We previously described use of the human parvovirus, adeno-associated virus (AAV), as a vector for transient expression in mammalian cells of the gene for chloramphenicol acetyltransferase (CAT). In the AAV vector, pTS1, the CAT gene is expressed under the control of the major AAV promoter p4o. This The human parvovirus, adeno-associated virus (AAV), replicates in mammalian cells in the presence of helper functions provided by adenovirus or herpesvirus (10). When molecular clones containing the entire AAV type 2 (AAV2) genome in bacterial plasmids are transfected into mammalian cells in the presence of adenovirus, the AAV genome is excised from the plasmid and replicated to produce infectious AAV particles (38,50,52). This facilitated both the genetic analysis of AAV (27,(51)(52)(53)59) and the development of AAV as a eucaryotic expression vector (26,60,61).The AAV genome contains three transcription promoters, P5, Pi9, and P40, which yield overlapping transcripts (11). Genetic analysis showed that a major AAV reading frame (orf-2) which is accessible from P40 transcripts codes for a major portion of the AAV capsid proteins (27,59). Mutations in this region (cap-) allow normal synthesis of duplex replicating form DNA but prevent synthesis of progeny single-stranded DNA and infectious particles (27,53,59). A second major open reading frame (orf-1) in the left half of the genome is apparently accessible from either p5 or P19 transcripts and codes for one or more proteins (rep) required for AAV DNA replication (27,53,59). Mutations in this region (rep-) are DNA negative but can be complemented. Deletion of both terminal palindromes results in a cis-dominant (ori-) defect which reflects the presence of the AAV replication origins in the terminal repeat sequences (3,4,25,51,53).
