Vertebrate parvoviruses share a similar genomic organization, with the capsid proteins encoded by genes on the right side and nonstructural proteins encoded by genes on the left side. The temporal and cell-specific appearances of these two types of gene products are regulated by a variety of genetic mechanisms. Rodent parvovirus structural proteins, for example, are encoded by a separate promoter which is positively regulated by nonstructural-gene products. In contrast, for the human B19 parvovirus, the analogous structural-gene promoter is nonfunctional, and both left- and right-side transcripts originate from a single promoter and are highly processed. Using a combination of sensitive RNA analyses of wild-type and mutant templates, we have found that the relative abundance of these alternatively processed transcripts appears to be governed by unique postinitiation events. In permissive cells, the steady-state level of right-side structural-gene transcripts predominates over that of left-side nonstructural-gene transcripts. In nonpermissive cells transfected with the B19 virus genome, nonstructural-gene transcripts predominate. Removal of 3' processing signals located in the middle of the viral genome increases transcription of the far right side. Disruption of a polyadenylation signal in this region makes readthrough of full-length right-side transcripts possible. These results suggest that the abundance of B19 virus RNAs is determined by active 3' processing and is coupled to DNA template replication
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