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SUMMARY: Recombinant plasmids between RI drd19 and P-lac and between R447b and P-lac were produced by phage PL25 transductions of the respective R factors to Providence P29 strains harbouring P-lac. The P-lacR1drd19 (the superior line indicating the transduced R factor) recombinant plasmid transfers lac+ and all determinants of the R factor to Proteus mirabilis strains and Escherichia coli at high frequencies which approach inter-Proteus transfer of the parental plasmids. The P-lacR447b recombinant transfers lac+ and only marker K of the parent R factor, at frequencies of P-lac inter-Proteus transfer. Both recombinants possess the compatibility property and possibly most other features of P-lac. Transductions of the P-lacR1drd19 recombinant with phages PL25 and 34 differ. Those with the former vector to P29 yield transductants which have markers ACKSu but not lac+ or S and do not transmit markers conjugally. Conjugal transferability is restored with transductions of the recombinant to P29 carrying a resident P-lac; these transductants then transmit lacR1drd19 with phage 34 to strains of P. mirabilis produce transductants which possess all the resistance determinants and which transfer these conjugally, but in which lac+ does not appear. A spontaneous lac segregant of the P-lacR1drd19 recombinant retained the high transfer rate and other properties of the parent. Transduction of this segregant to strains of P29 and P. mirabilis resulted in transductants which carried markers ACKSu and ACKSSu respectively but could not transfer them by conjugation. Transductions of the segregant to strains harbouring P-lac again produced recombinant plasmids at low rates which transmitted the above groups of resistance determinants and lac+ at frequencies similar to those of the original recombinant. Transductions of the P-lacR447b recombinant with either phage yielded transductants carrying K (and able to transmit it conjugally) but never any carrying lac+

Properties of Proteus and Providence Strains Harbouring Recombinant Plasmids between P-lac Rl drd 19 or R447b