Laboratory University of Wisconsin Madison, Wisconsin
I have been asked to write a short article on some biological aspects
of R NAdirected DNA polymerase activity that are relevant to the
theme of this workshop. Because I was not able to attend the workshop,
I shall write a general article without data or specific literature
references. At the end of the article, I have listed some of my
articles which give detailed references. Why are RNA tumor viruses
and RNA-directed DNA polymerase activity considered relevant to
the etiology of human leukemia? In animals, especially in mice and
chickens, much of the leukemia is caused by RNA tumor viruses. Furthermore,
RNA tumor viruses and other viruses with apparent RNA-directed DNA
polymerase activity are widely distributed in some normal animals
and have been isolated from many animals, although not yet from
man. It appears to be a reasonable working hypothesis that RNA tumor
viruses have some relevance to the etiology of human leukemia.
RNA tumor viruses are characterized by a medium-sized, enveloped
virion with a characteristic morphology and a density in sucrose
of about 1.16 g/ cm³ .Infectious RNA tumor virus virions contain
a 60-70S RNA complex and a DNA polymerase. The genome of an infectious,
strongly transforming RNA tumor virus, such as Rous sarcoma virus,
contains genes for virion structural proteins, also sometimes called
the virogene, which includes genes for two external glycoproteins,
three or more internal proteins, and a DNA polymerase. The internal
proteins and the DNA polyme rase have group-specific antigenicity.
The genes for neoplastic transformation are often called the oncogene
and may consist of at least four complementation groups. Thus, the
genome of an infectious, strongly transforming RNA tumor virus,
whether as virion RNA or as DNA provirus, equals the virogene plus
the oncogene. (There are also non-transforming RNA tumor viruses
that do not have an onco gene.)
RNA tumor virus replication involves attachment of the infecting
virus to recep tors of a sensitive cell, entrance into the cell
and uncoating, synthesis and integration of the DNA provirus, activation
of RNA transcription by normal passage of the infected cell through
the replicative cell cycle, synthesis of virus-specific RNA and
protein, formation of the virion core, and formation of the virion
by budding from the modified cell plasma membrane.
The major descriptive questions about RNA tumor viruses are what
genes exist in DNA proviruses in normal and neoplastic cells and
what products of these genes are formed in normal and neoplastic
cells. The major genetic questions are what is the origin of these
genes and what determines which products are formed.
The virion RNA-directed DNA polymerase has provided a way to answer
some of the descriptive questions. The DNA product of RNA-directed
DNA polymerase activity has allowed synthesis of radioactive DNA
products, which can be used in nucleic acid hybridization experiments
to find DNA and RNA sequences related to RNA tumor virus RNA. (Similar
experiments have been done with cellular messenger RNA' s. ) Viral
RNA has also been used in nucleic acid hybridization experiments
to find cellular DNA sequences related to RNA tumor virus RNA. Limitations
of this approach are the necessity to start with RNA of a known
RNA tumor virus and the apparent lack of nucleotide sequence homology
between the RNA from different RNA tumor viruses, even among those
from different groups growing in the same cells ( for example, avian
leukosis viruses and reticuloendotheliosis viruses) and among those
from antigenically related viruses (for example, mouse and feline
A similar requirement for a known RNA tumor virus exists when looking
for most of the virion structural proteins. Only known proteins
or proteins related to known ones can be studied. However, RNA-directed
DNA polymerase activity is characteristic of RNA tumor viruses and
thus can be looked for without starting with a known RNA tumor virus.
A limitation in the last approach is that endogenous RNA-directed
DNA polymerase activity with the biochemical characteristics of
RNA tumor virus RNAdirected DNA polymerase activity has been found
in normal, uninfected chicken embryos and in normal, uninfected
chicken fibroblasts and amnion cells in culture. This activity is
unrelated to known viruses and has no other properties of a virus.
It was isolated from apparently uninfected cells, which are not
producing virus. Therefore, without an infectious human RNA tumor
virus, it may be hard to answer the descriptive questions about
possible proviruses and their expression in normal and neoplastic
While there are fairly complete answers to some of the descriptive
questions about RNA tumor viruses, the genetic questions about RNA
tumor viruses are still unanswered in animals where many RNA tumor
viruses are known. The current hypotheses regarding the occurrence
of oncogenes are that oncogenes exist a) only in neoplastic cells
or b) in normal and neoplastic cells. The hypotheses as to the origin
of oncogenes are that they arise a) from the provirus of an infectious,
strongly transforming RNA tumor virus, b) from the vertically transmitted
provirus of a strongly transforming RNA tumor virus, c) from variational
processes involving normal cellular elements called protoviruses,
or d) from mutation of cell genes.
Much more work will have to be done to clarify these questions in
animals before we can hope to shed much light on the etiology of
human neoplasia. A further sobering thought is that, although RNA
tumor viruses have been known since the early 1900's to cause some
chicken leukemia, effective means of prevention or treatment of
this leukemia are still unknown.
Temin, H. M., and Baltimore, D. RNA-directed DNA synthesis and
RNA tumor viruses. Advan. Virus Res. 17: 129-186, 1972.
Temin, H. M., C.- y .Kang and s. Mizutani.: RNA-directed DNA synthesis
in viruses and cells. In "Possible Episomes in Eukaryotes. Proceedings
of Fourth Lepetit Colloq." L. Silvestri (ed.). North-Holland Publ.
Co., Amsterdam. 1973.
Temin, H. M. : The cellular and molecular biology of RNA tumor viruses,
especially avian leukosis-sarcoma viruses, and their relatives.
Advan. Cancer Res. 19, 1973.