It is almost twenty years since the first DNA tumor virus meeting was
held at Cold Spring Harbor. At this meeting studies on three tumor
viruses were discussed: the papovaviruses, the adenoviruses and the
herpesviruses. The present series Developments in Molecular Virology
chose to reverse this sequence by first publishing books on the
herpesviruses, followed by adenoviruses, and only now the papo-
vaviruses. All the DNA tumor viruses gained their original reputation by
serving as model systems in animal cells for studying gene expression
and gene regulation, but SV40 and polyoma have been the jewel in the
crown in these studies, as A phage was for the study of prokaryotes.
SV40 was the first DNA tumor virus to be completely sequenced that
enabled the definition of the cis controlling elements in DNA
replication and transcription. I am continuously fascinated by the
organization of the SV40 and polyoma genomes. Although they contain
about 5000 bp that encode for only 6 to 7 proteins, the mechanisms which
regulated their gene expression are varied and include almost any other
type of gene regulation found today to regulate eukaryotic genes. Just
to mention two: (i) the early promoter is a classical promoter that
contains the TAT A, CAAT and enhancer elements, while the late promoter
is devoid of these elements, and (ii) the mRNA can be structurally and
functionally monocistronic or dicistronic. This hints at the versatility
in the control of gene expression at the transcriptional and
translational levels.
