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transfer of the cloned genes Into receptive bacteria found in nature. Moreover, 
| the propagation of the phage can be blocked by many conditional mutations, which 
would be designed to block any secondary route of escape, mainly depending on trans- 
fer of the cloned DNA into another phage or bacterial host. It was recommended 
j further that the vector be designed in such a manner as to permit easy insertion 
| and monitoring of the foreign DNA and rapid assay of the safety features and give 
a high yield of cloned DNA (not less than 10^-1 molecules per ml) . There also was 
general agreement that host-phage systems other than E. coli should be considered, 
especially those restricted to very rare and unusual environments. Also, plasmids 
; derived from phage vectors and which give very high DNA yields while exhibiting 
safety features, e.g., Advcrots, should be considered as vehicles for cloned DNA. 
Szybalski and S. Brenner (Cambridge University) stressed that research on 
! recombinant DNA molecules may lend itself to very simple and inexpensive mechanical 
containment, e.g., a small sealed glove box, since all the vectors that carry such 
recombinant molecules possibly can be both created and destroyed in such a box, 
while development of special methods might permit study of many properties of the 
recombinant DNA, without ever removing it from the box. 
Thesg safety features were reflected in the subsequent presentations. 
F. Blattner and W. Williams (University of Wisconsin) described four specially con- 
structed A-cJ>80 phages which incorporate many of these safety features, and which 
they named Charon phages, for the mythical boatman of the river Styx. Some of these 
highly contained phages give yields of over 10 H particles/ml. R. Davis, J. Cameron 
and K. Struhl (Stanford University) found that X phages that carry foreign DNA 
never grow as well as the parental vector, which would select against their survival 
in the nature. They also reported that some eukaryotic genes could be expressed in 
E. coli, partially compensating for deficiencies in the histidine pathway or in 
polA or lig functions. These investigators surveyed over 1000 strains of E. coli 
isolated in the natural environment and did not find a single strain that could 
support propagation of the Xvir vector. 
V. Bode (Kansas State University) discussed the possibility of growing tail- 
free X heads. Such heads, which are packed with DNA, are very fragile, unless 
stored in 0.01 M putrescine buffer. Head yields close to 10-^/ml could easily be 
attained and, when required, heads could be quantitatively rejoined with separately 
supplied tails under special laboratory conditions. W. Arber, D. Scandella and J. 
Elliott (University of Basel) described bacterial host mutants that permit efficient 
infection only by phages with a full complement of DNA. This permits selecting for 
vectors that carry long fragments of foreign DNA. 
K. Matsubara, T. Mukai and Y. Takagi (University of Osaka and Kyushu University), 
and G. Hobom and P. Phillippsen (University of Freiburg and Stanford University) 
described various defective X plasmids (Xdv) that could be used as efficient vectors. 
Matsubara has shown that temperature-sensitive cro mutations permit obtaining 
between 1000 and 3000 cloned molecules per cell and at the same time result in 
killing of the carrier cells at body temperature. The mutations 0 ts and Pts were 
also evaluated as safety features. Phillipsen described many new Xdv plasmids 
created by cutting X DNA with Hindlll and Bconl restriction endonucleases followed by 
ligation. The final talk by F. Young, G. Wilson and M. Williams (University of 
Rochester) summarized the progress on the development of safer Bacillus subtilis 
host mutants and phages, especially <J>3, as vectors. New restriction nucleases, 
Bgl-1 and Bgl-2, were also described. 
[ 135 ] 
