Plasmid vectors for positive selection of DNA inserts controlled by the λ pl promoter, repressor and antitermination function

Hybrid plasmids consisting of pBR322 or pOP203-3 and the EcoRI-D fragment of λ DNA kill their bacterial host upon expression of a λ gene (probably the kil function) located either between or across the SalI sites. The plasmids from surviving hosts have acquired deletions that remove the λ kil gene or insertions that block the transcription of the kil gene. Some plasmids probably carry point mutations. Based on these findings, we constructed two vector plasmids, pKL1 and pHA10, which can be used for a direct positive selection of cloned fragments. These plasmids are particularly useful for the cloning and selection of N-unresponsive termination signals using BamHI and its isoschizomers. The DNA fragments cloned into these plasmids are under control of the strong pl promoter, which can be regulated by the λ represser, and the antitermination activity of the N gene product.