6 
and are conjugation-defective in matings with donors possessing conjugative plasmids 
in the P, W and 0 incompatibility groups but Con + as recipients for F, I and T 
group plasmids when compared to the dap+ gal + parent strain. Strains with endA 
mutations were also observed to exhibit increased transformation frequencies. 
Attempts to introduce temperature-sensitive polA alleles into strains to block rep- 
lication of ColEl cloning vectors at elevated temperatures and to cause DNA 
degradation at elevated temperatures in the presence of vecA and kthyA alleles have 
so far been unsuccessful since the NTG-induced polA alleles often do not have the 
same properties in the constructed strains as in the strains in which the allele was 
originally induced. Many mutations causing a Con” phenotype have been investigated, 
but many of these revert and/or do not exhibit a Con~ phenotype in matings with 
donors possessing conjugative plasmids of the incompatibility groups commonly found 
in enteric microorganisms. Some Con - mutants exhibit increased sensitivity to bile 
salts; thus, the mutant described by Bukhari may also exhibit a Con - phenotype. 
All of the strains constructed by the Curtiss group are SuII + and most have muta- 
tions abolishing restriction alone or both restriction and modification. Thus, 
sufficient information is now known to construct a usable safer E, coll K-12 host. 
Curtiss and collaborators are now introducing the A thy A and dna mutations into 
their dapD8 hbioH-asd kgal-chl r -uvnB hsr nalA x (for ease in monitoring) Su + A r <{>80 r 
strain to accomplish this objective. 
The final session involved a general discussion of some of the major points 
raised previously in the workshop. There was general agreement at this session that 
both plasmid-host and phage-host systems have been developed that should meet the 
criteria of an EK2 system specified by the National Institutes of Health guidelines 
for research on recombinant DNA molecules. Additional testing is required to con- 
firm the EK2 properties of these available systems, but it is anticipated that these 
vector-host systems will meet these tests. 
Donald R. Helinski Roy Curtiss III 
University of California, San Diego University of Alabama 
Stanley Falkow 
University of Washington 
Waclaw Szybalski 
University of Wisconsin 
[ 139 ] 
