Human herpesvirus-8 (HHV-8), also known as Kaposi's sarcoma-associated herpesvirus, encodes a protein, referred to as HHV8-Vcyc, with sequence similarity to human G1 cyclins, in particular of the D type. HHV8-Vcyc is expressed in Kaposi's sarcoma and functional analysis suggests that it can activate cyclin-dependent kinases (cdk) and thereby trigger inactivation of the retinoblastoma protein (pRb), indicating that HHV8-Vcyc may contribute to the oncogenic potential of HHV-8. We show here that HHV8-Vcyc can activate transcription of the human cyclin A gene in quiescent cells, a property shared with known transforming oncogenes. Transcriptional activation by HHV8-Vcyc depends on an E2F-binding site in the cyclin A promoter, and cdk6 kinase activity is required. The ability of HHV8-Vcyc to activate cyclin A gene expression is shared by D-type cyclins and cyclin E. Unlike D-type cyclins, HHV8-Vcyc is unable to trigger phosphorylation of the pRb-related protein p107 and fails to induce dissociation of p107 from E2F. Unlike cyclin E, HHV8-Vcyc fails to interact physically with E2F complexes on the cyclin A promoter. These results provide additional evidence for the notion that the HHV-8-encoded cyclin differs in several properties from cellular G1 cyclins.