mobilization of ColEl by Rldrdl9 by about five orders of 
magnitude. (F: 676-680). Derivatives of ColEl, including 
the recombinant DNA vectors pMB9 , pBR313, and pBR322, 
contain deletions which overlap the mob locus. These deletions 
can be complemented in trans by ColEl and ColK. Consequently, 
if a cell harbors both a mob- and a mob+ plasmid, both can be 
mobilized at high efficiency by a conjugative plasmid. The 
mob- phenotype of pBR322, for example, would be masked by ColEl 
(F: 676-680) . 
In assessing the probability of conjugational plasmid 
transfer, it is important to use wild bacterial strains in 
the experimental crosses, because differences in the surface 
characteristics of wild and laboratory strains may influence 
the probability of multiple matings.— ^ The frequencies of 
mobilization by Rldrdl9 reported by Dougan, Crosa, and Falkow 
(F: 676-680) ranged from 10“^- and 10 “ 2 for ColEl and RSF2124 
to 10~6 for pBR313 and pBR322 in both E. coli and healthy X1776 
These frequencies may be higher or lower than frequencies 
resulting from matings of wild bacterial strains. 
If the rate of infectious transmission is sufficiently 
high, plasmids can be maintained in a bacterial population even 
if they confer a selective disadvantage on the host organism. 
(F: 710-711). 
15 / J. W. Costerton, et al.. Scientific American, 238 (1978), 
86-95. 
[A- 193] 
