Vectors - The various animal vectors are based on one or the other virus, e.g., SV 40 vectors, bovine papillamovirus vectors, retrovirus vectors, etc., or on a transposable element, e.g., Drosophila P element vectors.

It may be pointed out that some of the these vectors, e.g., early or late region replacement SV40 vectors, retrovirus vectors etc., behave like viruses in that they produce virions, which are used to infect the host cells.

DNA insert of up to 40 kb can be placed between the two border sequences. The recombinant P DNA is injected intoDrosophila larvae alongwith a helper P element, which produces transposase. Transposase enables the transposition of recombinant P element (carrying the DNA insert) from the recombinant DNA into the Drosophila genome.

Bacculovirus vectors have been developed for transfection of insects. Two nuclear polyhedrosis viruses (NPV), e.g., AcNPV (Autographa californica NPV) and BmNPV (Bombyx mod NPV), have been exploited for this purpose. The NPV polyhedrin gene has a very strong promoter, and the polyhedrin protein is not needed for NPV replication.

Therefore, the general strategy is to replace the NPV polyhedrin coding sequence by the DNA insert so that the polyhedrin promoter drives the trans gene. The recombinant NPV DNAs form virions, infect silkworm larvae or cultured cells, and replicate to yield upto 50 μg vector DNA per larva.

Both circular and linearized vectors can integrate into the host genome, but the latter are far more readily integrated than the former. It has also been found that the presence of additional, vector. DNA along with the integrated gene construct interferes with the expression of introduced gene or transgene. Therefore, it is often desirable to introduce the transgene with a minimum of vector DNA associated with it.