Enclosed with this letter you will find copies of papers recently written by us and by Malcolm Martin's group at NIH.
As you will see from these preprints, both mouse and chick virus polymerases appear to synthesize double-stranded DNA, which
is principally of low complexity - representing less than 57. (our interpretation based on Fd RF and X standards) or about
20% (their value based on an SV 40 standard) of the viral 70S RNA genome. There is also at least one additional population
of double-stranded DNA molecules produced, this population being of greater complexity (12% of the genome in our hands, perhaps
100% in Martin's). Measuring the rate of reassociation of the large population (RR fraction, for "rapidly reassociating")
in the presence of unlabeled cell DNA's, we find about 20 copies of the RR sequences in both chick embryo DNA and in DNA
extracted from SR-RSV induced tumors. Salmon sperm DNA our viscosity control) has no copies. As you can see from their paper,
Gelb et al. also find multiple copies of their rapidly reassociating fraction in normal and transformed (including "non-producing")
rat and mouse cells, and in African green monkey kidney cells. We are about to look for the sequences in HeLa cells and may
look at other animal species as well. In addition, we are trying to determine the chromosomal location of the RR sequences
by using chick DNA extracted from chromosome populations fractionated in sucrose on the basis of size. Because the RR sequences
represent a small fraction of the viral genetic information, we are also using single-stranded DNA product (prepared from
RNA:DNA hybrids and presumably containing all the viral sequences) as a probe for viral information in normal and transformed
cells. Preliminary experiments in our lab and Martin's indicate that the transformed DNA does contain more viral sequences
than normal cells.
Of course, at this point the significance of the RR copies in normal chick cells is uncertain - we do not yet have any real
idea what the gene products of the RR DNA might be. But we are sure you will agree that it would be of particular interest
to know whether c/o' cells, which appear to lack viral antigens and cryptic RAV-60, are also lacking the RR sequences,
It is possible, of course, that we are not yet using the best RR probe, since ours is synthesized by SR-RSV polymerase; we
would be interested in your opinion about the feasibility of preparing sufficient amounts of RAV 60 to use for RR synthesis.
We consider the biochemical experiments we are suggesting with c/o' cells to be principally attempts to extent your biological
findings and Hanafusa's from another point of view. We are very pleased that you are willing to assist us in these experiments
by sending us the cells and hope we will soon be in touch about questions or results.