26 
Survival of Vectors. Because potentially useful vectors such as 
plasmids ancTviruses may be transferred from cell to cell, independent 
of the growth and survival of the initial recipient, it is also necessary 
to consider survival of the vectors. Plasmids and viruses occur widely 
in nature. Any particular plasmid or virus will normally multiply only 
within a limited number of species. Thus, for example, viruses that 
infect particular bacteria neither multiply nor cause disease in the cells 
of other bacterial species or complex organisms. In many instances, 
they do not even enter the cells of any organism other than the particular 
natural host. 
Only limited information concerning the effect of foreign DNA 
insertions on the survival or transferability of plasmid and viral 
vectors is available. In the case of plasmids, the factors contributing 
to their maintenance or loss from cells in natural environments, 
even without insertion of a foreign DNA, are not clearly understood (11). 
One exception is the selective advantage for maintenance provided 
by an antibiotic -resistance gene on the plasmid. Also, some plasmids 
are known to confer on host cells the ability to manufacture substances 
poisonous to other, closely related cells, thus giving the poison-producers 
special advantage in a competitive situation. Insertion of a foreign 
DNA fragment within the DNA sequence coding for the poison has 
been shown to eliminate production of the poison (4), thus decreasing 
the likelihood that the cells and their resident recombinant DNA will 
survive in nature. 
Experiments carried out during the last few years have yielded 
only minimal information on the stability of plasmids containing 
foreign DNA in E_. coli K1 2 host cells, or on the stability of the 
foreign fragmenT itself. For experimental purposes, cells containing 
recombined plasmids are generally grown under conditions 
especially designed to increase the stability of the plasmid (called 
"selective" conditions). For consideration of the loss of the plasmids 
in natural environments --the important point for matters of safety- - 
the stability of the plasmid or recombined DNA under ordinary, or 
nonselective, conditions needs to be known. A review of a limited 
number of unpublished observations indicates that because of the 
extensive diversity in the nature of the plasmids, and in the nature 
of the inserted DNA fragment, generalizations as to the rate of loss of 
the recombined, relative to the original plasmid are impossible. Whether 
an inserted DNA fragment provides an advantage or a disadvantage to 
the stability of the plasmid depends not only on the properties of the 
recombined DNA, but also on the site of insertion of the fragment into 
the vector, the relation of this site to the gene used to select for 
recipients of the plasmid, and the conditions of growth. 
