naked positive ssrna

[cage92]

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first aid say that naked nucleic acid of negative ssrna are not infectious do that mean that naked positive ssrna are infectious? i know that every negative ssrna should transform to positive ssrna to be infectious and what is the relation of envelop to the infectivity? hepatitis a and e are not enveloped and infectipus

 

[sloop]

first aid say that naked nucleic acid of negative ssrna are not infectious do that mean that naked positive ssrna are infectious? i know that every negative ssrna should transform to positive ssrna to be infectious and what is the relation of envelop to the infectivity? hepatitis a and e are not enveloped and infectipus

I don't have the book in front of me right now but:

Negative sense RNA is only infectious with the help of RNA-dependent-RNA-polymerase. These are packaged in the full virion, but the RNA cannot be taken and injected alone into a cell and cause infection. Positive sense is infectious because it acts as mRNA and transcribes the proteins it needs to replicate.

Envelopes are sort of a different matter and are mostly an indication of where the virus packages itself. The main implication is that nonenveloped viruses are resistant to drying and detergents and can often survive on fomites.

 

[aspiringmd1015]

yes, +rna are infectious, they are a "working copy" of the message. Enter the cell, ribosomes attack, proteins, enzymes, caspids etc are made directly.

 

[cage92]

so enveleped negative ssrna are not infectious?

 

[pd1112]

Correct, but don't think of it as naked vs. enveloped, just remember positive sense vs. negative sense. It just so happens that all of the relevant (and maybe all entirely but idk) negative sense ssRNA viruses are enveloped, but the toga-, flavi- & coronaviridae are enveloped positive sense ssRNA. So despite being enveloped, the naked genome of those three families is still considered infectious on its own.

Infectious in terms of virus genomes just means the genome by its self could be injected into a cell & produce fully functional virions. The genome is not as infectious as the full virion, because it doesn't have the attachment/tropic proteins that normally form the nucleocapsid.

 

[sloop]

so enveleped negative ssrna are not infectious?

Enveloped SS negative sense RNA virions can be infectious (many are, in fact—paramyxovirus is just one example). I will explain this, but it seems like the envelope thing is confusing you so, like pd1112, I would advise you to try to forget about envelopes for the moment.

The critical part to understanding this topic is the following: A particle/nucleic acid cannot be infectious if it does not lead to protein synthesis. Drill this into your mind because it is critical to understanding the rest of this. Now I'm going to take you through the SS RNA viruses to drill home this point starting from the simplest case and going to the more complex one.

SS (+) RNA

• SS (+) RNA is infectious because it is an "analog" of mRNA (remember that +/- is in relation to mRNA polarity). When you isolate the genome of a SS (+) RNA virus and inject it into a cell, the following happens:
  • The RNA first goes to the ribosome and, because it is like mRNA, codes for RNA-dependent-RNA-polymerase (an enzyme we don't have).
  • RNA-dependent-RNA-polymerase makes SS (-) RNA from the original SS (+) RNA
  • RNA-dependent-RNA-polymerase then makes more SS (+) RNA from the SS (-) RNA, thus replicating the genome (more copies of positive sense RNA)
  • Positive sense RNAs go to the ribosomes and code for viral particle proteins (capsids and whatnot) and these get packaged with the new copies of SS (+) RNA to make virions.
  • The conclusion here is that SS (+) RNA can be infectious when injected alone because it can code for its own RNA-dependent-RNA-polymerase

SS (-) RNA

• SS (-) RNA is not infectious because it is in the wrong direction to act as mRNA. Because of this, it cannot code for RNA-dependent-RNA-polymerase. It is only non-infectious when injected alone/without RNA-dependent-RNA-polymerase. When packaged in a virion, many viruses contain this enzyme which makes the genome infectious.
• SS (-) RNA viruses can be infectious if they contain RNA-dependent-RNA-polymerase in the viral particle. Here's what happens in this case:
  • Virus releases SS (-) RNA and RNA-dependent-RNA-polymerase into the cell
  • RNA-dependent-RNA-polymerase makes SS (+) RNA from the SS (-) RNA
  • SS (+) RNA codes for viral proteins (including the polymerase) and serves as the template for new SS (-) RNA
  • RNA-dependent-RNA-polymerase makes new SS (-) RNA from SS (+) RNA
  • SS (-) RNA, RNA-dependent-RNA-polymerase, and capsid proteins combine to form new virions which get released
• The conclusion here is that SS (-) RNA is only infectious when combined with RNA-dependent-RNA polymerase

Now, back to envelopes. First notice how this had NOTHING to do with our discussion about whether RNA from viruses is infectious. Envelopes are usually just an indication of how the virus gets packaged. For instance, herpesviruses (DNA viruses) have envelopes because they are assembled in the nucleus, bud from the nuclear membrane (forming the envelope) and then get sent to the ER/golgi for release. Other viruses just get packaged and released free. Envelopes are sensitive to detergents and drying and enveloped viruses generally do not survive long on fomites.

 

[cage92]

man thank you excellent explanation