In this article we will discuss about:- 1. Origin of Simian Virus 40 2. Organisation of Simian Virus 40 3. Replication.
Origin of Simian Virus 40 (SV40):
Simian virus 40 (SV40) belongs to polyomavirus family, the family of viruses that induce tumours in animals; indeed, the suffix oma means tumour. Simian virus 40 was first isolated from monkeys hence so named. SV40 was one of the first genetic elements to be studied by genetic engineering techniques and has been used extensively as a vector for transferring genes into eukaryotic cells.
Organisation of Simian Virus 40 (SV40):
SV40 is a naked icosahedral virus with a diameter of 45 nm. Its capsid contains 72 protein subunits. SV40 does not have enzymes in it unlike RNA viruses.
The genome of SV40 is a single molecule of dsDNA of 5243 base pairs and is very small. dsDNA of this virus is circular and exists in supercoiled configuration having the sedimentation coefficient of 21S. Total G+C content of the nucleic acid is 41%. A liner form of 14S sedimentation coefficient is formed after double-stranded break in the supercoil.
Complete base sequence of SV40 has been determined. Fig. 14.19 shows the genetic map of the virus in which it is clearly evident that the genes that encode VP1, VP2, and VP3 proteins overlap each other.
Replication of Simian Virus 40 (SV40):
When the virus enters the host cell, its genome migrates to the nucleus. SV40 genome replicates inside the nucleus of the cell, but the capsid protein are synthesized in the cytoplasm and migrate inside the nucleus where, finally, the assembly of the virus take place. The replication of SV40 genome can be divided into two distinct stages: early and late.
(i) Early stage:
The early region of the viral genome is transcribed to begin early protein synthesis during this stage within 12 hour of infection and before start of genome replication. A single RNA molecule, called primary transcript, is synthesized by the RNA polymerase of the host ceil and is processed into two mRNAs, a large one and a small one.
These mRNAs are capped in the cytoplasm and translated to yield two proteins, small-T and large-T. The large-T (the T-antigen) binds to the site on (origin of replication) on the parental DNA; this initiates the synthesis of viral genome.
(ii) Late stage:
Replication of viral genome starts in a bidirectional fashion from a single origin of replication. Late mRNA molecules are synthesized using the strand complementary to that used for early mRNA synthesis.
Transcription starts at a promoter near the origin of replication. This late RNA is then processed by splicing, capping, and polyadenylation to yield mRNA corresponding to the three coat proteins, namely, VP1, VP2, and VP3 (VP = virion protein).
Viral coat protein mRNAs are then transported to the cytoplasm and translated into the viral coat proteins, which are then transported back into the nucleus where the assembly of progeny viruses takes place. Release of progeny virus particles occurs by the lysis of the host cell.