Use of the lysis gene of bacteriophage phi X174 for the construction of a positive selection vector.

DNA fragments generated by a variety of restriction enzymes can easily be cloned in the small (3.2-kb) positive-selection vector pUH84, which contains the modified lysis gene of bacteriophage phi X174 under transcriptional control of the lac promoter. Plasmid pUH84 does not yield transformants after introduction into Escherichia coli unless the lysis gene is inactivated by insertion of foreign DNA into one of the unique PstI, SalI, AccI, HincII, BamHI, or EcoRI sites. This highly efficient positive selection of recombinants requires neither the use of a distinct host strain nor a special induction of the lysis function. Transcription of fragments cloned into pUH84 may be effectively regulated by the lac promoter provided the host cells are cotransformed with the newly constructed plasmid pUH7 which carries the IQ allele of the lac repressor gene.