tmRNA acts as which two types of RNA?

tmRNA functions as both a transfer RNA (tRNA) and a messenger RNA (mRNA). This dual role is quite unique and essential for the process of rescuing stalled ribosomes during protein synthesis. In instances where a ribosome is stuck due to a faulty mRNA that lacks a stop codon, tmRNA can step in to provide a tRNA-like function, delivering an amino acid to the growing polypeptide chain. At the same time, tmRNA also serves a messenger function by providing a sequence that codes for a degradation tag, allowing the incomplete or malformed protein to be targeted for degradation.

This combination of functionalities is crucial for maintaining the efficiency and accuracy of protein synthesis within the cell. The fact that tmRNA can facilitate both the addition of amino acids, as tRNAs do, and encode a meaningful peptide sequence, as mRNAs do, emphasizes its essential role in quality control during translation, particularly in bacterial systems. The other types of RNA listed in the choices do not share this dual functionality, which is why tmRNA is uniquely classified as both tRNA and mRNA.