Escherichia coli JA221 can suppress the UAG stop signal

Abstract
The Escherichia coli strain JA221 can suppress the UAG stop codon, although the existence of an amber suppressor tRNA has not previously been described for this strain. When using a plasmid to express alpha-sarcin, which has TAG as its stop signal, two proteins were obtained: a smaller protein corresponding in size to that of the expected protein, and a larger protein, which could be accounted for by the presence of a second stop codon (TGA) 18 base pairs downstream of the original. This feature of strain JA221 must therefore be considered when using this strain as a host for the production of recombinant proteins.