60 
With these aspects of biological containment in mind, the Guidelines 
specify the physical containment required for such experiments, as 
summarized in Table IV of Appendix E. The particular levels of physical 
containment depend on the source of the foreign DNA, whether defective 
polyoma or defective SV40 is the chosen vector, and finally on whether 
virus particles are produced. 
The Guidelines also contain recommendations for experiments in which 
plant cells serve as hosts for recombinant DNA. These cells might be 
single plant cells grown under laboratory conditions, or seedlings, plant 
parts, or small whole plants. This is the only instance, in fact, where 
the Guidelines address the question of recombinant DNA experiments 
with whole organisms. Directions are given for modification of the 
specifications for Pi, P2, and P3 physical containment in order to 
provide conditions appropriate for work with plants. 
Vectors for use in experiments with plants include plant organelle 
DNA, such as the DNA of chloroplasts, and DNA of plant viruses of low 
pathogenicity and restricted host range. These vectors offer moderate 
levels of biological containment, and the Guidelines specify the required 
physical containment levels (outlined in Table V of Appendix E). As 
before, the requirements are organized according to the source of the 
foreign DNA. 
Theoretically, there are a variety of organisms, both prokaryotes 
and lower eukaryotes, such as fungi and yeast, which will be useful 
hosts for experiments with recombinant DNAs. Some may offer the 
special advantage of not infecting humans, animals, or important 
ecological niches. A variety of technical developments are needed, 
however, before useful vectors are available for these systems. The 
growth characteristics of such hosts indicate that containment problems 
will be like those for E. coli K-12 hosts. The Guidelines urge develop- 
ment of these systems and point out that the biological and physical 
containment requirements for them can be derived from the detailed 
recommendations for E. coli K-12 systems. 
The diagram on the following page, from an article by Clifford 
Grobstein in Scientific American, illustrates the combinations of physical 
and biological containment required by the NIH Guidelines (9). 
