Transcription and expression of the herpes simplex virus tk gene inserted into proviral sequences of feline leukemia virus.

Recombinant DNA molecules containing the herpesvirus tk gene inserted near the middle of a cloned feline leukemia virus proviral genome, in the same transcriptional orientation as the long terminal redundancies (LTRs), were used to transform human tk- cells. Analysis of RNA from cloned lines indicates that the 5' LTR promotes a high level of transcription which, as a result of differing RNA splicing and polyadenylation pathways, results in three large, abundant RNAs, two of which contain the entire tk coding region. The tk promoter itself initiates transcription of a smaller, relatively rare tk mRNA, of the same length and abundance as found in cells transformed with the tk gene alone. Assays indicate that there is little if any thymidine kinase (TK) enzymatic activity contributed by the abundant LTR-promoted transcripts. This is presumably due to inefficient initiation of tk translation from the longer LTR-initiated transcripts because of upstream AUG codons in the viral sequences. RNA blots indicate that the viral LTR is stronger as a promoter than the tk promoter. The results also indicate that about one-third of the LTR-initiated transcripts are polyadenylated at the tk poly(A) site, while the rest use the poly(A) site of the 3' LTR.