OPPOSITE TRANSCRIPTIONAL EFFECTS OF CYCLIC AMP-RESPONSIVE ELEMENTS INCONFLUENT OR P27(KIP)-OVEREXPRESSING CELLS VERSUS SERUM-STARVED OR GROWING CELLS

The minute virus of mice, an autonomous parvovirus, requires entry of host cells into the S phase of the cell cycle for its DNA to be amplif ied and its genes expressed. This work focuses on the P4 promoter of t his parvovirus, which directs expression of the transcription unit enc oding the parvoviral nonstructural polypeptides, These notably include protein NS1, necessary for the S-phase-dependent burst of parvoviral DNA amplification and gene expression, The activity of the P4 promoter is shown to be regulated in a cell cycle-dependent manner, At the G(1 )/S-phase transition, the promoter is activated via a cis-acting DNA e lement which interacts with phase-specific complexes containing the ce llular transcription factor E2F. It is inhibited, on the other hand, i n cells arrested in G(1) due to contact inhibition, This inhibitory ef fect is not observed in serum-starved cells, It is mediated in cis by cyclic AMP response elements (CREs). Unlike serum-starved cells, confl uent cells accumulate the cyclin-dependent kinase inhibitor p27, sugge sting that the switch from CRE-mediated activation to CRE-mediated rep ression involves the p27 protein, Accordingly, plasmid-driven overexpr ession of p27 causes down-modulation of promoter P4 in growing cells, depending on the presence of at least two functional CREs, No such eff ect is observed with two other cyclin-dependent kinase inhibitors, p16 and p21. Given the importance of P4-driven synthesis of protein NS1 i n parvoviral DNA amplification and gene expression, the stringent S-ph ase dependency of promoter P4 is likely a major determinant of the abs olute requirement of the minute virus of mice for host cell proliferat ion.