Lecture 5: Suppressor Mutations and Phenotypic Analysis

Vocabulary practice cards covering suppressor mutations, screen versus select paradigms, complementation limits, and regulatory systems such as vancomycin resistance and metabolic suppression.

Starting strain

The initial organism or cell population used at the beginning of an experiment, such as a wild type strain for mutagenesis experiments or a mutant strain for complementation experiments.

Dominant mutation (in antibiotic resistance)

A mutation—such as an alteration in gyrA conferring ciprofloxacin resistance—where the resistant allele overrides the presence of a sensitive allele, meaning the phenotype cannot be complemented by simply introducing the wild-type gene because the resistant protein continues to function.

Tryptophan auxotroph

A mutant strain unable to synthesize tryptophan due to a mutation in any one of several biosynthetic genes (such as trpE, trpG, trpD, trpF, trpC, trpA, or trpB); strains sharing this phenotype may possess different genotypes and cannot be complemented by the same DNA fragments.

Suppressor mutation

A secondary mutation—arising spontaneously or following mutagenesis—in a strain that already contains an original mutation, which partially or fully restores a more wild type-like phenotype.

Intragenic suppressor

A suppressor mutation that occurs within the exact same gene as the primary mutation, functioning by altering the same codon to a different amino acid, altering a different codon in the gene, or restoring a shifted reading frame.

Intergenic suppressor

A suppressor mutation occurring outside the gene harboring the original mutation, acting in the same pathway, a different pathway, or globally via translational or regulatory mechanisms.

Informational (nonsense) suppressor

A class of intergenic suppressor mutation that alters translational machinery, specifically tRNAs, enabling them to recognize and insert an amino acid at premature stop codons.

Suppressors that alter the amount of mutant protein

Intergenic suppressors that compensate for a defect by increasing protein abundance through mechanisms like gene duplication, promoter mutations, transcription factor mutations, increased mRNA stability, or altered protein regulation and stability.

Suppressors that alter mutant protein activity

Intergenic suppressors that restore function by directly modulating protein performance through changes in interacting proteins, multi-protein complex subunits, or post-translational modifications.

Two-component signal transduction system

A regulatory mechanism, frequently encoded within a two-gene operon, composed of a membrane-associated sensor kinase (e.g., VanS) and a cytoplasmic response regulator (e.g., VanR) that coordinate adaptive gene expression in response to an extracellular signal.

VanS

A membrane-bound homodimeric sensor histidine kinase that autophosphorylates on histidine (His) residues upon detecting vancomycin and subsequently transfers the phosphate group to VanR.

VanR

A response regulator that receives a phosphate on an aspartate (Asp) residue from VanS; in its active, phosphorylated form, it binds upstream of the vanHAX promoter to stimulate RNA polymerase transcription of resistance enzymes.

VanHAX

A set of resistance enzymes transcribed downstream of VanR activation that enzymatically remodel the bacterial cell wall so that it can no longer be recognized or bound by vancomycin.

Pathway-related suppressor (GOGAT / GDH)

A suppressor mechanism where the failure of one metabolic route (such as a GOGAT block halting glutamate production and causing NADPH accumulation) is relieved by altering a parallel or connected enzyme (such as mutating GDH to convert 2-oxoglutarate to glutamate) to restore homeostasis.

"Bruce Rule" of suppressor isolation

The practical genetic guideline establishing that if an original mutant strain was isolated via screening, the suppressor of that mutation will be isolated via selection, and vice versa.

Temperature-sensitive (t.s.) suppressor selection

A selective procedure where a population harboring a temperature-sensitive lethal mutation in an essential gene is cultured at a non-permissive temperature (e.g., $42^{\circ}\text{C}$) so that only suppressor mutants capable of growth can survive.