21 
into the system. Similar experiments should also he done 
using a secondary recipient that is restrictive j or the 
recombinant plasmid as well as with primary donors possess- 
ing revressed confugative plasmids With incompatibility 
croup properties like those commonly found in enteric 
microorganisms. Since a common route of escape of plasmid- 
host systems in the laboratory might he hy accidental 
ingestion 3 it is suggested that the same types of experi- 
ments he conducted in suitable animal-model systems. In 
addition to these tests on survival of the cloned DNA 3 it 
would he useful to determine the survival of the host strain 
under nongrowth conditions such as in water and as a function 
of drying time after a culture has been spilled on a lab bench. 
O 
For EK2 phage vectors , no more than one in 10 recombinant phage 
particles should be able to perpetuate the cloned DNA fragment under 
non-permissive conditions designed to represent the natural environ- 
ment either (a) as a prophage or plasmid in the laboratory host used 
for phage propagation or (b) by surviving in natural environments 
and transferring the cloned DNA to a host (or its resident lambdoid 
prophage) with properties common to those in the natural environment. 
In terms of potential EK2 \-host systems 3 the following 
types of genetic modification should reduce survival of the 
cloned DU A. The examples given are for illustrative purposes 
[ 92 ] 
