5 
i 
Dr. Landy asked whether retroviruses carry gratuitous qenes, i.e., genes non- 
essential for virus replication. Dr. Mulligan replied they could, but these 
constructs are usually defective viruses. Retroviruses frcm chickens and a 
specialized class of retroviruses are exceptions. Rous sarcoma virus, for 
exanple, is replication competent yet carries in addition the src oncogene. 
The Human T-Lympho tropic Virus (HTLV) class of retroviruses carry a novel gene 
in addition to the standard viral gag , pol and env genes, although it is unlikely 
this gene is "gratuitous." In general, however, even though reccmbination 
occurs at high frequency in retroviruses, it is not likely the resulting 
virus would contain gratuitous genetic information because genes in the coding 
region of retroviruses cannot be easily substituted and infectious particles 
still produced. 
[Rapporteur's Note: There appears to be a maximum limit on the size of the 
RNA in infectious retrovirus virions. The molecular basis for this size limit 
is unknown. It could be related to problems of packaging viral FNA in the 
isocahedral core or to problems in reverse transcription or both. Typically 
in constructing a vector, the gene of interest replaces essential virion genes 
and renders virus replication defective. Added genes can be readily eliminated 
by spontaneous deletion. Same replication-defective, larger than normal 
constructs, are possible. 
In the special case of the Rous sarcoma virus, the virus adds the oncogenic 
src gene to the essential virion genes without disrupting vital functions. In 
all other kncwn cases, the inclusion of cellularly-derived transforming sequences 
is associated with loss of some or all of the coding regions found in replication- 
competent retroviruses. Thus, these modified viruses are replication-defective. 
Such viruses are generated at a very low frequency in nature.] 
Dr. Landy asked whether host cells can provide helper functions for retroviruses 
carrying foreign genes or if the helper function must also be transferred. 
Dr. Mulligan said specialized cell lines have been constructed that can provide 
all the viral functions necessary for encapsidation and transmission of recombinant 
genomes. The virus produced in these cell lines can only go through one replication 
cycle. The modified retrovirus might continue through a second round of replication 
in other cells if it can encounter and infect cells which possess the necessary 
helper function. 
Dr. Martin asked if DMA viruses such as SV40 can be rescued in tissue culture 
systems. Dr. Joklik replied they can be rescued but at very lew frequencies. 
Dr. Gottesman asked whether reccmbinants from tissue culture experiments could 
pose a danger to laboratory workers even though the possibility of rescue is 
low in experiments involving one-third of a viral genome. Would the virus 
infect human cells? Could the virus recombine with endogenous sequences and 
would the resulting virus be pathogenic? 
Dr. Anderson thought seme concern might exist for work with human cell lines. 
He did not think laboratory personnel would be endangered by work with cell 
lines other than human cell lines. 
[444] 
