A common first approach to study the function of a gene is to perform knock-out mutagenesis and observe the resulting phenotype. This is also termed reverse genetics. The bacterium E. coli has been the workhorse of molecular biology for the last 70 years or so, due to the ease of its culturing and its amenability to genetic manipulation1. Several methods have been developed to produce gene deletions in E. coli, including marker exchange mutagenesis2,3 and, more recently, recombineering using the λ Red or Rac ET systems4,5,6.
In a widely used system, coding sequences of individual genes are replaced by an antibiotic resistance cassette that can later be excised from the chromosome5,7. The coding sequences are replaced, for instance by a kanamycin (Kan) resistance cassette, which is flanked by flippase (FLP) recognition target (FRT) sites on either side. The FRT sites are recognized by the recombinase FLP, which mediates site-specific recombination between the FRT sites leading to the deletion of the Kan cassette. In this way, a full deletion of a given gene's coding sequence can be achieved, leaving behind only a minimal scar sequence of approximately 100 base pairs (bp) (Figure 1).
Just over a decade ago, the so-called Keio collection was developed. This is a bacterial library based on a standard laboratory E. coli K12 strain, where almost all non-essential genes were individually deleted by λ Red recombination7,8. The clones within this collection each have one coding sequence replaced with an excisable Kan resistance cassette. The Keio collection has proven to be a useful tool for many applications9. One such application is the production of deletion mutants in other E. coli strains. The Kan cassette from a given deletion clone can be mobilized by generally transducing bacteriophages, such as P110,11,12,13,14. A phage stock prepared from such a strain can then be used to infect a recipient E. coli strain of interest, where at a low but reliable frequency the Kan cassette-containing region can be incorporated into the recipient genome by homologous recombination (Figure 2). Transductants can be selected for the growth on the Kan-containing medium. Following this, if removal of the antibiotic resistance cassette is desired, the FLP recombinase can be supplied to the transductant strain in trans. After curing the FLP-containing plasmid, which carries an ampicillin (Amp) resistance marker, Kan and Amp-sensitive clones are screened for, and the correct excision of the wild-type coding sequence and the Kan cassette are verified by colony PCR.
Here, a detailed protocol is presented, describing each of the steps in producing a knock-out E. coli strain based on the strategy outlined above. As an example, a deletion of the tamA gene is demonstrated. tamA encodes an outer membrane β-barrel protein that is a part of the Transport and Assembly Module (TAM), which is involved in the biogenesis of certain autotransporter proteins and pili15,16,17. This knock-out strain was then used to examine the effect of the tamA deletion on the biogenesis of two trimeric autotransporter adhesins (TAAs), the Yersinia adhesin YadA and the E. coli immunoglobulin (Ig)-binding TAA EibD18,19.