The A enhancer of polyomavirus: Protein-protein interactions for the differential early and late promoter function under nonreplicating conditions

The A enhancer of the polyomavirus early promoter is a 110-bp domain locate d in the late region and it contains the major late RNA initiation site. Th e 'core' of this enhancer binds several cellular proteins, including protei ns PEA1, PEA2 and PEA3. Another element, NF-D/YY1, is also located in this enhancer. The A enhancer is known to stimulate the early promoter, contains auxiliary elements for replication and serves as the initiator for late tr anscription. It may also be involved in early-to-late switch. We were inter ested in investigating how the A-core- and NF-D-binding proteins regulate e arly and late promoter activity under nonreplicating conditions and how the protein-protein interactions affect the function of the individual element s. By point mutational analysis, we show that, except for PEA1, all other p roteins activate the early and late promoters differentially under nonrepli cating conditions. All three core-binding proteins, and the protein bound t o the NF-D site, are activators and have a combinatorial effect on early pr omoter activity. On late transcription, only PEA1 acts positively and inact ivation of the NF-D site is without any effect. In contrast, PEA2 and PEA3 have a repressor-like activity under nonreplicating conditions, indicating that these two proteins might be involved in repressing late transcription, probably early in infection. By increasing the spacing between two consecu tive elements, we further show that protein-protein interaction is importan t for enhancer function. Transactivation of the early promoter was affected by mutations in all four protein-binding sites. Responsiveness of these fa ctors in regard to the late promoter was parallel to their intrinsic promot er strength. The effects of middle and large T antigens are parallel for bo th the early and the late promoter, suggesting that the pathway(s) for tran sactivation function of these oncoproteins may overlap downstream. This stu dy with cloned viral promoter will be reflective of situations in vivo, at least partially, under nonreplicating conditions.