10         20         30         40         50         60
         |          |          |          |          |          |
MAGILFEDIF DVKDIDPEGK KFDRVSRLHC ESESFKMDLI LDVNIQIYPV DLGDKFRLVI - 60
ASTLYEDGTL DDGEYNPTDD RPSRADQFEY VMYGKVYRIE GDETSTEAAT RLSAYVSYGG - 120
LLMRLQGDAN NLHGFEVDSR VYLLMKKLAF

The three eukaryotic RNA polymerases (RNAPs) consist of a number of subunits and play central roles in the transcription of nuclear genes. Transcription is regulated by protein interactions between the subunits and/or transcription factors during the initiation, elongation, and termination processes. In eukaryotic cells, five subunits (RPB5, RPB6, RPB8, RPB10alpha and RPB10beta) are conserved in all the three RNAPs, which play essential roles for cell viability. RPB8 (hRPB8 or hRPABC17 in human) is a common subunit in all the three types of eukaryotic RNAPs, but is absent in both prokaryotes and archaebacteria, suggesting that it represents a unique feature in the eukaryotic transcription machinery. This subunit is highly conserved throughout the evolution and plays an essential role in eukaryotic cells. Although little is known about the exact function of the RPB8 subunit in transcription, the fact that RPB8 is one of the small subunits present in all three types of eukaryotic RNAPs suggests that it might be important for common characteristics shared by all RNAPs such as transcriptional efficiency, nuclear localization, enzyme stability, or coordinate regulation of rRNA, mRNA, and tRNA synthesis (Kang et al. 2006). DNA-dependent RNA polymerase catalyzes the transcription of DNA into RNA using the four ribonucleoside triphosphates as substrates. Common component of RNA polymerases I, II and III which synthesize ribosomal RNA precursors, mRNA precursors and many functional non-coding RNAs, and small RNAs, such as 5S rRNA and tRNAs, respectively.