⚡ Core Concept: Microbial Mutagenesis & High-Fidelity DNA Repair
Spontaneous and induced mutations in bacteria alter codon sequences, generating point mutations, insertions, or deletions. Bacterial DNA repair systems (Mismatch repair, Nucleotide Excision Repair, and the SOS response) balance genetic stability against adaptive hypermutation under severe environmental stress.
1. Mutational Mechanisms & The Ames Test
Mutations arise spontaneously via tautomeric shifts, depurination, or deamination (cytosine → uracil), or by chemical/physical mutagens (5-bromouracil base analogues, alkylating agents, UV pyrimidine dimers). The Ames Test evaluates carcinogenicity by measuring the reversion frequency of histidine-auxotrophic Salmonella enterica serovar Typhimurium (his– → his+) upon exposure to a mutagen in the presence of rat liver microsomal extract (S9 fraction).
2. The Bacterial SOS Response
Under severe genotoxic stress where replication forks stall at un-repaired lesions, ssDNA tracts accumulate and activate RecA. RecA* acts as a coprotease that stimulates the autocatalytic cleavage of the LexA repressor, derepressing over 40 SOS regulon genes, including error-prone translesion DNA polymerases (Pol IV / DinB and Pol V / UmuD’2C) which synthesize across damaged DNA at the cost of elevated mutation rates.