17 
however s most EK1 X vectors currently in use lack the att_ 
and int functions (25-27) thus reducing the probability of 
lysogenization to about 10~ 5 to 10~ 6 (36-38). The frequency 
for the conversion of X to a plasmid state for persistence 
— f) 
and replication is also only about 10 (39). Moreover 3 
the routine treatment of phage lysates with chloroform (40) 
should eliminate all surviving bacteria including lysogens 
and X plasmid carriers. Lysogenization could also occur when 
an infectious X containing cloned DM infects a \-sensitive 
cell in nature 3 and recombines with a resident lambdoid pro- 
phage. Although \-sensitive E. coli strains seem to be rare 3 
a significant fraction do carry lambdoid prophages (41-42) 
and thus this route of escape should be considered. 
While not exact s the estimates for containment 
afforded by using these host-vectors are at least as 
accurate as those for physical containment 3 and are » 
sufficient to indicate that currently employed plasmid 
and X vector systems provide a moderate level of bio- 
logical containment. Other nonconjugative plasmids 
and bacteriophages that 3 in association with E_. coli 
K-12 can be estimated to provide the same approximate 
level of moderate containment are included in the 
EK1 class. 
[ 88 ] 
