In a medical microbiology textbook I'm reading (Murray et al, 1994), the authors state the following:
The retrovirus genome has a 5' cap and is polyadenylated at the 3' end. Although the genome resembles a messenger RNA (mRNA), it is not infectious because it does not encode for a polymerase that can directly generate more mRNA. The genome consists of at least three major genes that encode for the enzymatic and structural proteins of the virus: gag (group-specific antigen), pol (polymerase), and env (envelope). At the end of the genome are long terminal repeat (LTR) sequences
From this, I interpreted that because viral RNA doesn't encode an RNA polymerase, it is not itself infectious. However, if a cell were to be injected with retroviral single-stranded RNA alone, wouldn't this RNA be translated into proteins (including reverse transcriptase) as if it were normal mRNA into a protein? If this were correct, then the single-stranded RNA has all the information in itself to propagate the virus. Single-stranded RNA needs to be reverse-transcribed into the host's DNA before being transcribed into mRNA that can be translated.
Is there something different between viral RNA and the mRNA transcript that makes one translatable and the other not?
Source: Murray, Patrick R et al, Medical Microbiology (1994), Second Edition.
I contacted the author of this chapter (Dr. Ken Rosenthal) and gained some more insight into how this hypothetical (injecting plain retroviral RNA into a cell) would play out. His answer is that yes producing new viruses possible but not probable. The reasoning for this is as follows:
- The main reason that a new virus would likely not be created is that for the genome to make a new virus, reverse transcriptase is needed. Without reverse transcriptase, cDNA can't be made. Suppose that a ribosome was able to make reverse transcriptase with naked retroviral RNA and this newly made reverse transcriptase stayed near the ribosome. It would have difficulties making new cDNA from the original RNA template. One of those challenges is that cDNA synthesis needs a specific primer on reverse transcriptase.
- The translated retroviral RNA would likely be degraded due to their finite lifespan in the cytoplasm.
- There would need to be enough viral proteins translated to form a capsid. This would be unlikely considering the lifetime of the RNA in
- Even if there were enough proteins in the cytoplasm, these proteins would then have to remove the RNA from the ribosome to encapsidate it. This is also unlikely.
Considering this, the microinjection of naked retroviral RNA would be an inefficient process to cause an infection.