ABSTRACT
We constructed and analyzed a series of deletion mutants in the noncoding regulatory region of tsa polyomavirus DNA to identify some of the sequences critical to the DNA replication origin and to the expression of the viral early genes in vivo. By using both transient and long-term assays under conditions where the influence of large T antigen (T-Ag) in replication or autoregulation was minimized, we observed no more than a 30% reduction in early gene expression upon removal of the CAAT or TATA elements or both. These assays demonstrated a predominant effect of upstream promoter or enhancer elements and indicated that removal of the CAAT or TATA boxes did not significantly affect viral early gene expression. Studies on the replicative ability of these mutants in mouse cells constitutively expressing the polyoma early proteins revealed that the removal of DNA sequences contained within a previously identified T-Ag high-affinity binding site (nucleotides 39 to 64) abolished viral DNA replication, whereas removal of two other high-affinity sites, closer to the early mRNA cap sites, did not. Furthermore, a deletion including this same high-affinity site plus a low-affinity binding site within the 32-base-pair palindrome of the origin core sequences eliminated the ability of the viral large T-Ag to efficiently repress early gene transcription. It is thus possible that the origin-proximal high-affinity T-Ag binding site is involved in both of the functions of large T-Ag, i.e., the initiation of viral DNA replication and the autoregulation of early gene transcription.
