GENE REGU~TION AND ONCOGENESIS BY ~TROVIRUSES* Harold E. Varmus,
Department of Microbiology & Immunology, University of California,
San Francisco, CA 94143.

  Retroviral DNA is equipped with long terminal repeats (LTRs)
that can influence expression of both viral and cellular genes.  I

will review our several efforts to define the biochemical basis
and biological consequences of these regulatory events.
(1) Determination of the nucleotide sequences of LTRs reveals sig-

nals and sites believed to affect initiation and polyadenylation af
viral RNAs.
(2)  The functional capacities of LTRs have been studied by con-
struction of recombinant plasmids, containing LTRs and cloned genes
in various arrangements, followed by their introduction into cul-

tured cells. As expected, only an LTR in the correct transcription-
a1 orientation upstream from a promoter-less gene can produce ex-
pression of that gene. However, whcn thc gene carries its own pro-
moter, the efficiency of DNA transfor~ati~n is cnhanccd many-fold
by the presence, in either orientation, of certain LTRs (e.g. that
of Rous sarcoma virus (RSV)), suggesting that some LTRs may en-
hance genc expression at distant, hctcrologous promoters in a non-
polar fashion, The LTR of mousc m~ary tumor virus (~V~ governs
expression of viral or linked cellular genes in a glucocorticoid-
responsive manner; the region responsible for hormonal regulation
has been localized to ca. 0.4 kb near the 3' end of the MMTV LTR.

(3) The induction of B-cell lymphomas by avian leukosis virus (ALV)
appears to require insertion of proviral DNA near -- c-myc, a putative
cellular oncogene. The viral inserts, sometimes comprising little
more than a single LTR, can enhance expression of c-myc in at  least
three arrangements: when positioned upstream from c-myc in the same
transcriptional orientation, the inserts serve as promoters for
transcription of -- c-myc; when positioned upstream in the opposite
orientation to c-myc or downstream in the same orientation, they
enhance expression from unidentified cellular promoter(s).

--

--

--

2.

In the last instance, the LTR provides a
adenylation. Interrupted - c-E loci have
DNA and reintroduced into cultured cells
of the enhancement phenomena.

nove,l site for poly-
been cloned from tumor
to identify determinants

(4) MMTV, like ALV, lacks a viral oncogene, yet regularly induces

tumors. By cloning the integration site from a mammary carcinoma
bearing a single Mt4W provirus, we have identified a 20 kb region
of the mouse genome in which proviruses are frequently found in

tumors. Adjacent transcriptional domains that may be affected by
these insertions are currently being sought.
(5) The activity of LTRs appears to be influenced by each other
and their chromosomal context. To understand these influences, we
are exploring the following situations: (a) a murine leukemia
virus LTR, placed within a transcriptionally-active RSV provirus
by infection, does not polyadenylate or initiate transcripts; (b)
ostensibly identical llSV proviruses at different chromosomal
sites display differential stability of gene cxprcssion; (c) cx-
pression of one such RSV provirus is successively diminished and
enhanced at high frequency; (d) expression of another RSV provirus
is affected only by rare spontaneous deletions which remove the 5'

LTR.