[Plasmid vectors of "insertion inactivation" of the trimethoprim resistance marker]

We demonstrate the possibility of using the T4 phage frd gene as an insertion inactivation marker within pBR322, in plasmids with changing copy number and expression of foreign genes under control. The structural part of the frd genes contains unique recognition sites for EcoRI and SalI endonucleases. Transformants with recombinant plasmids carrying the frd gene grow on media with up to 500 mkg/ml trimethoprim, whatever the gene dosage.