Three spliced mRNAs of TT virus transcribed from a plasmid containing the entire genome in COS1 cells

A permuted whole-genome construct of a TT virus (TTV), named VT416, had 3,852 nucleotides (nt) 98.2% similar to the prototype TA278 genome. To allow the transcription of TTV from the internal promoter, pBK*VT416(1.3G), carrying 1.3 units of VT416, was constructed. The poly(A)(+) RNAs expressed in COS1 cells 48 h posttransfection contained three TTV mRNA species 3.0, 1.2, and 1.0 kb in length, which were recovered in the 13 DNA clones from a lambda phage cDNA library. These mRNAs in the antigenomic orientation possessed in common the 3' terminus downstream of a poly(A) signal (A(3073)ATAAA) and the 5' terminus downstream of a cap site (C(98)ACTTC). A common splicing to join nt 185 with nt 277 was detected in all mRNAs. The coding region of the largest open reading frame (ORF) was maintained in 3.0-kb mRNA, because this splicing was located upstream of its initiation codon (A(589)TG). The second splicing was detected in 1.2-kb mRNA to join nt 711 with nt 2374 and in 1.0-kb mRNA to bind nt 711 to nt 2567. They linked a proposed ORF2 to another ORF for creating new ORFs over nt 2374 to 2872 in frame 2 and nt 2567 to 3074 in frame 3. The donor and acceptor sites of all three splicings matched the consensus sequence and were conserved in most of the 16 TTVs of distinct genotypes retrieved from the database. The observed transcription profile is unique to TTV among known members in the family Circoviridae.