TRANSDUCTION OF LOW-COPY NUMBER PLASMIDS BY BACTERIOPHAGE-P22

The generalized transducing bacteriophage of Salmonella typhimurium, P 22, can transduce plasmids in addition to chromosomal markers. Previou s studies have concentrated on transduction of pBR322 by P22 and P22HT , the high transducing mutant of P22. This study investigates the mech anism of P22HT transduction of low-copy number plasmids, namely pSC1O1 derivatives. We show that P22HT transduces pSC1O1 derivatives that sh are homology with the chromosome by two distinct mechanisms. In the fi rst mechanism, the plasmid integrates into the chromosome of the donor by homologous recombination. This chromosomal fragment is then packag ed in the transducing particle. The second mechanism is a size-depende nt mechanism involving a putative plasmid multimer. We propose that th is multimer is formed by interplasmidic recombination. In contrast, P2 2HT can efficiently transduce pBR322 by a third mechanism, which is in dependent of plasmid homology with the chromosome. It has been propose d that the phage packages a linear concatemer created during rolling c ircle replication of pBR322, similar in fashion to phage genome packag ing. This study investigates the role of RecA, RecD, and RecF recombin ation proteins in plasmid/plasmid and plasmid/chromosome interactions that form packageable substrates in the donor. We also examine the res olution of various transduced plasmid species in the recipient and the roles of RecA and RecD in these processes.