A suppressor mutation counters the effects of an original mutation by restoring the wild-type phenotype. There are two main types of suppressor mutations: intragenic mutations occur within the same gene and restore function through alternate amino acid substitutions, while intergenic mutations occur elsewhere in the genome and restore function through interacting gene products. Suppressor mutations are useful for studying protein-protein interactions and dissecting biological pathways.

1. Suppressor
Mutation
Name- DeepikaRana
RollNo.-1601
Deptt.- Microbiology(2nd )
M.D. University, Rohtak

2. •A suppressor mutation is
a mutation that counters
the phenotypic effect of a previous
mutation.
•It may either alleviates or reverts
the phenotypic effects of an already
existing mutation.
•Genetic suppression therefore restores
the phenotype seen prior to the original
background mutation.

3. For example, say a mutation causes
the codon 'AGA' (which codes for the amino
acid Arginine) to change to 'AGC' (which
codes for the amino acid Serine, a very
different amino acid to Arginine). A
suppressor mutation may be one that
changes the mutated codon to 'CGC' (which
also codes for Arginine), or to 'AAG' (which
codes for Lysine, a similar amino acid to
Arginine), thereby reversing the effect the
first mutation may have had on the function
of the protein.

6. Mechanisms of suppression

7. (a) A typical receptor-mediated signal transduction pathway is shown, involving a ligand, a cell surface receptor, a G
protein, a kinase cascade and a transcription factor that activates or represses gene expression in response to the
signal. (b) A null mutation in the gene encoding kinase 1 blocks the pathway, resulting in a mutant phenotype (i). The
null mutation can be suppressed by a mutation in a gene encoding a component (denoted by an asterisk), such as
kinase 3 (ii), which activates the downstream portion of the pathway independent of kinase 1 activity. Mutations in
upstream components, such as the G protein (iii), restore signaling through only part of the pathway, owing to the
kinase 1 defect. (c) When the pathway is inactivated by a partial loss-of-function mutation in kinase 1 (i), mutations in
either downstream (ii) or upstream (iii) components can activate the pathway enough to suppress the mutant
phenotype. Pathways can, therefore, only be ordered reliably when the starting mutation is a null allele.

8. Intragenic suppression
•Intragenic suppression, where a phenotype caused by a
primary mutation is ameliorated by a second mutation
in the same gene.
• The suppressing mutation might be a true revertant,
restoring the original DNA sequence; it might be an
alteration of the same codon, resulting in a less
detrimental amino acid at that position; or it might
affect a different codon, causing an amino acid change
at another position that now restores the function of
that protein closer to wild-type activity.
•Intragenic suppressors must be very tightly linked to
the original mutation, whereas extragenic suppressors
are unlikely to be tightly linked.
•Although intragenic suppressors provide valuable
information about the structure–function relationships
within a protein, they do not identify any new proteins
that are functionally related to the original mutant, and
therefore are generally not the goal when undertaking a
suppressor hunt.

9. Intergenic Suppression
• Also known as extragenic suppression relieves the effects of
a mutation in one gene by a mutation somewhere else within
the genome.
•The second mutation is not on the same gene as the original
mutation. Intergenic suppression is useful for identifying and
studying interactions between molecules, such as proteins.
• For example, a mutation which disrupts the complementary
interaction between protein molecules may be compensated
for by a second mutation elsewhere in the genome that
restores or provides a suitable alternative interaction between
those molecules.
•Several proteins of biochemical, signal transduction,
and gene expression pathways have been identified using this
approach.
•Examples of such pathways include receptor-
ligand interactions as well as the interaction of components
involved in DNA replication, transcription, and translation.

10. Suppression of frameshift mutations.
(a) A deletion in the nucleotide coding sequence can
result in an incomplete, inactive polypeptide chain.
(b) The effect of the deletion, shown in figure a, can be
overcome by a second mutation, an insertion in the
coding sequence. This insertion results in the production
of a complete polypeptide chain having two amino acid
replacements.
These may revert by the restoration of a deleted base for
example, but can also be suppressed by the addition or
deletion of further bases (not necessarily at the same
place as the original mutation) so that the total number of
bases added or lost is a multiple of three In this way the
original reading frame is restored leaving a limited
number of altered codons.
Whether this altered product has sufficient biological
function to result in observable suppression of the
original mutation will of course depend on the size and
nature of this altered sequence and its effect on the
function of the protein.

11. Nonsense suppression.
In E. coli, suppressor genes are known for
each of the three stop codons. They act by
reading a stop codon as if it were a signal for
a specific amino acid. There are, for
example, three well characterized genes that
suppress the UAG codon. One suppressor
gene inserts serine, another glutamine, and a
third tyrosine at the nonsense position. In
each of the three UAG suppressor mutants,
the anticodon of at RNA species specific for
one of these amino acids has been altered.
For example, the tyrosine(UAC and UAU)
suppressor arises by a mutation within a
tRNA Tyr gene that changes the anticodon
from GUA (30-AUG-50) to CUA (30-AUC-
50), thereby enabling it to recognize UAG
codons (Fig.). The serine and glutamine
suppressor tRNAs also arise by single base
changes in their anticodons.

12. Suppression of a nonsense
mutation.
A base substitution changes CAG to
the stop codon UAG, causing pre-mature
termination of translation.
This can be suppressed by a separate mutation
in a tRNA gene, giving rise to a tRNA that can
recognize the UAG codon. The original mutation
changes a glutamine codon (CAG) to a stop
codon (UAG). Suppression of this mutation can
occur by alteration of the glutamine tRNA gene
so that its anticodon now pairs with the amber
codon. Glutamine will therefore be inserted into
the growing peptide chain and the final product
will be identical with the wild-type protein.
Since there is more than one glutamine tRNA
gene, the cell does not lose the ability to
recognize genuine glutamine codons.
The importance of this type of suppression is
that the tRNA mutation is able to suppress any
corresponding mutation, not just the original
one it was selected for.

13. •Suppression of nonsense mutations is a simple approach to study the effect of
different amino acid substitutions at a specific position in a protein. Promega
sells a nice kit of nonsense suppressors for this type of analysis.
•Nonsense suppressors can also be used to incorporate unnatural amino acids
into proteins. For example, alpha-hydroxy acids with a wide variety of
substituent can be synthesized and attached to a nonsense suppressor tRNA,
allowing substitution of the unnatural amino acid for any chosen amino acid
residue in the protein.
•The unnatural amino acids can provide a probe for the role of that position in
the structure and function of the protein, etc.

14. Genetic tests for protein interactions. Suppressor mutations. Two proteins, A and B, normally
interact. A mutation in A prevents the interaction, causing a loss of function phenotype, but
this can be suppressed by a complementary mutation in B which restores the interaction.
•To show potential interactions between proteins by screening for suppressor
mutations, i.e. mutations in one gene that partially or fully compensate for a
mutation in another. The primary mutation causes a change in protein structure
that prevents the interaction, but the suppressor mutation introduces a
complementary change in the second protein that restores it. widely employed in
amenable organisms like Drosophila and yeast.
•Advantage- provide a short cut to functionally significant interactions, sifting
through the proteome for those interactions that have a recognizable effect on the
overall phenotype

15. •The analysis of any biological process by
classical genetic methods ultimately requires
multiple types of mutant selections to identify all
the genes involved in that process.
•One strategy commonly used to identify
functionally
related genes is to begin with a strain that already
contains a mutation affecting the pathway of
interest, selecting for mutations that modify its
phenotype.
• Modifiers that result in a more severe phenotype
are termed enhancers, while mutations that
restore a more wild-type phenotype despite the
continued presence of the original mutation are
termed suppressors.

16. •They also provide evidence between functionally
interacting molecules and intersecting biological pathways.
•Suppressor analysis is a commonly used strategy to
identify functional relationships between genes that might
not have been revealed through other genetic or
biochemical means.
•Suppressors have proven extremely valuable for
determining the relationship between two gene products in
vivo, even in the absence of cloning or sequence
information

17. There are two main reasons for the increased use
of suppressors.
•First, many genes are resistant to identification by more
direct genetic selections. A pre-existing mutation often
serves to sensitize that pathway, allowing the identification
of additional components through suppressor selections.
•Second, and perhaps more importantly, suppression of a
pre-existing phenotype establishes a genetic relationship
between the two genes that might not have been
established by other methods.

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•Prescott Harley, and Klein’s Microbiology seventh edition (2008)
Joanne M.Willey Hofstra University Linda M. Sherwood Montana State University Christopher
J.Woolverton Kent State University
•Principles of Gene Manipulation and Genomics SEVENTH EDITION (2006) S.B. Primrose and
R.M. Twyman
•Reversion and Suppression - SDSU College of Sciences
www.sci.sdsu.edu/~smaloy/MicrobialGenetics/topics/rev-sup/
•Suppressor mutation - Wikipedia, the free encyclopedia
https://en.wikipedia.org/wiki/Suppressor_mutation
•Mutation
www.cod.edu/people/faculty/fancher/genetics/Mutation.htm
•Suppression mechanisms themes from variations(Review Article)
Gregory Prelich Department of Molecular Genetics, Albert Einstein College of Medicine, 1300
Morris Park Avenue,Bronx, New York NY 10461, USA.