SUMMARY: Non-conjugative Tc, Sm, SmSu, SmSpc, SmSpcSu and TcSmSpcSu determinants (Tc, Sm, Spc and Su denote resistance to tetracycline, streptomycin, spectinomycin and sulphonamides, respectively) in wild-type strains were mobilized with transfer factors F, I and A2 and implanted in K12. F and I were transmitted at very high rates in all matings in which these K12 strains were used as donors; the rate of A2 transmission was not measured. When the Tc determinants, derived from 32 of the wild-type strains, were serially transferred between strains of K12 they were transmitted either at a very low rate in both first and second matings, at a very low rate in the first mating and at a very high rate in the second mating, or at a very high rate in both matings. In the donors that transmitted at the low rate, the Tc determinants were probably chromosomally located, a recombination event between them and the transfer factors being responsible for the formation of the donors that transmitted at the high rate; once they became extrachromosomally located, the Tc determinants continued to be transmitted at the high rate. Of the resistance determinants studied, Tc was the only one suspected of being chromosomally located.

F transmitted most of the four Sm and eight SmSpc determinants at high rates and the 14 SmSu determinants at much lower rates. I and A2 transmitted most of the three kinds of determinants at high rates but four of the K12 SmSpcA2 strains transmitted them at very low rates and two K12 Sm-SpcI strains and one K12 SmSuI strain did not transmit them at all. F and I did not establish linkage with Sm or SmSu but linkage was detected in the strains that transmitted at the high rate between them and SmSpc. The one TcSmSpcSu determinant studied was transmitted at two moderately low rates by F with which it was not linked. In the first matings following implantation in K12 by I or A2, it was transmitted at a very low rate, but in most subsequent matings its transmission rate was high; for the I strain this was certainly due to transfer factor linkage being established.


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