DNA intercalator stimulates influenza transcription and virus replication

The C-terminal domain (CTD) of RNAPII is important for cellular mRNA transcription, and interacts with several post-transcriptional factors for RNA maturation and nuclear export. The phosphorylation status of CTD is known to be a critical regulatory checkpoint for RNAPII transcription [1]. The hyperphosphorylated (transcriptionally engaged) form of RNAPII is designated as RNAPIIo, whereas its nonphosphorylated (transcriptionally inactive) form is designated as RNAPIIa. At the early stage of transcription, free RNAPIIa interacts with other general transcription factors on cellular DNA promoters to form a transcription pre-initiation complex, which is followed by transcription initiation [2]. The newly initiated RNAPIIa then proceeds to the promoter-proximal pause region, and the paused RNAPIIa is subsequently hyperphosphorylated, preferably on the serine 5 (Ser5) positions, by cyclin-dependent kinase (Cdk) 7. As transcription elongation proceeds, the serine 2 (Ser2) and Ser5 positions in the CTD of RNAPII are hyperphosphorylated by Cdk9 [3] and dephosphorylated by SCP1 [4], respectively. The Ser5-phosphorylation helps to recruit enzymes to cap the nascent RNA transcript, whereas the Ser2-phosphorylation facilitates the conversion of RNAPII into a productive elongating form.Influenza viral RNA synthesis is dependent on its host transcription machinery. Various RNAPII inhibitors such as α-amantin and actinomycin D (ActD) have been shown to inhibit influenza virus replication [5-7]. Chan et al. demonstrated that the influenza viral polymerase complex can inhibit RNAPII transcription elongation, but not initiation [8], a phenomenon that is similar to the transcriptional arrest of RNAPII. This transcriptional arrest may be related to direct interaction between vRNP and Ser5-phosphorylated RNAPIIo [9]. It has also been demonstrated that a robust polymerase complex is more capable of binding to RNAPIIo [10]. Recently, influenza viral polymerase has been proposed to induce t