4 
Dr. Joklik felt the NIH Guidelines should specify less than two-thirds of a 
viral genome as a defective virus; he felt this specification a reasonable 
limit for requiring IBC registration of experiments in tissue culture. He 
thought specifying a limit of less than one-third of a viral genome could 
create problems; for example, the Rhabdovirus L gene accounts for more than 
50% of the total Rhabdovirus genome. This gene could not be cloned without 
registrating the project with the IBC if a limit of one-third of the viral 
genome was specified. 
Dr. Martin said designating as defective some fractional portion of a 
gencme, e.g., less than two-thirds of a virus gencme, is artificial. Herpes 
viruses, for example, can almost grow in tissue culture with one-third of the 
genome deleted. Dr. Joklik agreed a large portion of the genome of certain 
viruses can be deleted, and the viruses will still replicate in certain cells 
in culture. Dr. Rapp added there is no evidence such a truncated virus could 
replicate in humans or other hosts. 
Dr. Gottesman asked the working group to discuss the possibility of rescue of 
defective viral genomes through recombination. 
Dr. Anderson said the presence of helper virus or endogenous viral sequences 
in a tissue culture system should be considered with retroviral systems. 
Two-thirds of the Moloney Murine Leukemia virus (MoMLV) gencme could recombine 
with helper virus to create an infectious retrovirus depending on the cell 
type used to culture the defective genome. 
[Rapporteur's Note: Retroviruses are animal viruses with a glycoprotein envelope 
and an FNA genome that replicates through a DNA intermediate. The DNA intermediate, 
the provirus, is stably integrated into cellular DNA. 
There are a number of ways in which retroviral genomes can acquire new genetic 
information. These include reverse transcription catalyzed recombination 
events (which necessitate the co-encapsidation of different RNA species) as well 
as recombi national events involving endogenous viral sequences.] 
Dr. Mulligan pointed out that in some cases very small regions of a retroviral 
genome can be acquired by defective genomes which restore infect ivity. For 
instance, cell lines have been constructed which permit the encapsidation of 
defective recombinant genomes. These cell lines contain a proviral gencme 
which gives rise to all the viral proteins necessary for the encapsidation 
process, yet lacks the sequnce necessary for the encapsidation of its own 
genomic RNA. With some of these cell lines, transfection of constructs 
containing an intact "packaging" sequence can lead to recombi national events 
reslting in the generation of fully infectious viruses. Dr. Anderson said 
this type of rescue would depend on the cells used to grow the defective 
retrovirus. 
Dr. Mulligan said the recombination frequency of retroviruses is approximately 
10“5 under optimal conditions. A lower frequency is observed when retroviruses 
recombine with endogenous viral sequences in the cell; recombination frequency 
then depends on the abundance of endogenous complementing sequences. 
[ 443 ] 
