Provided that the total length of the recombinant DNA molecule does not exceed about 105% of wild type (Bett et al., 1993), DNA can be inserted at several sites in the adenovirus chromosome to create a nondefective recombinant (see Fig. 2). Such vectors have been used to express foreign proteins in many studies, particularly for vaccine development studies where the ability of the virus to replicate postadministration may be advantageous. In addition to the commonly used serotypes 2 and 5, adenoviruses of other subgroups such as Ad4 and Ad7 have been used for protein expression (Imler, 1995). A property of nondefective vectors that may be useful in some situations is their ability to shut off host cell macro-molecular synthesis. This effectively enriches the expression of a recombinant protein regulated by late viral transcriptional and translational elements.
In many studies, the nonessential E3 region has been replaced by a foreign sequence of interest (Schneider et al., 1989; Morin et al, 1987; Dewar et al., 1989; Johnson et al., 1988), and the plasmid-
based systems now available for construction of recombinant viruses (see later) contain unique restriction sites to simplify cloning into the E3 locus. However, it may be advantageous to retain these sequences in viruses that are to be used for experiments in vivo because of the role of E3 in reducing the antiviral immune response (Wold, 1993; Ginsberg et al, 1989; Tollefson, 1998). Helper-independent viruses can also be created by inserting DNA between the E4 promoter and the inverted terminal repeat at the right end of the chromosome (Saito et al., 1985; Mason et al., 1990). However, unless another viral sequence (such as E3) is deleted, the packaging size constraint limits such insertions to about 1.8 kb.
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