21 
design of these experiments, the proficiencies of donor-ability are 
underestimates. 
§17. Triparental matings under permissive conditions: 
a) In triparental matings using X1753 as primary donor of 9 
different plasmids, xl^76 as primary recipient and secondary donor, and 
X1763 as secondary recipient to measure the R-mediated transfer of pSClOl 
to Xl?63 [ xl?53(R)-»-xl876(pSC101)^xl763, pSClOl scored by tet r l, only 
one plasmid failed to produce transconjugants . Additionally, some of 
the R-factors in the experiment which yielded transconjugants did not 
yield transconjugants with X1876 as primary (R) donor [ Xl 376 (R) (pSClOl)-*- 
X1763, pSClOl scored by tet r ] (Curtiss et^ £l L . , Tables 21 and 22). This 
suggests that X1876 may be a more proficient donor upon recent receipt 
of the R-factor than when it originally harbors the R-factor. Alternatively, 
pSC may be mobilized more efficiently upon receipt of the R-factor by some 
unknown catalytic mechanism. 
b) Although the frequency of R549 drd l transfer from Xl?83 to 
-4 
X1876 is 10 of that from X-L783 to X1763, the frequency of pSClOl 
mobilization mediated by R549drdl from X1783 to yl876 to X^-^^3 is 100 
to 1000 times more frequent than the frequency from Xl?8- to X1876 
[ X1783 (R549drdl)-+-yl876(pSCl 01)->-yl763 , pSClOl scored by tot r » y!783 (R549 drd l)->- 
Xl876(pSC101) , R54 9drd l scored] (Curtiss _et a]!. , p.25, Tables 18 and 22). 
These data are incrongruous , if not contradictory. They emphasize the point 
that the proficiencies of donor-ability for yl876 are underestimates 
(see this paper, §16 above) in the sense that X1876 is a more proficient 
recipient for R549 drd l than previously measured. 
[Appendix C — 209] 
