24 
This permits phage propagation in relatively small volumes 
and constitutes an a.dditiona.1 safety feature. 
The phenotypes and genetic stabilities of the mutations 
and chromosome alterations included in these X-host systems 
indicate that containment well in excess of the required 
—8 
10T or lower survival frequency for the cloned. DBA fragment 
should be attained. Obviously the presence of a.ll mutations 
contributing to this high degree of biological containment 
must be verified periodically by appropriate tests. Laboratory 
tests should be performed with the bacterial host to measure 
all possible routes of escape of cloned DBA. such as the 
frequency of lysogen formation 3 the frequency of plasmid 
formation and the survival of the lysogen or carrier bacterium. 
Similarly 3 the potential for perpetuation of the cloned DBA. 
fragment carried bu infectious phage particles could be tested 
by challenging typical wild-type E. coli strains or a X-sensitive 
nonpermissive laboratory K-12 strain 3 especially one lysogenic 
for a larhbdoid phage. 
In view) of the fact that accurate assessment of the proba- 
bilities for escape of infectious X grown on r~ m~ Su T hosts is 
dependent upon the frequencies of r~ 3 Su’ 3 and X-sensitive 
strains in nature , investiaators are encouraqed to screen E. 
coli strains for these properties . These data will also be 
useful in predicting frequencies of successful escape of plasmid 
cloning vectors harbored in r~ m~ Su + strains. 
[ 95 ] 
