How do small RNA viruses get the most bang for their replication buck?

Question

BlackTom AI · Accepted Answer

Overlapping reading frames, polyprotein synthesis with proteolytic cleavage, and programmed ribosomal frameshifting/readthrough are the principal tricks small RNA viruses use to pack maximum coding information into a tiny genome.

1) Relevant knowledge points and concepts
- Coding density: viruses are under pressure to encode many functional proteins in limited nucleotide length.
- Overlapping open reading frames (ORFs): a single nucleotide sequence can encode different proteins when translation initiates in different frames or at different start sites.
- Polyproteins: translation of one long ORF yields a single polyprotein that is proteolytically cleaved into multiple mature viral proteins (common in picornaviruses, flaviviruses).
- Programmed ribosomal frameshifting and readthrough: ribosomes are induced to shift reading frame or ignore a stop codon, producing a second protein from the same mRNA.
- Alternative strategies (less typical for many small RNA viruses): extensive alternative splicing is more characteristic of DNA viruses or retroviruses; segmented genomes increase packaging complexity.

2) Step-by-step reasoning
- Overlapping ORFs: by overlapping codons, the virus can add extra protein sequences without increasing nucleotide length. Example: gene A encoded in frame 0 and gene B encoded in frame +1 overlapping for several hundred bases.
- Polyprotein strategy: one initiation event yields a long polyprotein; viral/host proteases cleave it into many functional units. This avoids multiple promoters/translation starts and minimizes regulatory sequence overhead.
- Frameshifting/readthrough: a programmed sequence (slippery site and RNA structure) causes a fraction of ribosomes to shift into an alternative frame or bypass a stop codon, producing an extended product (e.g., gag-pol fusion in retroviruses; many RNA viruses use frameshifting to produce replicase subunits).

3) Intermediate reasoning examples
- If a 10 kb genome uses overlapping ORFs over 1 kb, that overlap encodes two proteins in 1 kb worth of nucleotides instead of 2 kb — doubling utility.
- A single polyprotein of 3000 aa cleaved into 10 proteins removes the need for 10 separate translation initiation signals.

4) Why the correct answer is correct
Each of the three named mechanisms directly increases the protein output per nucleotide, so together they let small RNA viruses “get the most bang for their replication buck.”

5) Why other options are incorrect (brief)
- Option 2 (e.g., extensive alternative splicing) is less applicable to many small positive-sense RNA viruses because they lack nuclear splicing machinery access; splicing is more typical of DNA viruses and retroviruses.
- Other possible strategies (large noncoding regions, redundant sequences) do not increase coding efficiency and so are not favored.

Conclusion: The most effective strategies are overlapping reading frames, polyprotein cleavage, and programmed frameshifting/readthrough — therefore the correct choice is 1, 3 and 4 above.

How do small RNA viruses get the most bang for their replication buck?Single choice

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