19 
no t be construed to encompass all possibilities . The presence 
of the non-conj uga tive plasmids ColEl- trp and pSClOl in an E_. 
coli K-12 strain possessing a mutation eliminating host- 
controlled restriction and modification ( hsdS) results in about 
20^ -fold reduction in mobilization to restriction-proficient 
recipients . The combination of the dapD8 3 A bioH—asd and b ga l— chi 
mutations in E. coli K-12 results in no detectable survivors in 
10 
feces of rats following feeding by stomach tube of 10 cells 
in milk and similarly leads to complete lysis of culture sus- 
pended in broth medium lacking diaminopimelic acid. E. coli 
K-12 strains with b thyA and deoC (dra) mutations undergo thymineless 
death in growth medium lacking thymine and give a 10 5 -fold 
reduced survival during passage through the rat intestine 
compared to wild-type thy + E_. coli K-12. (However 3 the h thyA 
mutation alone or in combination with a deoB ( drm) mutation only 
reduces in^ vivo survival by a factor of 10^.) Other host 
mutations 3 as yet untested 3 that might further reduce survival 
of the plasmid-host system or reduce plasmid transmission are: 
the combination polA (TS) recA (TS) b thyA which might interfere 
with ColEl replication and lead to DNA degradation at body 
temperatures ; Con~ mutations that reduce the ability of con- 
jugative plasmids to enter the plasmid-host complex and thus 
should reduce mobilization of the cloned DNA to other strains ; 
and mutations that confer resistance to known transducing phages. 
[ 90 ] 
