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.