title: A food-grade site-directed mutagenesis system for Streptococcus thermophilus LMG 18311.
abstract: OBJECTIVE
  To develop a general method for site-directed mutagenesis in the dairy starter strain Streptococcus thermophilus LMG 18311 which does not depend on antibiotic-resistance genes or other selection markers for the identification of transformants.
  
  
  RESULTS
  In a previous study, we demonstrated that Strep. thermophilus LMG 18311 can be made competent for natural genetic transformation by overexpression of the alternative sigma factor ComX. In the present study, we wanted to investigate whether the natural transformation mechanism of Strep. thermophilus LMG 18311 is efficient enough to make it feasible to perform site-directed mutagenesis in this strain without the use of a selection marker. Competent bacteria were mixed with a DNA fragment engineered to contain a nonsense and a frameshift mutation in the middle of the target gene (lacZ) and subsequently seeded on agar plates. By performing colony-lift hybridization using a digoxigenin-labelled oligonucleotide probe, we succeeded in identifying transformants containing the sought after mutation.
  
  
  CONCLUSIONS
  By exploiting the natural transformability of Strep. thermophilus LMG 18311 and standard molecular methods, we have demonstrated that the genome of this bacterium can be altered at preselected sites without introduction of any foreign DNA.
  
  
  CONCLUSIONS
  A food-grade site-directed mutagenesis system has been developed for Strep. thermophilus LMG 18311 that can be used by the dairy industry to construct starter strains with novel and/or improved properties.
authors.affiliation: Department of Chemistry, Biotechnology and Food Science, Norwegian University of Life Sciences, As, Norway.
authors.forename: T
authors.initials: T
authors.lastname: Blomqvist
authors.forename: H
authors.initials: H
authors.lastname: Steinmoen
authors.forename: L S
authors.initials: LS
authors.lastname: Håvarstein
mesh_terms: D018389:Codon, Nonsense; D003612:Dairying; D005516:Food Microbiology; D016368:Frameshift Mutation; D005798:Genes, Bacterial; D007763:Lac Operon; D016297:Mutagenesis, Site-Directed / Q000379:methods*; D009693:Nucleic Acid Hybridization; D048249:Streptococcus thermophilus / Q000235:genetics*; D014169:Transformation, Bacterial
chemical_list: D018389:Codon, Nonsense