A single nucleotide insertion mutation in a gene of phage T4
causes a shift in the readingframe
of all subsequent downstream codons, and potential incorrect amino acids.
The resulting protein is usually nonfunctional,
and a T4 phage with such a frameshift mutation cannot reproduce on E. coli.
If three insertions occur in tandem,
the reading frame is restored after the insertion.
Such a mutation produces a protein with enough functionality to restore the phage's ability to infect E. coli.
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