C-TERMINAL EXTENSION OF TRUNCATED RECOMBINANT PROTEINS IN ESCHERICHIA-COLI WITH A 10SA RNA DECAPEPTIDE

When murine interleukin-6 is overexpressed in Escherichia coli, a small population of molecules exhibits a novel C-terminal modification. Peptide mapping, electrospray ionization-mass spectrometry, and automated N- and C-terminal sequencing identified a peptide (''tag'' peptide), Ala-Ala-Asn-Asp-Glu-Asn-Tyr-Aza-Leu-Ala COOH, encoded by a small metabolically stable RNA of E. coli (10Sa RNA) attached to truncated C termini of the recombinant protein. A mutant strain of E. coli in which the chromosomal 10Sa RNA gene (ssrA) is disrupted does not produce this C-terminal modification, confirming that the tag peptide originates from the ssrA gene.