Final answer:
The rate-limiting step for the decay of nonsense codon-mRNA is the termination of translation when a nonsense codon is encountered. Release factors recognize these nonsense codons and cause the detachment of the amino acid from its tRNA, leading to the release of the protein.
Step-by-step explanation:
The rate-limiting step for the decay of nonsense codon-mRNA is the termination of translation when a nonsense codon (UAA, UAG, or UGA) is encountered. Release factors, which resemble tRNAs, recognize these nonsense codons and instruct peptidyl transferase to add a water molecule to the carboxyl end of the P-site amino acid. This reaction causes the P-site amino acid to detach from its tRNA, leading to the release of the newly made protein.