Evidence for a chromosome-borne resistance transposon (Tn916) in Streptococcus faecalis that is capable of "conjugal" transfer in the absence of a conjugative plasmid.

Abstract

S. faecalis strain DS16 harbors the conjugative hemolysin-bacterocin plasmid pAD1 (35 megadaltons) and the nonconjugative R-plasmid pAD2 determining resistance to streptomycin, kanamycin and erythromycin; a tetracycline resistance (Tetr) determinant is located on the chromosome. When strain DS16 was mated (on membrane filters) with the plasmid-free strain JH2-2, Tetr transconjugants could be obtained at a frequency of about 10-6/recipient. Analyses of transconjugants showed that some contained the Tetr determinant linked to pAD1. Subsequent studies showed that the Tetr determinant was located on a 10 Mdalton transposon, designated Tn916, which could insert into 2 other hemolysin plasmids: pAM.gamma.1 and pOB1. Derivatives of DS16 devoid of pAD1 were capable of transferring Tetr to recipient strains. Transconjugants (plasmid-free) from such matings could subsequently act as donors in the transfer of Tetr. Transposition and transfer were rec independent.