What amino acid is added to the polypeptide by tmRNAs?

tmRNA is known for its role in rescuing ribosomes that are stalled during protein synthesis due to the absence of a stop codon. It operates by providing both a tRNA-like function and an mRNA-like function, facilitating the addition of a specific amino acid to the incomplete polypeptide chain.

The amino acid commonly added by tmRNA is Alanine. When tmRNA is recognized by the ribosome during a stalled translation situation, it not only helps release the stalled ribosome but also adds a tag to the growing polypeptide chain, which typically starts the addition of a series of alanine residues. This addition serves as a signal for the degradation of the incomplete protein, allowing the cell to maintain protein quality control.

This process is crucial for cellular efficiency and helps prevent the accumulation of nonfunctional proteins, thus ensuring proper cellular function. The other amino acids mentioned, such as glycine, serine, and threonine, are not the primary amino acids that tmRNA adds in these instances.