Properties and products of the cloned int gene of bacteriophage P2

Fragments of DNA of the temperate phage P2, generated by treatment with the restriction enzyme PstI, have been cloned into the plasmid pBR322. One such fragment, which has its endpoints within phage genes T and C, carries the structural P2 int gene as well as its promoter and the phage att site. When introduced into a suitable bacterial host, the cloned fragment mediates the integration and excision of int- mutants of P2 and recombination within the phage att site in mixed infection. All these activities are independent of the orientation of the fragment within the plasmid. When introduced into minicells, the fragment produces, in addition to the products of genes D and U, a protein of 35-37,000 daltons identified as the int protein. A study of the map location of two amber int mutants, together with the sizes of the polypeptides they produce, indicates that the P2 int gene is transcribed from right to left on the P2 map, i.e. starting near gene C and proceeding toward att.