Efficient synthesis of adeno-associated virus structural proteins requires both adenovirus DNA binding protein and VA...

We have shown previously that replication of defective parvoviruses [adeno-associated viruses (AAV)] requires several early adenovirus (Ad) gene products [J. E. Janik, M. M. Huston, and J. A. Rose (1981) Proc. Natl. Acad. Sci. USA 78, 1925-1929]. To examine their possible roles in the transcription and translation of AAV mRNA, 293-31 cells, a human embryonic kidney cell line that constitutively expresses the Ad early region IA and IB gene products, were transfected with a pBR325 plasmid (pLH1) that contains a duplex AAV2 DNA segment (0.03-0.97 map units) which encompasses the promoters and coding sequences necessary for expression of all AAV polypeptides. When cells were transfected with pLH1 alone, both spliced and unspliced AAV-specific cytoplasmic RNAs accumulated. These transcripts were capable of directing synthesis of the three AAV capsid polypeptides in vitro, whereas in vivo synthesis of AAV protein was not detected by immunofluorescence or immunoprecipitation. When cells were cotransfected with pLH1 and intact Ad DNA, the level of cytoplasmic AAV RNA was enhanced and AAV protein was synthesized in vivo. Additional experiments demonstrated that in vivo AAV protein synthesis also could be induced when pLH1 was cotransfected with plasmids that contain the Ad DNA-binding protein (pDBP) and VA I RNA (p2BalM) genes; however, a low level of in vivo AAV capsid protein was occasionally detected in cotransfections with pLH1 and a plasmid that contains both VA I and VA II RNA coding sequences (p2SalC). Cotransfection of pLH1 and pDBP or pLH1 and p2SalC showed complex alterations in the steady-state patterns of AAV cytoplasmic transcripts. In both cases, increased levels of transcripts, particularly the 2.3-kb spliced species, were detected in comparison to levels seen in cells transfected with pLH1 alone. Despite these increases, however, there was little, if any, induction of AAV protein synthesis unless both the DNA-binding protein (DBP) and VA I RNA coding sequences were present in cotransfection with pLH1. We conclude that, in 293-31 cells, the Ad VA I RNA and DBP gene products regulate AAV capsid protein synthesis at least at two levels: (i) by increasing the steady-state levels of structural protein transcripts in the cytoplasm, especially the spliced species, and (ii) by enhancing the translation of these messages.