SV 40 Vectors

It is a spherical virus with a circular double stranded 5243 bp chromosome which encodes 5 proteins viz small T large T bothe early protein VP1 VP2 and VP3 (vp virion protein) has an origin or replication about 80 bp and is completed with hostones toform chromatin large T is essential for viral replication when VP1 VP2 VP3 form the viral capsid. In laboratory, it is multipliedincultured kidney cells of African green monkey, infected cells live after 4 days releasing upto 10 *5 virions Sv40 genome has been used to develop mainly 3 types of vector (1)transducing vectors, (2) plasmid vectors and (3) transforming vectors.
(1) SV 40 transducing Vectors. There vectors produce viral particles infecting monkey cells. They must have these 3 features (1) the sv 40 origin include the surrounding region containing the transcriptional regulatory signals i.e regions at which slicking and polyadenylation (2) size including the of the DNA insert between 3900 bp and 3500 bp for packaging into virons and (3) genes encoding large T, VP1 VP2 and Vp3. This leaves very little rooms for DNA inserts.
(2) Large replacement Vectors. The regions encoding VP1 Vp2 and Vp3 must be replaced in the vectors by DNA insert such a vector is called late region replacement vector. e.g SVGT-5. A vector of this type is used for infection of host cells in conjunction with another virus, called helper virus, which has the VP1 VP2 and VP3 genes intact but has defective large-T gene. In this casem onl those host cells that are infected by both the vectors and the helper virus will lyse and produce virions isnce cells infected by either the vectors the helper alone will not support packaging or replication . This feature is very useful since all the plaques formed or monkey cell monolayers contain the vector.
(3) Early replacement vectors. Alternatively the essential genes missing from the vector may be present within the genome of host cells. FOr example, COs (cv-1, origin of SV 40 CV-1 is a monkey cell line cell line of African green monkey kidney cell cultures contains in its genome the gene for large T of SV40. Therefore, a vector having the origin of replication and genes for VP1 VP2 and Vp3 will replicate and produce virions in COA cell line cells. In such a case, no helper virus is required. Since in such a vector the early genes large T are replaced by the DNA insert, is is called early region replacement vector.
SV 40 plasmid Vectors. These vectors replicate in monkey cells but do not get packaged into virions they contain the origin of replication and the large T encoding gene (large T gene is not necessary for multiplication in COA cells. Obviously, there is not size limit on such vectors, and some of them are E. coli and monkey shuttle vectors e.g pSV2, pSV3 etc. These vector produced high copy number per cell. The shuttle vectors are used to propagate the recombinant vector in E.coli which are then introduced into monkey cells to study the expression of DNA inserts.