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5, 6-dichloro-1-β-D-ribofuranosylbenzimidazole (DRB) is a transcriptional elongation inhibitor.

Cyclin-dependent kinases (CDKs) belong to a family of protein kinases involved in regulating the cell cycle, transcription, mRNA processing, and the differentiation of nerve cells.[1] They are expressed in all known eukaryotes. With cyclin, CDK shows kinase activity. CDKs are serine-threonine kinases [1].

DRB inhibited the activity of several carboxyl-terminal domain (CTD) kinases including casein kinase II, Cdk7, Cdk8, and Cdk9 with the IC50 of 4-10 μM, ~20 μM, ~20 μM, and 3 μM) [2-5]. In HeLa cells, DRB (75 μM) inhibited 60-75% of nuclear heterogeneous RNA (hnRNA) synthesis. DRB (75 μM) reduced the appearance of labeled cytoplasmic poly(A)-containing messenger RNA (mRNA) by approximately 95%. DRB inhibited the initiation of hnRNA chains, but did not directly interfere with labeling of poly(A) [6]. DRB inhibited influenza virus multiplication in the chorioallantoic membrane in vitro [7]. DRB inhibited a HeLa protein kinase whihc phosphorylated an RNA polymerase II-derived peptide [8]. DRB can also inhibit HIV transcription (IC50 = ~4 μM) by targeting elongation enhanced by the HIV-encoded transactivator Tat.

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