14 
The nonconjugative plasmid vectors cannot promote 
their own transfer 3 hut require the presence of a con- 
jugative plasmid for mobilization and transfer to other 
bacteria. When present in the same cell with derepressed 
conjugative plasmids such as F or Ri der d!9 3 the non- 
conjugative ColEl 3 ColEl- trp and pSClOl plasmids are 
transferred to suitable recipient strains under ideal 
laboratory conditions at frequencies of about 0.b 3 10~~ 
to 10~ c 3 and 10~ : per donor cell 3 respectively . These 
frequencies are reducea by another factor of 10 6 to 10' 
if the conjugative plasmid employed is repressed with 
respect to expression of donor fertility. 
The experimental transfer system which most closely 
resembles nonconjugative plasmid transfer in nature is a 
triparental mating. In such matings 3 the bacterial cell 
possessing the nonconjugative plasmid must first acquire 
a conjugative plasmid from another cell before it can trans- 
fer the nonconjugative plasmid to a secondary recipient. With 
ColEl 3 the frequencies of transfer are 10~~ and 10~~ to 10~ * 
when using conjugative plasmid donors possessing derepressed 
and repressed plasmids 3 respectively . Mobilization of ColEl- 
trp and pSClOl under similar laboratory conditions is so low 
as to be usually undetectable (31). Since most conjugative 
plasmids in nature are repressed for expression of donor 
fertility 3 the frequency at which nonconjugative plasmids are 
mobilized and transferred by this sequence of events in_ vivo 
[ 85 ] 
