F plasmid provides a function that promotes rec A-independent site-specific fusions of pSC101 replicon

Abstract

Recently, it has been reported that the tetracycline (Tc) resistance plasmid pSC101 (ref. 1) can undergo integrative recombination (that is, fusion) with a plasmid or bacteriophage genome that lacks extensive DNA sequence homology with pSC101. Such fusion of pSC101 with a second replicon can occur in the absence of the bacterial rec A gene function, and seems to involve DNA sequences on pSC101 that closely resemble the inverted repeat termini of the transposable genetic element, Tn3 (refs 2, 3). In both of the observed instances of replicon fusion involving pSC101, the fertility plasmid F was present in the bacterial cell; in one case an autonomously replicating F plasmid was used to support the infection of the male-specific bacteriophage fl (ref. 2), whereas in the other F was integrated into the chromosome of an Hfr bacterial strain being used to study mobilization of a non-conjugative plasmid⁴. However, in many studies with the pSC101 plasmid carried out in the absence of F in our laboratory and elsewhere, pSC101 has not been observed to undergo fusion to a second concurrently present replicon. These observations suggested to us that the F plasmid might carry a function that enables pSC101 to undergo replicon fusion. We report here results indicating that the F plasmid does, in fact, provide a trans-acting function that promotes rec A-independent site-specific recombination by pSC101, and that the function carried by F is located in the segment of the plasmid that includes the tra genes and the γδ sequence.