In Gram-positive quorum sensing, such as that observed for Staphylococcus aureus, RNA III regulates expression by coding for a

Question

BlackTom AI · Accepted Answer

RNA III is a regulatory antisense RNA produced from the agr P3 transcript in Staphylococcus aureus and functions by base-pairing with target mRNAs to control their translation and stability.

1) Relevant knowledge points and concepts
- Gram-positive quorum sensing in S. aureus uses the agr locus (agrBDCA and promoters P2/P3). Accumulated autoinducing peptide (AIP) activates the AgrC/AgrA two‑component system.
- Activated AgrA drives transcription from P3, producing RNAIII (~514 nt), the primary effector of the agr regulon.
- Antisense RNAs are noncoding (or primarily noncoding) RNAs that regulate gene expression by complementary base pairing with target mRNAs, altering translation initiation or mRNA stability.

2) Step-by-step reasoning
- Step 1: agr operon senses cell density via AIP → AgrC/AgrA activation.
- Step 2: AgrA upregulates the P3 promoter → synthesis of RNAIII.
- Step 3: RNAIII interacts by complementary base pairing with specific mRNAs (for example, spa mRNA encoding protein A and rot mRNA encoding a repressor), blocking ribosome binding or altering mRNA structure.
- Step 4: These interactions either repress translation or expose ribosome binding sites to activate translation (e.g., modulating hla expression), thereby changing virulence gene expression.

3) Intermediate molecular steps (examples)
- RNAIII base pairs with the 5′ UTR of spa mRNA → prevents ribosome binding → reduced protein A production.
- RNAIII disrupts inhibitory secondary structure of hla mRNA → increases alpha-toxin translation.

4) Why "Antisense RNA" is correct
RNAIII functions primarily through sequence-specific antisense interactions with target mRNAs rather than by encoding a regulatory protein. Its regulatory effects arise from base-pairing-mediated modulation of translation and mRNA stability, the defining mechanism of an antisense RNA.

5) Why other options are incorrect
- A protein transcription factor: RNAIII does not act chiefly by encoding a regulatory protein (although it contains a short ORF for δ-hemolysin, its regulatory role is RNA-mediated).
- A riboswitch: riboswitches are cis-acting metabolite-sensing elements in untranslated regions, not trans-acting regulatory RNAs like RNAIII.
- microRNA: miRNAs are eukaryotic small RNAs processed by Dicer; bacterial RNAIII is a different bacterial antisense sRNA.

6) Conclusion
Because RNAIII controls target gene expression via complementary base pairing and translational control, it is an antisense RNA. This confirms the provided correct answer: Antisense RNA.

In Gram-positive quorum sensing, such as that observed for Staphylococcus aureus, RNA III regulates expression by coding for aSingle choice

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