Targeting of CDK8 to a promoter-proximal RNA element demonstrates catalysis-dependent activation of gene expression

During transcription of mRNA genes, there is a correlation between the phosphorylation state of the C-terminal domain (CTD) of the large subunit of RNA polymerase II (RNAP II) and the ability of the RNAP II complex to processively transcribe the gene. To examine the involvement of CTD phosphorylation in modulation of RNAP II function, we have analyzed the ability of a known CTD kinase, human Cdk8, to modulate HIV-1 LTR-driven gene expression upon directed targeting to a promoter-proximal nascent RNA element. The results indicated that Cdk8, when localized to an RNA element, activates gene expression in a catalysis-dependent manner. Also, Cdk8 targeted to RNA was observed to act in a synergystic manner with DNA-targeted Sp1 but not with DNA-targeted HIV-1 Tat, suggesting that RNA-targeted Cdk8 acts on similar rate limiting post-initiation events as Tat. As recent observations suggest that Tat/TAR-mediated transcription of the proviral genome of HIV depends on specific phosphorylation of RNAP II in its CTD by the Tat-associated kinase (TAK/p-TEFb/Cdk9), our results indicate that Cdk8 shares with Cdk9 the ability to modulate transcription upon targeting to a nascent RNA element.