Mechanism of Tax-induced overactivation of replication origins. (A) Accessibility of chromatin dictates the fate of replication origins. Acetylated chromatin replicates early in S phase, whereas more condensed chromatin replicates later in S phase. Tax-mediated acetylation of late origins increases chromatin accessibility and provokes a late-to-early switch of replication timing. (B) Similarities between the mechanisms of transcriptional activation and origin firing: Tax forms complexes with transcription factors (TF) or MCM proteins (M) and p300, leading to histone acetylation at gene promoters (PROM) and replication origins (ORI), respectively.