxP site was inserted. The resulting plasmid, pFG23dXllox, can be used for generation of infectious vims by cotransfection of 293Cre cells with aplasmid such as pFG1731ox (Figure 9). Optionally, viral genes, such as those encoding fibre or genes of E4 can be mutated in pFG23 dX 1 lox and the resulting mutations rescued into vims. Because Ad sequences 5' of the lox site (counterclockwise in the diagram) are not necessary when Cre-mediated site specific, rather than homologous, recombination is used to generate infectious vims, viral sequences between a unique Bst 11071 site and a BsiWl site immediately 5' of the lox P site were deleted to generate pFG23dXlloxc. One skilled in the art would appreciate, based on the instant disclosure, that just as Cre recombinase may be provided by inserting a Cre expression cassette in one or another or both of the cotransfecting plasmids to facilitate recombination between plasmids designed to rescue mutations or insertions in El, similarly, one may insert said expression cassette into either or both of the plasmids to be recombined as diagrammed in figure 9 so that site specific recombination can be achieved in host cells that do not express the recombinase constitutively. In a preferred embodiment, the shuttle plasmid thus modified would be further modified to contain a junction of ITRs as the results shown in Table 4 indicate that said junction results in a significant improvement in efficiency of vims production. As in the examples illustrated in Figures 8c and 8e, said plasmids would most often be designed so that the Cre expression cassette would not be rescued into the infectious viral genomes that are thus generated. Examples illustrating rescue of mutations into infectious vims are not meant to b e limiting as one skilled in the art could readily appreciate that the methods described herein are equally employed to rescue insertions of foreign DNA into the viral genome. An example of a suitable plasmid that is readily constructed is pFG23dXlLacZlox. Figure 11 is a diagrammatic representation of said plasmid wherein a foreign DNA, such as a gene encoding bacterial lacZ, is inserted between the lox ? site and the fibre gene. In this example, not meant to be limiting, an expression cassette encoding β-galactosidase is inserted into the Cla I site adjacent to the loxP of pFG23dXllox (Figure 10) for subsequent rescue into infectious vims by the method illustrated in Figure 9. It will be appreciated by those skilled in the art that other foreign DNAs could readily be rescued into infectious vims genomes by the methods illustrated above. Said foreign DNA segment could be a separate expression cassette or a fusion of sequences encoding peptide sequences to sequences encoding fibre, said peptide sequences representing, for example, a ligand to a cell surface receptor such that the rescued vims expressing a modified fibre would have novel and useful cell attachment properties. This example is not meant to be limiting as it will be appreciated by one skilled in the art that lox P sites can readily be introduced into other positions of the viral DNA for substitution of other virion genes with mutated counteφarts. These examples are not meant to be limiting as one could constmct a plasmid similar to pFG1731ox from which other viral genes have been deleted such as, for example, those of E 1 such that the resulting vimses generated by Cre-mediated recombination are E 1 deleted vimses. EXAMPLE 9 USE OF ENGPNEERED ADENO VIRUSES PRODUCED ACCORDING TO THIS INVENTION The use of the two plasmid system in combination with Cre-mediated site-specific recombination is not meant to be limiting as one skilled in the art will readily appreciate that, as taught for the generation of vimses carrying E 1 mutations, deletions and insertions, one could employ viral DNA isolated from suitably engineered vimses for the manipulation of the viral genome by Cre-mediated recombination. For example, as illustrated in Figures 12 and 13, 293Cre cells are cotransfected with DNA extracted from a vims containing a floxed fibre gene in such a way as to retain either or both terminal proteins, TP. Optionally the DNA is digested with restriction enzymes that cut sequences between the lox sites prior to cotransfections. It will be apparent to those skilled in the art, based on the instant disclosure, that the right most lox site is not needed and may optionally be deleted or omitted if DNA-TP is to be cut with restriction enzymes prior to cotransfection. As with the two plasmid method, the method of figures 12 and 13 is employed to rescue mutations in the fibre gene or in E4 or to rescue foreign DNA inserts as in Figure 13. To confirm that it is possible to insert into the adenovims genome lox sites that flank a gene such as that encoding fibre, the plasmid shown in Figure 14, called pAB14flox, was constmcted. This plasmid contains a lox site inserted into the unique Blp I site in pAB14, which is located between the 3' terminus of the fibre gene and the coding regions of E4 genes. A second lox site was inserted into the Xbal site upstream of fibre. PAB14flox (fibre flanked by lox sites) was rescued into infectious vims by cotransfection with pFG173 (described in Hanke, T., Graham, F.L., V. Lulitanond and D.C. Johnson, "Heφes simplex vims IgG Fc receptors induced using recombinant adenovims vectors expressing glycoproteins E and I," Virology 177: 437-444, 1990. PFGl 73 is available from Microbix Biosystems) as illustrated in Figure 15, to produce Adfloxfibre. In two experiments, 293 cells were cotransfected with pAB14flox and pFG173, and two plaque isolates were obtained in each experiment (from 8 cotransfected dishes of 293 cells in experiment 1 , and from 4 dishes in experiment 2). Two plaques were expanded and analyzed and shown to have the expected DNA stmcture as illustrated in Figure 15. Upon transfection of 293 Cre cells with DNA-TP complex of an Ad vims, such as Adfloxfibre-TP depicted in Figure 15, said floxed fibre gene is excised by site-specific recombination between similarly oriented lox ? sites, resulting in noninfectious viral DNA (as fibre is an essential component of the virion) as illustrated in Figure 12. Cotransfection of said 293Cre cells with a plasmid containing a single lox ? site upstream of fibre, such as pFG23dXllox, optionally carrying a fibre or E4 gene mutation or insertion of foreign DNA, results in high-efficiency site-specific recombination between the plasmid and viral DNA and results in a vims whose fibre gene is derived from the plasmid as illustrated in Figure 12 or Figure 13. Therefore, it will be readily appreciated by one skilled in the art that mutations, deletions or other modifications engineered in and around the fibre gene of the plasmid, are rescued into the infectious vims genome. As an example, not meant to be limiting, the combination of plasmid, vims DNA and recombinase as illustrated in Figures 12 and 13 leads to high-efficiency substitution of wild-type fibre with modified fibre genes for production of mutant vimses whose virion capsids contain altered fibre. As a further example of the utility of this approach, a foreign DNA segment is introduced into a plasmid, such as pFG23dXllox, between the lox site and the coding sequences of fibre, such that said foreign DNA segment is rescued into vims by cotransfection of 293Cre cells with DNA prepared from Adlox2fibre (Figure 13). As in the examples described previously for use of the two plasmid system, said foreign DNA segment could be a separate expression cassette or could be a fusion of peptide sequences such as a ligand to a cell surface receptor. Table 5 provides results documenting the efficiency with which Cre mediated recombination can be used to generate infectious vims by cotransfection of 293Cre cells as illustrated in figure 9. ft is apparent that the efficiency of rescue is comparable to that shown in Tables 1 and 2 and is several fold higher than the efficiency of homologous recombination (pFG173 + pFG23dXl). EXAMPLE 10 USE OF ALTERNATE ADENOVIRAL VECTOR SYSTEMS ACCORDING TO THIS INVENTION Those skilled in the art will recognize, based on the instant disclosure, that in the system described herein according to figure 8b, the left most lox site is not essential when the viral DNA is digested with enzymes such as those depicted, namely AsuII and or Swal. It will also be recognized that enhanced rescue of mutations or inserts into the viral genome by cotransfection of cells with a plasmid plus a viral DNA fragment with TP does not require a TP at both ends so the large viral DNA fragment generated by AsuII and/or Swal digestion and having a TP at the right end only is sufficient for this system to operate efficiently. Similarly in the systems disclosed according to figures 12 and 13, only the lox site 5' of fibre is necessary if the viral DNA-TP is cleaved with one or more enzymes that cut to the right, e.g. in fibre or in E4. If there are not naturally occurring restriction sites suitable for this puφose, such sites may easily be engineered by those of ordinary skill in the art, based on the present disclosure. For example we have identified a Blp I site between the 3' end of fibre and the coding sequences for E4 that can be used to insert a synthetic DNA. As illustrated in figure 14 we inserted a lox DNA sequence into this site but we could easily have introduced DNA containing a restriction endonuclease site that is not present elsewhere in the viral genome, and said restriction site could be rescued into an infectious vims as illustrated in figure 15. It will further be recognized, based on the present disclosure, that the combination of Cre- lox with the two plasmid system will have widest application because of its simplicity: only readily prepared plasmid DNA is required, no restriction enzyme digestions are required, no possible background of parental vimses has to be contended with, and the system is more than adequately efficient for most pi-uposes. Nonetheless, when enhanced levels of infectivity are required, utilization of the methods disclosed herein for use of viral DNA incoφorating bound terminal protein may also benefit through combination with the site-specific recombination techniques taught herein. EXAMPLE 11 USE OF HEAD-TO-HEAD ITR JUNCTIONS TO ENHANCE THE EFFICIENCY OF RECOMBINATION BY HOMOLOGOUS RECOMBINATION OR SITE-SPECIFIC RECOMBINATION As noted above in this invention disclosure, the efficiency of rescue of genes into Ad recombinant vectors can be markedly enhanced by engineering the plasmids so that recombination is mediated by Cre recombinase rather than by homologous recombination. Constmction of Ad vectors by Cre-mediated recombination between two plasmids cotransfected into 293Cre4 cells is highly efficient. Unfortunately, this system requires El -complementing cell lines expressing Cre, which are currently not as widely available as are the parental 293 cells. This method would be more widely applicable if high efficiency Cre-mediated vector rescue could be achieved using the ubiquitous 293 cells or other El -complementing cell lines. Therefore, we modified the system by introducing a Cre expression cassette into the shuttle plasmid pCA361oxΔ to generate pCA361oxΔCreR and pCA361oxΔCreT (Fig. 8c). The plasmids were designed so that the Cre cassette is not incoφorated into the recombinant vector after recombination owing to its location within the plasmids, but should permit transient Cre expression following cotransfection thus abrogating the need for a Cre-expressing cell line. The validity of this approach was tested by comparing the vector rescue efficiencies following cotransfection of 293 cells with pBHGloxΔE 1,3 and shuttle plasmids with or without the Cre expression cassette. The results of typical experiments are presented in Table 3 and Table 6. In Table 6 the numbers of plaques generated following cotransfection of 293 cells with pBHGloxΔEl ,3 and pCA36 or pCA361ox were similar and are typical of the efficiency of vector rescue by homologous recombination. No plaques were generated with pC A361oxΔ since all Ad sequences downstream of the loxP site have been deleted to virtually preclude vector rescue by homologous recombination. In contrast to pCA361oxΔ, pCA351oxΔCreT was able to mediate vector rescue indicating that Cre-mediated vector rescue could be achieved using 293 cells by including a Cre-expression cassette in the shuttle plasmids. Similar numbers of plaques were generated with pCA361oxΔCreR (Table 3). However, the efficiencies of vector rescue using pCA361oxΔCreT or pCA361oxΔCreR were considerably lower than that obtained using 293Cre4 cells (Table 3) suggesting that constitutive Cre expression from 293Cre4 cells resulted in more efficient Cre-mediated vector rescue than was obtained via transient Cre-expression from the transfected shuttle plasmid. Although we had no direct measure of Cre levels in 293Cre4 cells compared to 293 cells transiently transfected with pC A361oxΔCreR, it was possible that Cre recombinase levels in 293Cre4 cells were higher than levels in transfected 293 cells. Therefore we asked whether it might be possible to increase the efficiency of Cre-mediated vector rescue by increasing the copy number of the shuttle plasmid. To do this we replaced the single ITR junction in pCA361oxΔCreT with a head-to-head ITR junction to generate pCA351oxΔCreITR (Fig. 8c). The rationale for this modification was based on the observation that an ITR junction can serve as an efficient origin of Ad DNA replication resulting in linearization and, presumably, amplification of plasmid DNA (Graham, 1984 EMBO J. 3:2971). Therefore, following cotransfection of 293 cells with pBHGloxΔE 1,3, which also bears an ITR junction as well as encoding all traw-s-acting factors required for Ad DNA replication, pCA351oxCreITR was expected to linearize and replicate, which we anticipated should also result in increased Cre expression (more copies of the expression cassette). The validity of this approach was tested by comparing the vector rescue efficiencies following cotransfection of 293 cells with pBHGloxΔE 1 ,3 plus various shuttle plasmids. The results of a typical experiment are presented in Table 4. The numbers of plaques generated using pCA361ox and pCA361oxΔCreT were consistent with the results presented in Table 6. In contrast to the results with pCA361oxΔCreT, the number of plaques generated with pC A35 loxΔCrelTR was ~ 10-fold higher (Table 4) indicating that replacement of the single ITR with an ITR junction resulted in a significant increase in the efficiency of Cre-mediated vector rescue using 293 cells. The results presented in Table 4 indicated that a shuttle plasmid containing a Cre expression cassette and an ITR junction resulted in very significantly improved rescue efficiency compared to a similar plasmid that had only a single ITR. However, in the absence of direct measurements of Cre protein in transfected cells we could not conclude that this was due to enhanced Cre expression or due to some other mechanism. As a control experiment we constmcted a shuttle plasmid without the Cre cassette and containing an ITR junction for comparison with a similar plasmid having a single ITR (Figure 2a). Vims rescue in this experiment (Table 7) was dependent on homologous rather than Cre-mediated recombination since none of the plasmids used contained lox sites and no Cre was expressed in the cotransfected cells. Suφrisingly, the shuttle plasmid with an ITR junction gave about 10 fold higher efficiency of recombinant vims rescue compared to an otherwise identical plasmid with a single ITR. Thus the improved rescue using pCA351oxCreITR compared to pCA361oxΔCreT may be entirely due to the use of an ITR junction rather than to increased levels of Cre. Consequently, recombinant vims rescue using the two plasmid approach is markedly improved (approximately 10 fold greater efficiency) by incoφoration of ITR junctions into the shuttle plasmids. This allows either for greater numbers of plaques to be produced or allows for vector rescue to be achieved using lower concentrations of plasmid DNA in the cotransfections or fewer numbers of cells or both. Expression of Cre can be from the cotransfected cells, eg through use of 293Cre4 cells or the like, or by expression from Cre cassettes cloned in one or the other of the cotransfected plasmids. For example, not meant to be limiting, a Cre cassette can be inserted into the genomic plasmid such as one based on pBHGloxΔEl,3. An example of one method of constmction of such plasmids is illustrated in Figure 4c wherein a Cre expression cassette was inserted in either of two orientations into a pBHGlO derivative, pBHGloxΔEl,3, to generate pBHGloxΔEl ,3 Cre and pBHGloxΔEl ,3 CreR. Combining use of ITR junctions with the Cre-mediated recombination system of AdVeclO improves the efficiency of recombinant vims production by at least 100 fold over the original two plasmid method (Tables 8-13). For example, the data presented in Table 8 show that cotransfection of 293 cells with the genomic plasmid, pBHGloxΔE 1,3, plus the shuttle plasmid pCA361ox did not produce any plaques, in this experiment, because of the low efficiency of rescue by homologous recombination when the shuttle plasmid contains only a single ITR. In contrast addition of an ITR junction resulted in relatively high efficiency of rescue (15 plaques/dish), and cotransfection of 293Cre cells with pCA361ox + pBHGloxΔEl, 3 (which allows for rescue by Cre mediated recombination) resulted in 11 plaques/dish. The best efficiencies were obtained by cotransfecting 293Cre cells with the shuttle plasmid containing an ITR junction: 113 plaques/dish. That recombination was due to action of Cre can be seen from the results obtained when the shuttles are deleted of Ad sequences rightward of the expression cassette thus preventing homologous recombination. In this case, no plaques were obtained following cotransfection of 293 cells but over a hundred plaques/dish were obtained in 293Cre cells when the plasmid contained an ITR junction. Thus a preferred embodiment of the invention is the combination of site specific recombination with use of shuttle plasmids that contain ITR junctions in addition to lox sites. Nonetheless, those skilled in the art will appreciate that even the simple addition of an ITR junction to the shuttle plasmids used in homolgous recombinations results in a marked improvement in rescue efficiency over use of plasmids containing a single ITR. Thus when use of a site specific recombinase might be impractical or undesirable, use of shuttle plasmids with ITR junctions provides a simple and relatively efficient means to constmct Ad vectors containing foreign DNA insertions. The results presented in Tables 9 and 10 demonstrate that Cre recombinase can be provided by the cotransfecting shuttle plasmid, for example pCA351oxΔCreITR, or by the cotransfected host cells, eg. 293 Cre cells. In these and several additional experiments, plaques were isolated, expanded on 293 cells and analyzed for viral DNA stmcture and for expression of β-galactosidase from the expression cassette cloned in pCA35 and pCA36 and derivative plasmids. Over 60 independent plaque isolates were analyzed in this way and in every case the viral DNA stmcture was that predicted for recombinant viral vectors generated by recombination between the genomic plasmid DNA and the shuttle plasmid DNA and all recombinant vimses expressed β-galactosidase. Thus the methods described herein provide for accurate and reliable constmction of Ad vectors containing and expressing a foreign DNA. The results presented in Tables 11 and 12 indicate that for high efficiency rescue the Cre recombinase can be expressed by the recipient host cells, such as 293Cre4 cells, or by the shuttle plasmid, such as pCA351oxΔCreITR, or by the genomic plasmid such as pBHGloxΔEl ,3 Cre or by a combination of these. It will be seen by those skilled in the art that the result of recombination between said plasmids is a vims that does not contain the Cre expression cassette. The results also provide further evidence for the importance of providing an ITR junction on the shuttle plasmid whether rescue is via homologous recombination or via site specific recombination. Thus, as is amply illustrated by the results of numerous cotransfection experiments presented in Tables 8-13, the Cre recombinase may be expressed from either of the cotransfecting plasmids or from the host cells, such as 293Cre4 cells. The efficiency of rescue of recombinant vimses can be remarkably high, in some experiments, such as those illustrated by Tables 9 and 10, resulting in too many plaques to be countable. Although a Cre cassette may be in either the genomic plasmid or in the shuttle plasmid, if Cre expression is provided by the cotransfected plasmid DNA rather than from the transfected host cells, it is a preferred emodiment of the invention that the Cre cassette be present in the genomic plasmid for two reasons: firstly, the shuttle plasmids may then be as small as possible with as many cloning sites as possible for ease of insertion of foreign DNAs, and secondly, the results illustrated in Tables 11-13 suggest that the efficiency of rescue is greater for those transfections of 293 cells in which Cre recombinase is provided by the genomic plasmid, specifically the plasmid pBHGloxΔE 1,3 Cre. Those skilled in the art will appreciate that many different shuttle plasmids can be constmcted and used to generate recombinant viral vectors by the methods outlined above. As an example, not meant to be limiting, the series of shuttle plasmids illustrated in Figures 5a, 5b and 5d were constmcted from readily available plasmids using methods commonly employed by those skilled in the art. Among the shuttle plasmids constmcted areplasmids suchaspΔElSplAloxITR(MCS) andpΔElSplBloxITR(MCS) (Figure. 5a) that have ITR junctions, polycloning sites (MCS), lox sites and Ad sequences for homologous recombination. Said plasmids can be used for vims rescue by either homologous or site specific recombination by the methods outlined above. Other useful plasmids are pDCl 11-114 (Figure 5b) also having ITR junctions, polycloning sites, lox sites, or having ITR junctions and Ad sequences for homologous recombination (pDC 113 and pDCl 14) if Cre mediated recombination is not required or not desirable. Figure 5d and Figure 6 illustrate the constmction of several shuttle plasmids that have a promoter (the murine cytomegaloviras immediate early gene promoter, MCMV), a polycloning region, and polyadenylation signal (SV40) for insertion and expression of cDNAs in Ad vectors. These plasmids provide for considerable versatility in vector isolation. For example, pDC115 and pDCl lό (Figure 5d), having lox sites but no Ad sequences rightward of the lox sites, can be used for vims rescue by cotransfection of cells in the presence of Cre recombinase. Plasmids pDC 117 and 118 on the other hand, having no lox sites but having Ad sequences for homologous recombination, can be used for recombinant vims isolation in the absence of Cre recombinase, and the efficiency of rescue is high due to the presence of ITR junctions. The use of shuttle plasmids with ITR junctions and the use of Cre mediated site specific recombination and the combination of these two improvements significantly increase the efficiency and reliability of the two plasmid system for Ad vector constmction. These new methods will permit the production of expression vector libraries comprising large numbers of recombinant adenovims vectors capable of expressing different cDNAs for analysis of protein function in mammalian cells and in animals. Vectors produced by the methods disclosed herein can also find use as recombinant viral vaccines and in gene therapy. The enhanced efficiency of recombinant vims isolation using shuttle plasmids containing ITR junctions is not limited to production of vectors with insertions or substitutions or mutations in the El region. One skilled in the art will recognize that the ease and efficiency of engineering alterations into other regions of the vims genome would be similarly enhanced by introduction of ITR junctions in place of single ITRs in plasmids containing Ad sequences from the right end of the genome. For example, not meant to be limiting, in such plasmids as pFG23dXllox or pFG23dXlLacZlox illustrated in Figures 10 and 11 the single ITR could be readily substituted with ITR junctions to improve the efficiency of rescue of, for example, fibre mutations or foreign DNA inserts in cotransfected cells by methods such as that diagrammed in Figure 11a. For example, an ITR junction from pBHGlO was introduced into pFG23dXllox to produce pFG23dXlloxITR. Cotransfection of 293Cre4 cells with pFG1731ox and pFG23dXlloxITR resulted in approximately 50-60 plaques per dish, about 5-10 fold higher than is routinely obtained with pFG23dXllox. Furthermore, the vims rescues illustrated in Figures 9, 12 and 13 would be similarly increased in efficiency if the second plasmid were engineered to contain ITR junctions such as those illustrated for pFG23dXllox ITR in Figure 11a. EXAMPLE 12 PRODUCTION OF RECOMBINANT VECTOR USING FLP-FRT RECOMBPNATION 293 cells and 293 Cre cells transformed with an expression plasmid actively encoding FLP recombinase and demonstrated to express functional FLP have been disclosed in copending Patent application (docket no. AdVec3IB, USSN 09/351,819). As FLP is capable of catalyzing site specific recombination between fit sites (Figure 16 A) in a manner analogous to Cre mediated site specific recombination between loxP sites it will be appreciated by those skilled in the art that genomic plasmids and shuttle plasmids carrying frt sites at loci previously occupied by loxP sites can be used in an analogous system as that disclosed in copending application (AdVeclO, USSN 09/263,650 and AdVeclOIA, USSN 09/415,899) to rescue foreign DNA into Ad vectors by site specific recombination. Accordingly, this invention enables the production of vimses and vectors generated by FLP-mediated site specific recombination between two cotransfected plasmid DNAs in 293FLP cells. An example of a genomic plasmid derived from pBHGlO (Bett, A. J., Haddara, W., Prevec, L. and Graham, F.L "An efficient and flexible system for constmction of adenovims vectors with insertions or deletions in early regions 1 and 3." Proc. Natl. Acad. Sci. US 91: 8802-8806, 1994., available from Microbix Biosystems) that contains an appropriately designed fit site is illustrated in Figure 16B. The minimal fit DNA sequence is a 34bp DNA segment that is readily produced as a synthetic deoxyoligonucleotide that can be inserted into plasmid or viral DNA (see Figure 16A). The genomic plasmid pBHGfitΔEl,3 was constmcted in two stages. First a synthetic oligonucleotide carrying an fit site comprising annealed oligonucleotides ABI 0352 and AB10353 was inserted into the Bglfl site of pΔElSPl A (Bett, A. J., Haddara, W., Prevec, L. and Graham, F.L "An efficient and flexible system for construction of adenovims vectors with insertions or deletions in early regions 1 and 3." Proc. Natl. Acad. Sci. US 91: 8802-8806, 1994., available from Microbix Biosystems) to introduce a fit site immediately 5' of the Ad5 pIX gene. Second, an EcoRV-Bstl 1071 fragment spanning said fit site and pIX gene was substituted for a Bstl 1071 fragment in pBHGlO. The resulting genomic plasmid, pBHGfrtΔEl, 3, like pBHGlO, encodes all viral functions necessary for replication but lacks the packaging signal required for viral DNA encapsidation into virions. Generation of infectious vims from pBHGfrtΔE 1,3 and like plasmids requires that the packaging signal be reconstituted, by for example, recombination between pBHG plasmids and a second plasmid selected from a series of plasmids that contain the left end of the Ad genome including an ITR and a packaging signal. This constitutes the basis for the "two plasmid" rescue method for constmction of Ad vectors (Bett, A. J., Haddara, W., Prevec, L. and Graham, F.L "An efficient and flexible system for constmction of adenovims vectors with insertions or deletions in early regions 1 and 3." Proc. Natl. Acad. Sci. US 91 : 8802-8806, 1994). We have shown previously (Ng et al, 1999, 2000) that site specific recombination catalyzed, for example, by Cre recombinase, is more efficient than homologous recombination. We now demonstrate in the following that FLP mediated recombination is likewise highly efficient in generating recombinant vimses from cells cotransfected with a genomic plasmid and a shuttle plasmid both of which contain frt sites. Figure 17A illustrates the construction of a shuttle plasmid, pCA35ΔfrtITR, made by insertion of a synthetic fit site comprised of oligonucleotides ABI 9509 and ABI 9510 between a Bglll site and an Ehel site in pCA35ITR, a shuttle plasmid containing an expression cassette for the E. coli β-galactosidase gene. The resulting plasmid, pCA35ΔfrtITR, comprises an ITR junction, the viral packaging signal, the β-galactosidase expression cassette and an fit site in the same orientation as the fit site in pBHGfrtΔE 1,3. FLP mediated recombination between said fit sites, as illustrated in Figure 17B, results in generation of a recombinant adenovims expression vector containing the transgene, the E. coli β-galactosidase gene in this example, inserted in the El region of said vims. The results of a typical experiment demonstrating that FLP functions with high efficiency to produce recombinant viral vectors in cotransfected 293FLP cells are illustrated in Table 14. We have previously demonstrated (Ng et al, 2000; AdVec 10IA, USSN 09/415,899) that in Cre mediated rescue of Ad vectors the recombinase need not be expressed constitutively by the host cells that are cotransfected but can be transiently expressed from an expression cassette cloned either into the genomic plasmid or into the shuttle plasmid. This system has certain advantages over a system utilizing recombinase expressing host cells since rescue can be performed in any suitable host cell including a host cell that has not previously been engineered to express a recombinase. Thus for certain puφoses, insertion of a recombinase gene into one of the cotransfected plasmids is a preferred embodiment of the invention. Therefore we constmcted a FLP expressing genomic plasmid, pBHGfitΔE 1 ,3FLP, derived from pBHGfrtΔEl, 3 as illustrated in Figure 18. First, pBHGfrtΔEl,3 was modified by insertion of a synthetic oligonucleotide containing a Swal restriction site to generate pBHGfrtΔEl, 3poly2. Next, a FLP recombinase expression cassette contained within a Hpal-Sall fragment of pdelElCMVFLP (obtained from Volker Sandig, Merck Research Laboratories, Inc) was inserted into the Swal site of pBHGfrtΔEl ,3poly2 as illustrated. The final plasmid, pBHGfrtΔE 1,3FLP, thus contains a FLP expression cassette that results in transient expression of FLP in cotransfected cells that catalyzes site specific recombination between the frt site in pBHGfrtΔE 1,3FLP and a frt site in a cotransfected shuttle plasmid such as pCA35ΔfrtITR to generate an infectious Ad vector identical to that illustrated in Figure 17B. The genomic plasmid, pBHGfrtΔEl,3FLP, is designed such that the FLP expression cassette is not introduced into the final recombinant vims. Those skilled in the art will appreciate that the foregoing examples are not meant to be limiting. Thus one skilled in the art could readily constmct genomic plasmids analogous to any or all of the plasmids previously described as containing loxP sites or Cre expression cassettes wherein said loxP sites and said Cre expression cassettes are substituted with fit sites and FLP expression cassettes, respectively. Thus one skilled in the art could readily constmct genomic plasmids pBHGl lfrt, pBHGdXlPfrt and pBHGE3frt by the methods taught in the constmction of pBHGl llox, pBHGdXIPlox, pBHGE31ox. Results of typical experiments in which the efficiencies of Cre-lox mediated or FLP-frt mediated recombination were compared are provided in Table 15. It is apparent from the numbers of plaques obtained that the two systems function with similar high efficiencies. Thus the two site specific recombinase systems can be used interchangeably for rescue of viral vectors. It will therefore be appreciated by those skilled in the art that where, in copending applications 09/251,955; 08/719,217; 08/486,459; 09/286,874; 09/263,650; 09/415,899; 09/250,929; the use of Cre-lox recombination is disclosed for manipulation of the Ad genome, for constmction of vectors, for the rescue of fibre mutations or other viral gene modifications, that one could readily substitute FLP for Cre and frt for loxP. Therefore the present invention is not limited to use of FLP and fit for isolation of first generation Ad vectors with insertions of foreign DNA in the El region but can be used for modifications of other regions of the viral genome by methods disclosed in copending applications as noted above. It will be further appreciated that the present invention is not limited to use of the site specific recombinases Cre and FLP but is more generally suitable to employment of any site specific recombinases. To maximize the utility and versatility of the FLP-frt system for isolation of first generation Ad vectors several additional shuttle plasmids were constmcted that permit those skilled in the art to employ the invention in efficient and simple production of Ad expression vectors containing foreign DNA. Said shuttle plasmids are small, easily grown and can be readily used for insertion of expression cassettes or cDNAs for rescue into vectors by the methods disclosed herein. Figure 19 illustrates the constmction of plasmids pDC511, 512, 515 and 516 that can be readily utilized by those skilled in the art for insertion of expression cassettes with genes under the control of a variety of promoters (pDC511 and 512) or for insertion of cDNAs to be expressed under control of the MCMV promoter (pDC515 and 516). Figure 19A. The plasmid pDC511 was constmcted by replacing the 2347 bp Sall/Psh-AI fragment from pDC411 with an oligo bearing an frt site AB19818: 5' TCGACGGATCCGAAGTTCCTATTCTTACTAGAGTATAGGAACTTCGACTA 3' and AB19819: 5' TAGTCGAAGTTCCTATACTCTAGTAAGAATAGGAACTTCGGATCCG 3'). The plasmid pDC512 was constmcted by replacing the 2352 bp BamHI/Ps-liAI fragment from pDC412 with an oligo bearing an fit site (ABI 9816: 5' GATCCGAAGTTCCTATTCTTACTAGAGTATAGGAACTTCGACTA 3' and ABI 9817: 5' TAGTCGAAGTTCCTATACTCTAGTAAGAATAGGAACTTCG 3'). Figure 19B. The plasmid pDC515 was constmcted by inserting the 728 bp Xbal/Bglll fragment from pMH4 containing the MCMV promoter, polycloning region and S V40 poly A into the XbaLBamHI sites of pDC512. Figure 19C. The plasmid pDC516 was constmcted by ligating together the 3229 bp Xbal BamHI fragment from pDC512 with the 568 bp Xbal/Sall fragment from ρDC316 and the 160 bp Sall/Bgl-II fragment from pMH4. The examples described in the present invention are not meant to be limiting as it will be appreciated that those skilled in the art can readily constmct a variety of similar shuttle plasmids with other promoters and alternate polycloning sites by the methods taught herein. Accordingly, as can be understood from the foregoing disclosure, when implementing hthlTR junctions according to this invention, recombination may be via site specific mechanisms, for example Cre-mediated recombination as indicated in Figure 1 la, or by FLP-mediated recombination as indicated in Figure 17B, or by homologous recombination if the cotransfecting plasmids are constmcted so as to have overlapping Ad sequences that can allow for recombination to produce an infectious viral genome. The above examples are not limiting. Thus one skilled in the art could readily constmct shuttle plasmids analogous to any or all of the plasmids previously described as containing loxP sites or Cre expression cassettes wherein said loxP sites and said Cre expression cassettes are substituted with fit sites and FLP expression cassettes, respectively. Similarly, one skilled in the art could readily constmct shuttle plasmids pΔE 1 sp 1 Afit, pΔE 1 sp 1 AfrtΔ, pΔEl splBfrt, pΔEl splBfrtΔ, pMH4frt, pMH4frtΔ, pMH4frtΔlink, pCAl 3fit, pCAl 3frtΔ, pCA14frt, pCA14frtΔ, pCA36fit, pCA36frtΔ, pCA36frtΔFLPR, pCA36frtΔFLPT, pCA35frt and pCA35fitΔFLPITR by the methods taught in the constmction of pΔElsplAlox, pΔElsplAloxΔ, pΔElsplBlox, pΔElsplBloxΔ, pMH41ox, pMH41oxΔ, pMH41oxΔlink, pCA131ox,pCA131oxΔ,pCA141ox,pCA141oxΔ,pCA361ox,pCA361oxΔ, pCA361oxΔCreR, pCA361oxΔCreT, pCA351ox and pCA351oxΔCreITR respectively. Those skilled in the art will also appreciate that similar manipulations to those described above may be carried out on any particular portion of the adenoviral genome that does not result in dismption of key functions. Thus, for example, on the right end of the genome, rescue of fibre mutations, E3 insertions and the like come within the scope of this invention when implemented according to the methodology disclosed and claimed herein. Table 1. Cotransfections on 293 and 293Cre4 cells for rescue of LacZ vectors (± loxP) Plasmid combo μg Plaques/dish Plaques/dish DNA (293 cells) (Totals) (293Cre4 cells) (Totals) pCA36:pBHG10 5:5 0, 0, 0, 0 0,1,2,0 5:10 0, 0, 0, 1 1,0,0,0 10:10 2,0,1,1 1,2,0,0 (5) (7) pCA36:pBHGloxΔEl,3 5:5 0, 0, 0, 1 0, 0, 0, 0 5:10 0, 0, 0, 1 0, 0, 0, 0 10:10 0, 0, 2, 1 0, 0, 0, 0 (5) (0) pCA361ox:pBHG10 5:5 1,3,1,0 0, 1, 0, 1 5:10 0, 1,0,0 0,0,1,2 10:10 0, 0, 0, 0 0, 1, 1,0 (6) (7) pCA361ox:pBHGloxΔEl,3 5:5 1,0,0,1 15,14,20,20 5:10 0,0,0,0 11,15,12,16 10:10 0,0, 1, 1 18, 9, 10, 8 (4) (168) Table 2. Cotransfections on 293 and 293Cre4 cells for rescue of LacZ Vectors (± loxP) Plasmid combo μg Plaques/dish Plaques/dish DNA (293 cells) (Totals) (293Cre4 cells) (Totals) pCA36:pBHGloxΔEl,3 5:5 1 , 1 , 2, 6, 2, 3 1,1,2,1,2,3 (15) (10) pCA361ox:pBHGloxΔEl,3 5:5 1,2,2,2,2,1 41,44,41,41,44,31 (10) (242) pCA361oxΔ:pBHGloxΔEl,3 5:5 0,0,0,0,0,0 41,36,55,34,24,40 (0) (230) FG140 1 72,72 150,115 Table 3 Efficiency of Ad vector rescue by cotransfection with pBHGloxΔEl,3 and various shuttle plasmids' 1 Cell line Shuttle plasmid Plaques/dish Average/dish 293 pCA361ox 6, 2, 3, 3, 5 3.8 pCA361oxΔ 1,4,0,0,0 1.0 pCA361oxΔCreR 2,2,4,3,2 2.6 pCA361oxΔCreT 9, 4, 4, 7, 3 5.4 293Cre4 pCA361oxΔ 23, 28, 22, 28 25.3 a 5μg of all plasmids were used in cotransfections. Table 4. Efficiency of Ad vector rescue by cotransfection of 293 cells with pBHGloxΔEl,3 and shuttle plasmids encoding Cre\ Cell line Shuttle plasmid Plaques/dish Average/dish 293 pCA361ox 2,3,1,0,1 1.4 pCA361oxΔ 1,0,0,0,0 0.2 pCA361oxΔCreT b 3,1,5,2,4 3.0 pCA351oxΔCreITR b 21,20,42,34,40 31.4 "All cotransfections performed with 5 μg of the indicated shuttle plasmid and 5μg of pBHGloxΔEl ,3 b Plasmids illustrated in figure 8c. Table 5. Efficiency of rescue of fibre and E4 genes into Ad by cotransfection with pFG1731ox and pFG231ox a Number of plaques (average/dish) Plasmids μgDNA 293 cells 293Cre4 cells pFG1731ox b : pFG23dXlloxc c 5:5 0,0,0,0 (0) 33,27,39,26 (31) 2:2 0,0,0,0 (0) 9,15,10,9 (11) pFG173 : pFG23dXl 5:5 0,0,0,0 (0) 0,0,1 (0.3) pFG140 95 93 "Cotransfections as diagrammed in figure 9 ^Diagrammed in figure 9b "Diagrammed in figure 10 Table 6. Recombinant virus rescue following cotransfection of 293 cells with shuttle plasmids with or without a Cre expression cassette 293 cells were cotransfected with 5 μg of pBHGloxΔE 1,3 and 5 μg of the indicated shuttle plasmid or 1 μg of pFG140 Table 7. Efficiency of Ad vector rescue by cotransfection of 293 cells with pBHGlO and shuttle plasmids with a single ITR or an ITR junction Table 8. Cotransfections on 293 and 293Cre4 cells for rescue of LacZ vectors (± lox, ± ITR junction, ± CRE) Shuttle Plasmid Plaques/dish Plaques/dish (293 cells) (Average) (293Cre4 cells) (Average) pCA361ox 0,0,0,0 (0) 13,15,3,13 (11) pCA351oxITR 8,13,21,19 (15) 111,131,100,130 (113) pCA361oxΔ 0,0,0,0 (0) 10,8,9,12 (10) pCA351oxΔ-rTR 0,0,0,0 (0) 91,127,141,118 (119) pFG140 b 75 83 All cotransfections 5μg shuttle plasmid + 5μg pBHGloxΔEl,3 lμg/dish Table 9. Cotransfections on 293 and 293Cre4 cells for rescue of LacZ vectors (± lox, + ITR junction, ± CRE) Number of plaques /dish (average/dish) Plasmids μg DNA /dish 293 cells 293Cre4 cells pCA36:pBHGloxΔEl,3 5:5 ND 0,3 (1.5) pCA361ox:pBHGloxΔEl,3 2:2 ND 9,3 (6) 5:5 2,0,0,0 (0.5) 30,31,30,30 (30.25) pCA351oxΔCrerTR:pBHGloxΔEl,3 2:2 -ND 71,60,56,79 (66.5)* 5:5 36 100,96 (98) pCA351oxΔITR:pBHGloxΔEl,3 2:2 ND 55,64,75,63 (64.25)* 5:5 0 120,113 (116.5) pCA351oxrTR:pBHGloxΔEl,3 2:2 ND 53,54,61,66 (58.5)* 5:5 ND 130,126 (128) pFG140 (DC) 1 92 178 pFG140 (CE) 1 94 118 * 5 plaques picked from each of these cotransfections and analyzed. All + for β-gal and all had predicted viral DNA structure Table 10. Cotransfections on 293 and 293Cre4 cells for rescue of LacZ vectors (± lox, ± ITR junction, ± CRE) Number of plaques/dish (average/dish) Plasmids* ugDNA/dish 293 cells (average) 293 Cre4 cells (average) pCA36 5 1,0,0,0 (0.3) 1,0,0,0 (0.3) pCA361ox 5 1,1,1,0 (0.8) 10,18,6,7 (10.3) 5 pCA361oxΔ 0,0,0,0 6,4,3,0 (3.25) pCA361oxΔCreT 5 2,1,2,2 (1.8) 4,4,2, (3.3) pCA351oxΔCreITR 5 14,23,25,23 (21.3) 116,79,83,100 (94.5) pCA351oxΔITR 5 0,0,0,0 (0) 65,62,64,51 (60.5) pCA351ox-TTR 5 4,3,4,0 (2.8) 114,101,75,79 (92.25) pFG140 (DC) 1 40,31 (35.5) 106,92 (99) pFG140 (CE) 1 21,19 (20) 44,42 (43) *cotransfections with 5μg pBHGloxΔEl,3 except for pFG140 Table 11. Cotransfections on 293 and 293Cre4 cells for rescue of LacZ vectors (± lox, ± ITR junction, ± CRE) Number of plaques/dish (average/dish) 'Cotransfections with 5 μg each plasmid dish except 1 μg/dish for pFG140 'Not done a Not done ''Too numerous to count Table 13. Cotransfections on 293 and 293Cre4 cells for rescue of LacZ vectors (CRE expressed from plasmids, by 293 cells, or both) 1 Too numerous to count TABLE 14 . Cotransfections on 293, 293Cre or 293FLP cells for rescue of LacZ Number of plaques /dish (average/dish) Plasmids μg DNA /dish 293 cells 293Cre cells 293FLP cells pCA35ITR:pBHG10 0.5:0.5 3, 2, 4, 2 (2.8) ND ND 2:2 20, 17, 13, 10 (15) ND ND pC A361ox:pBHGfrtΔE 1 ,3 0.5:0.5 4, 6, 4, 7 (5) ND ND 2:2 24, 18, 11, 22 (19) ND ND P CA351oxΔlTR:pBHGloxΔE 1 ,3 0.5:0.5 ND 60, 84, 83, 106 (83) ND 2:2 ND > 150 ND pCA35frtΔrTR:pBHGfrtΔE 1 ,3 0.5:0.5 ND ND 37, 41, 48, 52 (45)* 2:2 ND ND 60, 68, 52, 49 (57) pFG140 0.5 95 ND ND * 6 plaques picked from these cotransfections and analyzed. All + for β-gal and all had predicted viral DNA structure 12 . Vector rescue efficiency by Cre versus FLP-mediated recombination using genomic plasmids expressing recombinases 3 . a 60 mm dishes of 293 cells were cotransfected with 2 μg of each plasmid and plaques were counted 10 days post-cotransfection. 13

Claims: WHAT IS CLAIMED IS: 1. A method for making an infectious adenovirus which comprises contacting a cell with or introducing into a cell: (a) a first nucleic acid sequence encoding adenovirus sequences which, in the absence of intermolecular recombination, are insufficient to encode an infectious, replicable or packageable adenovirus; and (b) a second nucleic acid sequence encoding adenovirus sequences which, in the absence of adenoviral replication factors provided in trans or intermolecular recombination with said first nucleic acid sequence, are insufficient to encode an infectious, replicable or packageable adenovirus; provided that (i) said first and said second nucleic acid sequences each comprise a head-to-head ITR junction; (ii) said first nucleic acid and said second nucleic acid each comprise recombinase recognition sites and wherein said first and said second nucleic acids are contacted with a recombinase which recognizes said recombinase recognition sites; or (iii) said first and said second nucleic acid sequences each comprise a head-to-head ITR junction and said first nucleic acid and said second nucleic acid comprise recombinase recognition sites and wherein said first and said second nucleic acids are contacted with a recombinase which recognizes said recombinase recognition sites; whereby said first and said second nucleic acids recombine to form said infectious adenovirus. 2. The method according to claim 1 wherein said first nucleic acid sequence is a plasmid containing a circularized adenovirus DNA molecule encoding adenovirus sequences which, in the absence of intermolecular recombination, are insufficient to encode an infectious, replicable or packageable adenovirus. 3. The method according to claim 2 wherein said plasmid includes a bacterial origin of DNA replication, an antibiotic resistance gene for selection in bacteria, a deletion or modification in El that renders the adenoviral sequences insufficient to form infectious virus, an expression cassette encoding a site-specific recombinase, and combinations thereof. 4. The method according to claim 2 wherein said adenovirus DNA has a deletion of an adenoviral packaging signal, or wherein said packaging signal is flanked on either side by at least one site-specific recombinase recognition site. 5. The method according to claim 4 wherein said adenovirus DNA comprises (i) a deletion of, (ii) a modification in, or (iii) sequences flanked with a site-specific recombinase recognition site, of an adenoviral gene selected from the group consisting of adenoviral El sequences extending beyond said packaging signal, adenoviral fibre gene sequences, adenoviral E3 gene sequences, adenoviral E4 gene sequences, and combinations thereof. 6. The method according to claim 5 wherein said adenovirus DNA has a lox site, an frt site or both located 5' of a pIX gene. 7. The method according to claim 2 wherein said plasmid is selected from the group consisting of pBHGloxΔEl, 3, pBHGl llox, pBHGdXIPlox, pBHGE31ox, pFG1731ox, pBHGloxΔEl, 3Cre. 8. The method according to claim 1 wherein said first nucleic acid sequence is a segment of adenoviral DNA comprising covalently linked adenoviral terminal protein TP. 9. The method according to claim 8 wherein said first nucleic acid sequence is AdLC8cDNA-TP, or adfloxfibreDNA-TP. 10. The method according to claim 1 wherein said second nucleic acid sequence is a plasmid comprising: (i) said head-to-head ITRjunction, and apackaging signal contained within the leftmost approximately 350 nt of the adenovirus genome; (ii) a polycloning site or a foreign DNA or an expression cassette; and optionally (iii) a lox P or an frt site 3' of said polycloning site, foreign DNA, or expression cassette. 11. The method according to claim 10 wherein said plasmid is selected from the group consisting of pΔElsplAlox, pΔElsplAloxΔ, pΔElsplBlox, pΔElsplBloxΔ, pMH41ox, pMH41oxΔ, pMH41oxΔlink, pCA131ox, pCA131oxΔ, pCA141ox, pCA141oxΔ, pCA361ox, pCA361oxΔ, pCA361oxΔCreR, pCA361oxΔCreT, l ft2 pCA351ox, pCA351oxΔCreITR, pDCl l l, pDC112, pDC113, pDC114, pDC115, pDCl lό, pDC117, pDCl lδ, combinations thereof and modifications thereof wherein such modifications comprise inclusion of a head-to-head ITR junction. 12. A recombinant adenovirus vector system comprising: (a) either (i) a first nucleic acid sequence encoding adenovirus sequences which, in the absence of intermolecular recombination, are insufficient to encode an infectious, replicable or packageable adenovirus, said first nucleic acid sequence comprising at least one site-specific recombinase recognition target site which is recognized by a site-specific recombinase or (ii) a first nucleic acid sequence encoding adenovirus sequences which are sufficient to encode an infectious, replicable or packageable adenovirus covalently linked to adenoviral terminal protein, said first nucleic acid sequence comprising at least one site-specific recombinase recognition target site which is recognized by a site-specific recombinase, wherein contact of said first nucleic acid with said site-specific recombinase results in excision of sequences from said replicable adenovirus such that, in the absence of intermolecular recombination, said infectious, replicable or packageable adenovirus is rendered replication or packaging defective, or (iii) where a head-to-head ITR junction is present in a second nucleic acid sequence, sufficient overlapping adenoviral nucleic acid sequences such that homologous recombination with sequences in said second nucleic acid sequence may occur; (b) a second nucleic acid sequence encoding adenovirus sequences which, in the absence of adenoviral replication factors provided in trans or intermolecular recombination with said first nucleic acid sequence, are insufficient to encode an infectious, replicable or packageable adenovirus, said second nucleic acid sequence comprising (i) at least one head-to-head ITR junction and sufficient adenoviral sequences to permit homologous recombination with said first nucleic acid sequences, (ii) at least one recombinase recognition target site sufficiently identical with said recombinase recognition target site in said first nucleic acid as to be recognized by the same site-specific recombinase which recognizes said site-specific recombinase recognition target site in said first nucleic acid, or (iii) combinations thereof; wherein said first and said second nucleic acid sequences, in combination and following homologous or site-specific intermolecular recombination, result in production of an infectious adenovirus, and wherein a site-specific recombinase which recognizes said site-specific recombinase recognition target sites either (i) is expressed by a cell into which said first and said second nucleic acids are introduced, (ii) is operatively encoded by said first nucleic acid, said second nucleic acid or both, (iii) is provided in trans through expression from a third nucleic acid or is provided in trans as an active protein, or (iv) is absent and recombination occurs through homologous recombination, provided that in this case, the head-to- head ITR junction is present. JQ 4 13. The recombinant adenovirus vector system of claim 12 comprising: (a) a first plasmid selected from the group consisting of pBHGloxΔE 1,3, pBHGl llox, pBHGloxΔE 1,3 Cre, and pBHGloxΔE 1,3 CreR, containing a circularized adenovirus DNA molecule and optionally including a bacterial origin of DNA replication and an antibiotic resistance gene for selection in bacteria and having a deletion or modification of the packaging signal, of additional El sequences, of E3, E4 or fibre, and optionally having a lox ? site located adjacent the pIX gene, E3, E4 or fibre of the virus, said plasmid optionally encoding Cre, FLP or another site-specific recombinase; (b) a second plasmid selected from the group consisting of pΔElsplAlox, pΔElsplAloxΔ, pΔElsplBlox, pΔElsplBloxΔ, pMH41ox, pMH41oxΔ, pMH41oxΔlink, pCA131ox, pCA131oxΔ, pCA141ox, pCA141oxΔ, pCA361ox, pCA361oxΔ, pCA361oxΔCreR, pCA361oxΔCreT, pCA351ox, pCA351oxΔCreITR, pDCl l l, pDC112, pDC113, pDC114, pDC115, pDCl lό, pDC117, pDC118, combinations thereof and modifications thereof wherein such modifications comprise inclusion of a head-to-head ITR junction: (i) all or most of the left ITR and the packaging signal contained within the leftmost approximately 350 nt of the Ad genome or a head-to- head ITR junction; (ii) a polycloning site or a foreign DNA or an expression cassette; and optionally, (iii) a lox ? or fit site 3' of said polycloning site or foreign DNA or expression cassette; and (c) a cell line that is normally able to support replication of adenovirus and which optionally expresses the recombinase Cre, FLP or another site specific recombainse that is able to catalyse site-specific recombination between said lox P sites, said frt sites, or both. 14. The recombinant adenovirus vector system of claim 12 wherein said cell further expresses adenoviral El. 15. The recombinant adenovirus vector system of claim 12 wherein said first plasmid and said second plasmid are cotransfected into said cell to produce an infectious virus vector comprising a left end, a polycloning site, foreign DNA, or an expression cassette derived from said second plasmid, joined to the remaining portion of the viral DNA derived from said first plasmid. 16. The recombinant adenovirus vector system of claim 12 wherein said cell is co- transfected with a first DNA, extracted in such a manner as to retain covalently bound adenoviral terminal protein TP, from a virus selected from the group consisting of AdLC8, AdLC8cluc, AdLC8cCE199, comprising a packaging signal flanked by loxP sites, and a second DNA comprising a packaging signal wherein said second DNA is selected from the group consisting of pΔElsplAlox, pΔElsplAloxΔ, pΔElsplBlox, pΔElsplBloxΔ, pMH41ox, pMH41oxΔ, pMH41oxΔlink, pCA131ox, pCA131oxΔ, pCA141ox, pCA141oxΔ, pCA361ox, pCA361oxΔ, pCA361oxΔCreR, pCA361oxΔCreT, pCA351ox, pCA351oxΔCreITR, pDCl l l, pDC112, pDC113, pDC114, pDC115, pDCl lό, pDC117, pDC118, combinations thereof and modifications thereof wherein such modifications comprise inclusion of a head-to-head ITR junction, whereby said lox P sites flanking said packaging signal of said first DNA are acted upon by Cre recombinase expressed in said cells to induce excision of said packaging signal, producing a noninfectious virus genome incapable of packaging its DNA into virions unless joined by Cre-mediated recombination to the lox P site of said second DNA to reconstitute a packaging signal therein. 17. The recombinant adenovirus vector system of claim 16 wherein, prior to said co- transfection, said first DNA is cleaved with a restriction enzyme that cuts between said loxP sites. 18. A kit for construction of recombinant adenovirus vectors comprising: (a) either (i) a first nucleic acid sequence encoding adenovirus sequences which, in the absence of intermolecular recombination, are insufficient to encode an infectious, replicable or packageable adenovirus, said first nucleic acid sequence comprising at least one site-specific recombinase recognition target site which is recognized by a site-specific recombinase, (ii) a first nucleic acid sequence encoding adenovirus sequences which are sufficient to encode an infectious, replicable or packageable adenovirus covalently linked to adenoviral terminal protein, said first nucleic acid sequence comprising at least one site-specific recombinase recognition target site which is recognized by a site-specific recombinase, wherein contact of said first nucleic acid with said site-specific recombinase results in excision of sequences from said replicable adenovirus such that, in the absence of intermolecular recombination, said infectious, replicable or packageable adenovirus is rendered replication or packaging defective, or (iii) sufficient adenoviral sequences to permit homologous recombination with similar sequences in a second nucleic acid sequence; (b) a second nucleic acid sequence encoding adenovirus sequences which, in the absence of adenoviral replication factors provided in trans or intermolecular recombination with said first nucleic acid sequence, are insufficient to encode an infectious, replicable or packageable adenovirus, said second nucleic acid sequence comprising (i) at least one recombinase recognition target site sufficiently identical with said recombinase recognition target site in said first nucleic acid as to be recognized by the same site-specific recombinase which recognizes said site-specific recombinase recognition target site in said first nucleic acid, (ii) a head-to- head ITR junction and sufficient adenoviral sequences to permit homologous recombination with similar sequences in said first nucleic acid, or (iii) combinations thereof; and (c) a cell wherein, when said component (a) and said component (b) are cotransfected and recombined through homologous recombination or the action of a recombinase which recognizes said recombinase recognition sites, an infectious recombinant adenovirus vector is produced. 19. The kit according to claim 18 wherein component (a) is selected from the group consisting of pBHGloxΔE 1,3, pBHGl llox, pBHGdXIPlox, pBHGE31ox, and pBHGloxΔEl, 3Cre. 20. The kit according to claim 18 wherein said component (b) is selected from the group consisting of pΔE 1 sp 1 Alox, pΔEl sp 1 AloxΔ, pΔE 1 sp 1 Blox, pΔE 1 sp 1 BloxΔ, pMH41ox, pMH41oxΔ, pMH41oxΔlink, pCA131ox, pCA131oxΔ, pCA141ox, pCA141oxΔ, pCA361ox, pCA361oxΔ, pCA361oxΔCreR, pCA361oxΔCreT, pCA351ox, pCA351oxΔCreITR, pDCl l l, pDC112, pDC113, pDC114, pDC115, pDCl lό, pDC117, pDC118, combinations thereof and modifications thereof wherein such modifications comprise inclusion of a head-to-head ITR junction. 21. The kit according to claim 18 wherein said cell of (c) is selected from the group consisting of 293 cells, 293 cells expressing Cre, PER-C6 cells expressing Cre, 911 cells expressing Cre, and wherein said recombinase recognition sites are lox sites. 22. The recombinant adenovirus vector system according to claim 12 wherein an adenoviral gene mutation is rescued into said adenoviral vector recombinant. 23. The recombinant adenovirus vector system according to claim 22 wherein said adenoviral gene mutation rescued into said adenoviral vector recombinant is a mutation in the adenoviral fibre gene, the adenoviral E4 gene, the adenoviral E3 gene, or combinations thereof. JLUV 24. The recombinant adenovirus vector system according to claim 12 wherein said first nucleic acid sequence comprises a recombinase recognition site and a deletion in the adenoviral fibre gene. 25. The recombinant adenovirus vector system of claim 12 comprising: (a) a first adenovirus vector having a fibre gene flanked by loxP, frt sites, or both loxP and fit sites; (b) a plasmid comprising a bacterial origin of replication, a bacterial antibiotic resistance marker, a deletion in the adenoviral fibre gene, a foreign DNA insert, the right end of the Ad genome encompassing the fibre gene, including a single loxP or frt site located to the left of the fibre gene and a foreign DNA insert between the loxP or frt site and the fibre gene, and optionally also comprising a head-to-head ITR junction. 26. An adenoviral vector selected from the group consisting of pBHGloxΔE 1,3, pBHGl llox, pBHGdXIPlox, pBHGE31ox, pFG1731ox, and pBHGloxΔEl,3Cre. 27. An adenoviral vector selected from the group consisting of pΔElsplAlox, pΔElsplAloxΔ, pΔElsplBlox, pΔElsplBloxΔ, pMH41ox, pMH41oxΔ, pMH41oxΔlink, pCA131ox, pCA131oxΔ, pCA141ox, P CA141oxΔ, pCA361ox, pCA361oxΔ, pCA361oxΔCreR, pCA361oxΔCreT, pFG23dXllox, pAB141oxΔ, pAB14flox, pCA351oxΔCreITR, and derivatives thereof comprising at least one head-to-head ITR junction. 28. A cell comprising the adenoviral vector of claim 26. 29. A cell comprising the adenoviral vector of claim 27. 30. A cell into which has been introduced a first vector selected from the group consisting of pBHGloxΔE 1,3, pBHGl llox, pBHGdXIPlox, pBHGE31ox, pFGl 731ox, and pBHGloxΔEl ,3 Cre, and a second vector selected from the group consisting of pΔElsplAlox, pΔElsplAloxΔ, pΔElsplBlox, pΔElsplBloxΔ, pMH41ox, pMH41oxΔ, pMH41oxΔlink, pCA131ox, pCA131oxΔ, pCA141ox, pCA141oxΔ, pCA361ox, pCA361oxΔ, pCA361oxΔCreR, pCA361oxΔCreT, pCA351ox, pCA351oxΔCreITR, pDCl l l, pDC112, pDC113, pDC114, pDC115, pDCl lό, pDC117, pDC118, combinations thereof and modifications thereof wherein such modifications comprise inclusion of a head-to-head ITR junction. 31. A method of vaccinating or administering gene therapy to a recipient in need of such treatment which comprises administering to said recipient an effective amount of an adenovirus produced by site-specific recombination of: (a) either (i) a first nucleic acid sequence encoding adenovirus sequences which, in the absence of intermolecular recombination, are insufficient to encode an infectious, replicable or packageable adenovirus, said first nucleic acid sequence comprising at least one site-specific recombinase recognition target site which is recognized by a site-specific recombinase, (ii) a first nucleic acid sequence encoding adenovirus sequences which are sufficient to encode an infectious, replicable or packageable adenovirus covalently linked to adenoviral terminal protein, said first nucleic acid sequence comprising at least one site-specific recombinase recognition target site which is recognized by a site-specific recombinase, wherein contact of said first nucleic acid with said site-specific recombinase results in excision of sequences from said replicable adenovirus such that, in the absence of intermolecular recombination, said infectious, replicable or packageable adenovirus is rendered replication or packaging defective, or (iii) a first nucleic acid comprising sufficient adenoviral sequences to permit homologous recombination with similar sequences in a second nucleic acid sequence; (b) a second nucleic acid sequence encoding adenovirus sequences which, in the absence of adenoviral replication factors provided in trans or intermolecular recombination with said first nucleic acid sequence, are insufficient to encode an infectious, replicable or packageable adenovirus, said second nucleic acid sequence comprising (i) at least one recombinase recognition target site sufficiently identical with said recombinase recognition target site in said first nucleic acid as to be recognized by the same site-specific recombinase which recognizes said site-specific recombinase recognition target site in said first nucleic acid, (ii) sufficient adenoviral sequences to permit homologous recombination with similar sequences in said first nucleic acids and at least one head-to-head ITR junction, or (iii) combinations thereof; wherein said first and said second nucleic acid sequences, in combination and following site-specific intermolecular recombination, result in production of an infectious adenovirus, and wherein, optionally, where site-specific recombination is to be employed, a site-specific recombinase which recognizes said site-specific recombinase recognition target sites is either (i) expressed by a cell into which said first and said second nucleic acids are introduced, (ii) operatively encoded by said first nucleic acid, said second nucleic acid or both, or (iii) is provided in trans through expression from a third nucleic acid or is provided in trans as an active protein. 32. The method according to claim 31 wherein said first nucleic acid is selected from the group consisting of pBHGloxΔEl ,3, pBHGl 1 lox, pBHGdXIPlox, pBHGE31ox, pFG1731ox, and pBHGloxΔE 1,3 Cre, and wherein said second nucleic acid is selected from the group consisting of pΔElsplAlox, pΔElsplAloxΔ, pΔElsplBlox, pΔElsplBloxΔ, pMH41ox, pMH41oxΔ, pMH41oxΔlink, pCA131ox, pCA131oxΔ, pCA141ox, pCA141oxΔ, pCA361ox, pCA361oxΔ, pCA361oxΔCreR, pCA361oxΔCreT, pCA351ox, pCA351oxΔCreITR, pDCl l l, pDCl 12, pDCl 13, pDCl 14, pDCl 15, pDCl 16, pDCl 17, pDCl 18, combinations thereof and modifications thereof wherein such modifications comprise inclusion of a head-to-head ITR junction. 33. A composition comprising the recombination product of a first vector selected from the group consisting of pBHGloxΔE 1,3, pBHG 11 lox, pBHGdX 1 Plox, pBHGE3 lox, pFG1731ox, pBHGloxΔE 1,3 Cre, and a second vector selected from the group consisting of pΔElsplAlox, pΔElsplAloxΔ, pΔElsplBlox, pΔElsplBloxΔ, pMH41ox, pMH41oxΔ, pMH41oxΔlink, pCA131ox, pCA131oxΔ, pCA141ox, „ „ „ 113 pCA141oxΔ, pCA361ox, pCA361oxΔ, P CA361oxΔCreR, pCA361oxΔCreT, pCA351ox, pCA351oxΔCreITR, pDCl l l, pDC112, pDC113, pDC114, pDC115, pDCl lό, pDC117, pDC118, combinations thereof and modifications thereof wherein such modifications comprise inclusion of a head-to-head ITR junction, wherein said first vector and said second vector are contacted optionally in the presence of Cre recombinase. 34. The composition according to claim 33 wherein said first and said second vectors are contacted inside a cell and said recombination product is harvested from said cell. 35. The recombinant adenovirus vector system according to claim 12 comprising: (a) either (i) a first nucleic acid sequence encoding adenovirus sequences which, in the absence of intermolecular recombination, are insufficient to encode an infectious, replicable or packageable adenovirus, said first nucleic acid sequence comprising at least one site-specific recombinase recognition target site which is recognized by a site-specific recombinase or (ii) a first nucleic acid sequence encoding adenovirus sequences which are sufficient to encode an infectious, replicable or packageable adenovirus covalently linked to adenoviral terminal protein, said first nucleic acid sequence comprising (A) at least one restriction enzyme recognition site such that upon restriction of said nucleic acid with a restriction enzyme which recognizes said site, a site- specific recombinase recognition target site remains intact, but said nucleic acid is rendered replication, packaging or infection deficient, or (B) wherein said nucleic acid comprises at least one site-specific recombinase recognition site which is recognized by a site- specific recombinase, wherein contact of said first nucleic acid with said site-specific recombinase results in excision of sequences from said replicable adenovirus such that, in the absence of intermolecular recombination, said infectious, replicable or packageable adenovirus is rendered replication or packaging defective; (b) a second nucleic acid sequence encoding adenovirus sequences which, in the absence of adenoviral replication factors provided in trans or intermolecular recombination with said first nucleic acid sequence, are insufficient to encode an infectious, replicable or packageable adenovirus, said second nucleic acid sequence comprising at least one recombinase recognition target site sufficiently identical with said recombinase recognition target site in said first nucleic acid as to be recognized by the same site-specific recombinase which recognizes said site-specific recombinase recognition target site in said first nucleic acid, and at least one head-to-head ITR junction; wherein said first and said second nucleic acid sequences, in combination and following site-specific intermolecular recombination, result in production of an infectious adenovirus, and wherein a site-specific recombinase which recognizes said site-specific recombinase recognition target sites is either (i) expressed by a cell into which said first and said second nucleic acids are introduced, (ii) operatively encoded by said first nucleic acid, said second nucleic acid or both, or 1 -j _ (iii) is provided in trans through expression from a third nucleic acid or is provided in trans as an active protein. 36. An improved adenovirus vector system comprising two plasmids, neither of which alone comprises sufficient adenoviral sequences to produce infectious adenovirus when introduced into a cell but which, when both plasmids are introduced into a cell, recombine to form an infectious recombinant adenovirus, the improvement comprising: (a) inclusion of at least one head-to-head ITR junction in at least one of the plasmids, or (b) inclusion, either in said first plasmid, said second plasmid, in both said first and said second plasmids or into a cell into which said first and said second plasmids are introduced, sufficient sequences to encode an active site- specific recombinase, and inclusion in said first and said second plasmid of recombinase recognition sequences, such that upon contact of said first and said second plasmids with said site-specific recombinase, site-specific recombination between said recombinase recognition sequences in said first plasmid and said second plasmid occurs, or (c) combinations of (a) and (b). 37. A two-plasmid system for making an infectious adenoviral vector wherein each plasmid alone comprises insufficient adenoviral sequences to encode an infectious adenoviral vector wherein, upon recombination, an infectious adenoviral vector is produced, provided that each plasmid of said two-plasmid system comprises (a) a head-to-head ITR junction; (b) a recombinase recognition site such that upon contact of both plasmids of said two-plasmid system with a site-specific recombinase, site-specific recombination between the plasmids of said two-plasmid system occurs, or (c) wherein said two plasmid system comprises both (a) and (b). 38. The recombinant adenovirus vector system of claim 12 comprising: (a) a first plasmid selected from the group consisting of pBHGfrtΔE 1,3, pBHGl lfrt, pBHGdXIPfrt, pBHGE3frt and, pBHGfrtΔEl ,3FLP containing a circularized adenovirus DNA molecule and optionally including a bacterial origin of DNA replication and an antibiotic resistance gene for selection in bacteria and having a deletion or modification of the packaging signal, of additional El sequences, of E3, E4 or fibre, and optionally having a frt site located adjacent the pIX gene, E3, E4 or fibre of the virus, said plasmid optionally encoding Cre, FLP or another site-specific recombinase; (b) a second plasmid selected from the group consisting of pCA35ΔfrtITR, pDC511, ρDC512, pDC515, pDC516 and combinations thereof and modifications thereof wherein such modifications comprise inclusion of a head-to-head ITR junction: (i) all or most of the left ITR and the packaging signal contained within the leftmost approximately 350 nt of the Ad genome or a head-to- head ITR j unction; (ii) a polycloning site or a foreign DNA or an expression cassette; and optionally, (iii) a lox P or fit site 3' of said polycloning site or foreign DNA or expression cassette; and Λ „ m 117 (c) a cell line that is normally able to support replication of adenovirus and which optionally expresses the recombinase Cre, FLP or another site specific recombainse that is able to catalyse site-specific recombination between said lox P sites, said fit sites, or both. 38. The kit according to claim 18 wherein component (a) is selected from the group consisting of pBHGfrtΔEl,3, pBHGl l frt, pBHGdXlPfrt, pBHGE3frt and, pBHGfrtΔEl,3FLP. 39. The kit according to claim 18 wherein said component (b) is selected from the group consisting of pCA35ΔfrtITR, pDC511, pDC512, pDC515 and pDC516, combinations thereof and modifications thereof wherein such modifications comprise inclusion of a head-to-head ITR junction. 40. The kit according to claim 18 wherein said cell of (c) is selected from the group consisting of 293 cells, 293 cells expressing FLP, PER-C6 cells expressing FLP, 911 cells expressing FLP, and wherem said recombinase recognition sites are fit sites. 41. A method for recombining DNA comprising combining use of Cre-lox with FLP- fit. 42. The method according to claim 41 wherein said FLP-frt system is employed to rescue cassettes comprising molecular switches regulated by Cre-lox, or vice versa. 43. The method according to claim 41 wherein said FLP-frt system is employed to rescue a Cre expression cassette, or vice versa. 44. A nucleic acid construct selected from the group consisting of: pBHGl l frt, pBHGdXlPfit and pBHGE3frt by the methods taught in the construction of pBHGl llox, pBHGdXIPlox, pBHGE31ox, pΔElsplAfrt, pΔElsplAfrtΔ,pΔElsplBfrt,pΔElsplBfrtΔ,pMH4frt,pMH4frtΔ,pMH4frtΔlink, pCA13frt, pCA13frtΔ, pCA14frt, pCA14frtΔ, pCA36fit, pCA36frtΔ, pCA36fitΔFLPR, pCA36frtΔFLPT, pCA35fit and pCA35frtΔFLPITR.

Inventors: GRAHAM FRANK L, NG PHILIP, PARKS ROBIN, GRAHAM, FRANK, L., NG, PHILIP, PARKS, ROBIN
Applicants: GRAHAM FRANK L, MERCK & CO INC, NG PHILIP, PARKS ROBIN, GRAHAM, FRANK, L., MERCK & CO., INC., NG, PHILIP, PARKS, ROBIN
Application year: 2000
Main classification: C12N15
IPC codes: A61K35, A61K48, C12N5, C12N7, C12N15