 Suppressor mutation

A suppressor mutation is a second genetic change that reduces or counteracts a specified effect of another change. The resulting phenotype can move toward the original functional state while the original altered DNA sequence remains present. Suppression can act through another position in the same gene or through a different gene; recovery of one readout need not restore every property of the organism.

A constructed reading frame example makes the distinction visible. A one-nucleotide insertion changes the downstream grouping of codons. A second, later one-nucleotide deletion can restore the original grouping after the second change, because the net length change is $1-1=0$. The sequence between the two changes can still specify different amino acids. Two additional inserted nucleotides would likewise give a net change of three, restoring phase downstream without recreating the original nucleotide sequence. Whether function recovers remains an experimental question.

Suppressor screens also identify mutations that bypass a defined regulatory phenotype. In a 2002 SecM study, mutations in 23S ribosomal RNA and ribosomal protein L22 relieved ribosome stalling. The affected positions faced the ribosomal exit tunnel constriction, linking the altered response to the environment around the nascent polypeptide.^[ref] Such a screen finds a functional relationship; by itself it does not prove that every suppressor physically contacts the original altered component.