Question

how is -ssRNA transcribed into +ssRNA?

More specifically, is -ssRNA transcribed into +ssRNA using

1. RNA polymerase encoded by the host cell

or

2. RNA polymerase encoded by the virus

or does it transcribe it using dna polymerase?

because I thought that if a virus contains -ssRNA then host ribosomes cannot translate it until the -ssRNA is replicated into +ssRNA by viral dependendent polymerase, so does this mean its option 1?

Answer 1

Yes, option 1 is correct. RNA dependent RNA polymerase (RdRP) is quite common in negative sense RNA viruses. Example Family Orthomyxoviridae., Influenza virus, Polio virus etc.

Existence of this enzyme was confirmed by use of Actinomycin D. Actinomycin D is known to block cellular DNA dependent RNA synthesis. The Virus( Polio virus) showed resistance to this drug. It made people think that there must be some other polymerase at work which should be RNA dependent (as genetic material of virus is RNA only).

RdRP is essentially packed in virion particle at the time of release from the host. When virus infects the new host. It enters inside the cell (mostly by endocytosis) and makes the positive sense RNA. a viral protein VPg (viral protein genome linked) acts as primer during synthesis of positive strand. During elonagation subsequent NTPs are added.This positive sense RNA can now act as m RNA and direct the host's ribosome for protein synthesis.

Use of DNA polymerase is impossible in synthesis of positive strand as DNA polymerase synthesizes DNA strand only.