Transcription is a complex process that relies on the cooperative interaction between sequence-specific factors and the basal transcription machinery. The strength of a promoter depends on upstream or downstream cis-acting DNA elements, which bind transcription factors. In this study, we investigated whether DNA elements located downstream of the JCV late promoter, encompassing the late leader peptide region, which encodes agnoprotein, play regulatory roles in the JCV lytic cycle. For this purpose, the entire coding region of the leader peptide was deleted and the functional consequences of this deletion were analyzed. We found that viral gene expression and replication were drastically reduced. Gene expression also decreased from a leader peptide point mutant but to a lesser extent. This suggested that the leader peptide region of JCV might contain critical cis-acting DNA elements to which transcription factors bind and regulate viral gene expression and replication. We analyzed the entire coding region of the late leader peptide by a footprinting assay and identified three major regions (region I, II and III) that were protected by nuclear proteins. Further investigation of the first two protected regions by band shift assays revealed a new band that appeared in new infection cycles, suggesting that viral infection induces new factors that interact with the late leader peptide region of JCV. Analysis of the effect of the leader peptide region on the promoter activity of JCV by transfection assays demonstrated that this region has a positive and negative effect on the large T antigen (LT-Ag)-mediated activation of the viral early and late promoters, respectively. Furthermore, a partial deletion analysis of the leader peptide region encompassing the protected regions I and II demonstrated a significant down-regulation of viral gene expression and replication. More importantly, these results were similar to that obtained from a complete deletion of the late leader peptide region, indicating the critical importance of these two protected regions in JCV regulation. Altogether, these findings suggest that the late leader peptide region contains important regulatory elements to which transcription factors bind and contribute to the JCV gene regulation and replication.