DNA-dependent RNA polymerases (EC 2.7.7.6) are ubiquitous enzymes necessary for the transcription of genomic DNA into RNA. Most RNA polymerases are multimeric enzymes and are composed of a variable number of subunits. However the two largest subunits (generally known as α and β chains) are present in all known multimeric RNA polymerases. The β chain is currently known in:

• Eubacterial RNA polymerase β chain (gene rpoB).
• Plants and algal chloroplast RNA polymerase β chain (gene rpoB).
• Eukaryotes RNA polymerases I, II and III second largest subunits.
• Archebacterial RNA polymerase subunit B (gene rpoB) or B' (gene rpoB1).
• Poxviruses RNA polymerase 132 Kd subunit (gene RPO132).
• Baculoviruses RNA polymerase subunit LEF-8.
• African swine fever virus RNA polymerase subunit EP1242L.
• Fungal plasmid-encoded RNA polymerases.

The β chains are proteins of from 900 to 1400 amino acid residues. A well conserved region of 13 residues that contains two conserved lysines and which is thought to be part of the active site is located in the C-terminal part of all β chains [1]. We have used this region as a signature pattern.