Question about viruses

[JP2740]

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The Princeton book tells us to figure out what single stranded DNA and double stranded RNA viruses need to encode or carry (if anything). I believe double stranded RNA viruses just need to encode RNA dependent RNA polymerase because the host ribosome can translate the (+) strand. As for single stranded DNA, I thought it depends which strand it is. If it is transcribed into the (-) RNA, then it needs to carry RNA dependent RNA pol to replicate this into a (+) strand. If the single stranded DNA is transcribed into the (+) RNA then it only needs to encode for RNA dependent RNA pol. Did I get this right?

 

[PiBond]

You are correct about the double stranded RNA virus only needing to encode RNA dep RNA pol. When it's injected into the host, the (+) RNA strand acts as mRNA and will thus be translated into the appropriate product. For the single stranded DNA the host will carry its own DNA polymerase so it is likely that the virus will be able to replicate itself and thus create its complementary strand using host enzymes and dNTPs. Thus the virus does not need to carry any enzymes because the host will have the appropriate ones on hand. The virus only carries stuff that the host doesn't normally have, like RNA dep RNA polymerases

 

[JP2740]

But would it be possible that the RNA complement to the DNA be a (-) strand and therefore the virus would need to carry RNA dependent RNA polymerase in order to synthesize the (+) strand before translation?

 

[ilovemcat]

First note that ss-DNA can either code template DNA (antisense strand) for (+) mRNA or (complement DNA strand) for (-) mRNA.

If DNA contains the antisense strand (template), than RNA-dependent-RNA pol will not be carried since the mRNA itself will code for mRNA.

RNA-dependent RNA pol will first replicate (+) mRNA by encoding (-) mRNA. It will than use (-) mRNA as the template to code more (+) mRNA.


If ss-DNA contains the sense (non-template) strand for mRNA, then the virus must carry RNA-dependent RNA pol in order to translate the (-) mRNA strand.

The reason for this is because (-) mRNA codes useless polypeptides. So RNA-dependent RNA pol is needed to convert that to (+) mRNA first (which will encode more RNA-dep RNA pol). After several copies of (+) mRNA has been made, it will than use the (+) mRNA as a template to code more (-) mRNA which will then be packaged into the newly made viruses.


A ds-RNA virus has both strands, one of which will includes the useful (+) mRNA therefore there's no need to carry RNA-depend RNA polymerase.

In short:
ds-RNA (RNA dependent RNA Pol not carried)
ss-DNA containing template strand codes (+) mRNA (RdRP not carried)
ss-DNA containing non-template strand codes (-) mRNA (RdRP is carried)

I hope this makes sense.