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.

References:
[1].  Zandomeni, R.O. Kinetics of inhibition by 5,6-dichloro-1-β-D-ribofuranosylbenzimidazole on calf thymus casein kinase II. Biochemistry Journal 262, 469-473 (1989).
[2].  Yankulov, K.,Yamashita, K.,Roy, R., et al. The transcriptional elongation inhibitor 5,6-dichloro-1-β-D-ribofuranosylbenzimidazole inhibits transcription factor IIH-associated protein kinase. The Journal of Biological Chemisty 270(41), 23922-23925 (1995).
[3].  Rickert, P.,Corden, J.L., and Lees, E. Cyclin C/CDK8 and cyclin H/CDK7/p36 are biochemically distinct CTD kinases. Oncogene 18, 1093-1102 (1999).
[4].  Schang, L.M. Cyclin-dependent kinases as cellular targets for antiviral drugs. Journal of Antimicrobial Chemotherapy 50, 779-792 (2002).
[5] Sehgal P B, Darnell J E, Tamm I.  The inhibition of DRB (5, 6-dichloro-1-β-d-ribofuranosylbenzimidazole) of hnRNA and mRNA production in HeLa cells[J]. Cell, 1976, 9(3): 473-480.
[6] Tamm I, Tyrrell D A J.  Influenza virus multiplication in the chorioallantoic membrane in vitro: kinetic aspects of inhibition by 5, 6-dichloro-1-β-D-ribofuranosylbenzimidazole[J]. The Journal of experimental medicine, 1954, 100(6): 541.
[7] Stevens A, Maupin M K.  5, 6-Dichloro-1-β-d-ribofuranosylbenzimidazole inhibits a HeLa protein kinase that phosphorylates an RNA polymerase II-derived peptide[J]. Biochemical and biophysical research communications, 1989, 159(2): 508-515.