[Su.sup.+] strains insert an amino acid in response to an amber docon and may restore activity to any altered regulatory gene resulting from an amber mutation
. Phage forming a clear streak on [Su.sup.0] (nonsuppressor strain) but not on [Su.sup.I], [Su.sup.II], or [Su.sup.III] (suppressor strains) are amber mutants.
The only bacteria that survived were those that incorporated an amino acid--either a natural or an introduced one--at the location of the amber mutation.
These bacteria already had an amber mutation in a gene that encodes barnase, a protein that kills bacteria.