Variants of lambda repressor and cytochrome b(562) translated from messenger RNAs without stop codons were modified by carboxyl terminal addition of an ssrA-encoded peptide tag and subsequently degraded by carboxyl terminal-specific proteases present in both the cytoplasm and periplasm of Escherichia coli, The tag appears to be added to the carboxyl terminus of the nascent polypeptide chain by cotranslational switching of the ribosome from the damaged messenger RNA to ssrA RNA.