A plasmid cloning vehicle allowing a positive selection for inserted fragments

We describe a plasmid cloning vehicle, pTR262, which allows a strong positive selection (resistance to tetracycline) for transformants bearing plasmids which have DNA insertions. pTR262 is derived from plasmid pBR322 and contains the cI gene and adjacent regulator region o R p R or the bacteriophage λ. The expression of the tetracycline resistance ( tet-r) gene(s) in pTR262 requires transcription from p R and is repressed by the cI gene product, λ repressor. Insertion of a DNA fragment into the HindIII or BclI sites in pTR262 inactivates the c gene and allows expression of the tet-r gene(s) in the host bacterium. A 100-fold increase in the number of tetracycline-resistant transformants is obtained when HindIII- or BclI-generated fragments are added to a ligation mixture containing HindIII- or BclI-digested pTR 262 DNA.