Bacterial RNA polymerase subunit ω and eukaryotic polymerase subunit RPB6 are sequence, structural, and functional homologs and promote RNA polymerase assembly

Bacterial DNA-dependent RNA polymerase (RNAP) has subunit composition β′βα^Iα^II. The role of ω has been unclear. We show that is homologous in sequence and structure to RPB6, an essential subunit shared in eukaryotic RNAP I, II, and III. In Escherichia coli, overproduction of ω suppresses the assembly defect caused by substitution of residue 1362 of the largest subunit of RNAP,β′. In yeast, overproduction of RPB6 suppresses the assembly defect caused by the equivalent substitution in the largest subunit of RNAP II, RPB1. High-resolution structural analysis of the ω-β′ interface in bacterial RNAP, and comparison with the RPB6-RPB1 interface in yeast RNAP II, confirms the structural relationship and suggests a "latching" mechanism for the role of ω and RPB6 in promoting RNAP assembly.