The genome of the autonomous parvovirus minute virus of mice (MVM) is organized
in two overlapping transcription units: the genes coding for the two non-structural
proteins (NS-1 and NS-2) are transcribed from a promoter (P04) located at map unit 4,
whereas the promoter controlling the capsid protein genes (P39) lies at map unit 39. We
studied the effect of viral proteins on the activity of the P39 promoter in vivo. By sitedirected
mutagenesis we constructed clones encoding only one of the two NS proteins.
The activity of the P39 promoter was measured in HeLa or EL-4 cells transfected with
these clones, either by an RNase protection assay or by following the expression of a
reporter gene, CAT (which codes for chloramphenicol acetyltransferase), placed under
the control of this promoter. We found that the P39 promoter of strain MVMi is
activated in trans by a viral gene product, and evidence to suggest that NS-1 is the only
viral gene product responsible for this trans-activation. We also determined that the
mechanism of trans-activation is very rapid, since all species of viral mRNAs appear
together in non-synchronized infected EL-4 cells within a 2 h interval.