Published
in: "Embryonic and Fetal Antigens in Cancer",
vol. 2, pp. 5-7, (Anderson NG, Coggin JH, eds.), National Technical
Information Service, U.S. Dept. Commerce, Springfield, VA., 1972:
"Mated Models of Gene Regulation in Eukaryotes".
Paul R. Herstein and John H. Frenster.
Division of Oncology, Department of Medicine, Stanford University
School of Medicine,
Stanford, California 94305.
Introduction:
SV40-Transformed Cells:
DNA-Transfected Cells:
Murine Leukemia RNA:
Gene Regulation in Eukaryotes:
Mated Model of Gene Regulation:
Origin of De-Repressor
RNA Species:
Transfer
of De-Repressor RNA to Daughter Cells:
DNA
Strand Separations during Gene Transcription:
Support:
References:
Other Sites:
Feedback:
The re-expression of embryonic information
within the adult neoplastic cell promises not only to yield important immunologic
antigens for the diagnosis and therapy of the neoplastic state (1),
but also to provide unique biologic data regarding normal embryogenesis,
gene regulation, and RNA synthesis (3). Both viral and
chemical carcinogens regularly induce re-expression of embryonic information,
and by such induction can be utilized as defined and specific probes of
normal gene regulation (13).
Mammalian cells transformed
by SV40 virus regularly re-express normal embryonic transplantation anigens
(5). After SV40 virus transformation of such cells,
SV40 is linked to host-cellular DNA in an alkali-stable association (26),
but can be rescued as infectious SV40 virus from the virus-free transformed
cells by fusion with permissive cell lines (23). Host
cells abortively transformed by SV40 virus appear to possess SV40 in looser
association with host-cellular DNA (27), and may fail
to express embryonic transplantation antigens even though still possessing
the complete SV40 genome in multiple copies (27). These
data indicate that the presence of the complete viral genome may be a necessary
but not sufficient condition for the re-expression of these embryonic antigens
(27); they also suggest that the physical details of
the non-covalent association between SV40 DNA and host-cellular DNA may
determine whether or not host DNA is de-repressed and embryonic antigens
re-expressed (13). In this viral-induced neoplasm,
SV40 DNA may be playing the role normally played by de-repressor RNA in
activating specific gene loci by non-covalent association with the anti-coding
strand of DNA at the particular gene loci (14).
Similarly, the addition of allospecific
Drosophila
melanogaster DNA to young D. melanogaster embryos results in
a transformation of the host cell phenotype that is restricted, specific,
and unstable (10), and which suggests a non-covalent
physical association between the donated DNA and particular sequences of
the host-cellular DNA (10).
Finally, the restricted host range
for expression of the group-specific antigens associated with cellular
transformation by murine leukemia viruses indicates that specific host
DNA sequences are required for the expression of group-specific antigens;
it also suggests both that the antigens are coded on the cellular genome
and that a non-covalent physical association is occurring between the viral
RNA and/or DNA and particular target sequences of host-cellular DNA which
are de-repressed to allow expression of the group-specific antigens (29).
These experimental data
indicating the ability of exogenous DNA or RNA to de-repress specific loci
in the host-cellular genome suggest a close relationship to the normal
mechanisms of gene regulation in animal cells (14),
which may be subverted to allow re-expression of otherwise repressed embryonic
information (13). A comparison of molecular models
of gene regulation indicates that the operon model of gene regulation in
bacteria (21) cannot be applied directly to more complex
organisms (25). Eukaryotic organisms utilize species
of nuclear RNA to direct transcription of specific gene loci (7,
11,
20,
22),
overcoming the gene repression imposed by polycationic histones which are
not found in bacteria (2). The de-repressor model of
such gene regulation in eukaryotes (14) accounts for
selective gene transcription that is locus- and strand-specific, but does
not discuss gene-gene interactions. The reiterated-sequence model (4)
accounts for repeated DNA sequences and the high turnover rates of nuclear
RNA in eukaryotes, but it ignores both molecular mechanisms and feedback
elements found in normal gene doasge compensation (17,
19).
Recent data concerning cleavage of newly-synthesized nuclear RNA (8,
18)
now suggest a posible complementarity among these models of gene regulation
which can be used in their fruitful mating Fig. 1).
Fig.
1. Mated Model of Nucleic Acid Interactions During Gene Regulation in
Eukaryotes. De-repressor RNA (dRNA) binds to the anti-coding strand
(anti-o) of operator locus, permitting transcription of operator (o) and
structural gene (sg) loci. The de-repressor RNA of a given operon is complementary
in base sequence to the operator portion of the direct transcription product.
The direct transcription product is cleaved to form messenger RNA (mRNA)
and operator RNA (oRNA). Cleavage can occur directly or after formation
of heterometric duplex RNA by base-pairing of de-repressor RNA with the
operator portion of the transcription product. By such duplex formation,
de-repressor RNA is selectively removed from the operator locus, thus providing
feedback inhibition of transcription of the operon. With consumption of
messenger RNA and with degradation of operator RNA, de-repressor RNA is
released from the duplex, thus providing positive feedback de-repression
of transcription of the operon. Different structural genes may share operators
with common base sequences, and thus be equally sensitive to given species
of de-repressor RNA, both during gene transcription and during its selective
inhibition.
The recent studies
of heterogenous nuclear RNA indicate its role as a precursor of messenger
RNA via cleavage of the direct transcription product (8,
18).
Other recent studies indicate that the direct transcription product is
often in a partially duplex RNA state before its cleavage (28).
The time and mode of origin of de-repressor RNA species are not as yet
determined, but may include
(1) universal transcription during oogenesis of all anti-coding
DNA strands in the genome, with asymmetric tissue distribution among mitotic
daughter cells during subsequent embryogenesis (15,
16).
(2) tissue-selective transcription during adult life on non-contiguous
regulatory genes (4), and
(3) the introduction of viral RNA or DNA which can mimic host-cellular
de-repressor RNA and by such mimicry subvert the gene regulatory system,
inducing inappropriate gene de-repression and re-expression of embryonic
information (3, 13).
Finally,
in considering the mated model of gene regulation (Fig. 1), it should be
noted that the long unpaired RNA strand of heterometric duplex RNA provides
a molecular mechanism for base-pairing of the RNA duplex to structural
gene loci during cell division, thus providing a mechanism for ensuring
that all de-repressor RNA species of a dividing cell are equally and symmetrically
distributed to both daughter cells. Such symmetrical distribution during
adult life would appear to be necessary to account for the long-term stability
of the tissue-specific and x-chromosome-specific gene expression patterns
observed in multiple successive generations of cloned adult eukaryotic
cells in vitro (9, 12).
The
existence of DNA strand separations during seletive gene transcription
(12) as indicated in Fig. 1 has recently received additional
theoretical (6) and experimental (24)
support.
Support:
Supported in part by grants CA-10174 and AM-01006 from the National
Institutes of Health, and by a Research Scholar Award from the Leukemia
Society.
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During Selective Transcription and Asynchronous Replication", in: "The
Cell Nucleus", (Klouwen HM, ed.), London: Taylor and Francis, Ltd., 1966,
pp. 27-46.
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of Heterochromatin and Euchromatin", In: "Handbook of Molecular Cytology",
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Additional References:
0. Blumenthal T, Evans D, Link CD, Guffanti A, Lawson
D, Thierry-Mieg J, Thierry-Mieg D, Chiu WL,
Duke K, Kiraly M, and Kim SK, "A Global
Analysis of Caenorhabditis elegans Operons". Nature vol. 417,
no. 6891, pp. 851-854 (June 20, 2002).
1. Frenster JH, and Herstein PR, "Gene De-Repression",
New
Eng. J. Med. 288: 1224-1229 (1973).
2. Frenster JH, "Selective Control of DNA Helix
Openings during Gene Regulation", Cancer
Res. 36: 3394-3398 (1976).
3. Frenster JH, "Selective Gene De-Repression by
De-Repressor RNA", in: "Eukaryotic Gene Regulation",
vol. 1, pp. 131-143 (1980), ed: Kolodny GM, CRC Press, Boca Raton, FL,
USA.
4. Frenster JH, "Activation of DNA Transcription
within Repressed Chromatin", 14th John Innes Symposium, "Chromosome
Dynamics and Expression", p. 51, (2001).
5. Frenster JH, "Yeast RNA Re-Programming of Already-Active
Mammalian Chromatin", RNA 2002, p. 592, The RNA
Society, 2002, Bethesda, MD, USA.
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