The genome of polyomaviruses is divided into two coding regions: the e
arly and the late region. A relatively short regulatory sequence, enco
mpassing the origin of viral DNA replication (ori), separates the two
regions encoding the structural genes. In mouse polyomavirus (Py) in p
articular, the early DNA codes for three antigens: large, middle and s
mall T-antigen (L-T, M-T and S-T, respectively). Large T antigen binds
ori and thus regulates both viral DNA transcription and replication.
Middle T antigen has been shown to mediate malignant transformation in
non-permissive cells in vitro. No defined function has been assigned
to the small T antigen although this gene product is thought to act sy
nergistically both with L- and M-T antigens. The viral late region of
Py encodes also three different genes whose products form the viral ca
psid during the productive infection cycle in permissive cells. Py ear
ly region was thought to be the only part of the genome necessary to c
ode for proteins of functional and regulatory significance. The viral
late region, on the other hand, was for a long time considered a simpl
e reservoir of structural information, since it codes for capsid prote
ins and was supposedly devoid of functional control properties. This s
hort review is focused on recent works from our and other laboratories
, reporting evidence that in Py also the late region has a functional
role since late sequences are involved in the control of viral DNA rep
lication and in capsid assembly. Results indicating that this might be
true for the cognate simian virus SV40 will be also reviewed.