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The genome of the human parvovirus B19 contains a transcriptional promoter (BP06) at map position 6, upstream from the nonstructural protein genes. By cotransfecting HeLa cells with this promoter cloned before the chloramphenicol acetyltransferase (CAT) gene together with a plasmid containing almost the whole B19 genome, we showed that BP06 is transactivated by a B19 gene product. The transactivating viral protein was identified as the nonstructural protein NS-1. NS-1 synthesized in a wheat germ extract specifically stimulates transcription from BP06 in vitro. NS-1 of the minute virus of mice (MVM) activates the analogous MVM promoter, MP04. NS-1, therefore, has a positive feedback effect on the activity of its own promoter. Moreover, NS-1 of MVM activates the human BP06. We have identified, in the genome of B19, a second transcriptional promoter activity at map position 44, before the capsid protein genes. This promoter, BP44, was identified by cloning fragments of B19 DNA upstream of the CAT gene, transfecting the DNA into HeLa cells, and measuring CAT expression. The strength of the BP44 promoter is similar to that of the capsid gene promoter, MP39, of MVM. In (nonpermissive) HeLa cells, the BP44 promoter is not activated by NS-1. Thus, the BP06 promoter apparently does not determine the tissue specificity of B19 virus but BP44 could do so.

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 ad 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 site-directed 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.

Two strains of minute virus of mice (MVM) show different host cell specificities. The prototype strain MVM(p) grows in fibroblasts, whereas the immunosuppressive variant MVM(i) grows in T lymphocytes. In this study, we have mapped on the viral genome a cell type-specific determinant: it is located between 69 and 85 map units in a region coding for the viral capsid proteins. The DNA of MVM(p) does not replicate in lymphocytes. MVM(i) cannot help MVM(p) grow in lymphocytes; thus the determinant acts in a cis fashion. We did not detect viral mRNA during a restrictive infection of lymphocytes with MVM(p). However, when the same cells were transfected with cloned DNA, both MVM(p) and MVM(i) DNAs were transcribed with the same efficiency from both promoters and the RNA was processed normally. Therefore, the specificity determinant is not a cell type-specific enhancer.