3 
The morning session on bacteriophage vectors was followed by a session on 
plasmid vectors that was chaired by D. Helinski (University of California, San 
Diego). Helinski presented the following properties as highly desirable character- 
istics of a safer plasmid vehicle: (a) non-conjugative; (b) non-mobilizable or 
poorly mob iliz able by a conjugative plasmxd; (c) possesses little or no extraneous 
genetic information; (d) poorly recombines or does not recombine with the chromo- 
some of the host cell; (e) provides no selective advantage to the host cell or the 
selective property is conditional; and (f) possesses mutations that restrict its 
maintenance to a specific host, prevent replication at mammalian body temperature 
and/or provide the plasmid with the capability of killing any cell to which it 
might be transmitted other than the host cell. V. Hershfield (University of Cali- 
fornia, San Diego) described the properties of a variety of derivatives of the ColEl 
plasmid and the broad-host range, P-type plasmid, RK2. One of the ColEl derivatives, 
ColEl-trp, constructed in collaboration with C. Yanofsky and N. Franklin (Stanford 
University) provides the means to use the tryptophan genes of E. coli as a selective 
marker in transformation with recombinant DNA in situations where it is desirable 
to avoid antibiotic resistance genes. In addition, Hershfield described collabora- 
tive work with H. Boyer that resulted in the development of a mini-ColEl plasmid 
and derivatives of this plasmid (mini-ColEl-kan and mini-ColEl-irp) as cloning 
vehicles. Finally, she described the temperature-sensitivity properties of trp and 
kan derivatives of a temperature-sensitive replication mutant of ColEl isolated by 
J. Collins (Molecular Biology Institute, Stockheim) and hybrid ColEl plasmids carry- 
ing the EcoKL generated Cts fragment of bacteripphage X-trp6l. 
J. Carbon (University of California, Santa Barbara) described a replica plating 
method that greatly facilitates the detection of E. coli, clones bearing ColEl plas- 
mids. The procedure, which utilizes the F^ plasmid to promote the transfer of a 
hybrid ColEl plasmid to a suitable auxotrophic recipient, was successful in identi- 
fying clones bearing hybrid plasmids carrying a number of different regions of the' 
E. coli chromosome. The contributions of A. J. Clark and collaborators (University 
of California, Berkeley) were relevant to the problem of the mobilization and subse- 
quent transfer of non-conjugative plasmids carrying foreign DNA of a potentially 
hazardous nature. Clark described the variations in transmission frequencies be- 
tween the non-conjugative plasmids pSClOl, pML31, pSC138 and a number of pSClOl 
hybrids containing various EcoKL fragments of F when the conjugal transfer of these 
plasmids was promoted by several different conjugative plasmids. 
I. C. Gunsalus and collaborators (University of Illinois) and A. Chakrabarty 
(General Electric Research and Development Center) described the properties of a 
variety of plasmids isolated from Pseudomonas putida. These contributions were 
followed by a discussion on the merits of developing plasmid-host systems involving 
Pseudomonas strains that naturally exhibit unusual growth requirements. Similar 
studies with plasmids isolated from Bacillus megaterium by B. Carlton (University 
of Georgia) from B. subtilis by P. Lovett (University of Maryland) and other natur- 
ally occurring Bacillus species by W. Goebel and K. Bernhard (Microbiology Institute, 
Wurzburg) were discussed and their further development as plasmid-host cloning sys- 
tems was explored. It was clear from these presentations that considerable progress 
has been made recently in the identification and characterization of a variety of 
plasmid elements that occur naturally in Pseudomonas and Bacillus species. Several 
of the plasmids described show considerable promise as plasmid cloning systems 
involving a host other than E. coli. 
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