Signature-tagged mutagenesis

Signature-tagged mutagenesis (STM) is a genetic technique used to study gene function. Recent advances in genome sequencing have allowed us to catalogue a large variety of organisms genomes, but the function of the genes they contain is still largely unknown. Using STM, a scientist may infer what function the product of a particular gene has by disabling it and observing the effect on the organism. The original and most common use of STM is to discover which genes in a pathogen are involved in virulence in its host, so that better medical treatments can be designed.

Basic Premise
The gene in question is inactivated by insertional mutation; a transposon is used which inserts itself into the gene sequence. When that gene is transcribed and translated into a protein, the insertion of the transposon affects the protein structure and (in theory) prevents it from functioning. In STM, mutants are created by random transposon insertion and each transposon contains a different 'tag' sequence that uniquely identifies it. If an insertional mutant bacterium exhibits a phenotype of interest, such as susceptibility to an antibiotic it was previously resistant to, then the scientist will sequence its genome and run a search (on a computer) for any of the tags used in the experiment. When a tag is located, the gene that it disrupts is also thus located (It will reside somewhere between a start and stop codon which mark the boundaries of the gene).

Scientists may use STM to discover which genes are critical to a pathogen's virulence by injecting a 'pool' of different random mutants into an animal model such as a mouse and observing which of the mutants survive and proliferate in the host. Those mutant pathogens that didn't survive in the host must have had an inactivated gene that was needed for virulence. This is hence an example of a negative selection method.