ε, a new subunit of RNA polymerase found in gram-positive bacteria.

RNA polymerase in bacteria is a multisubunit protein complex that is essential for gene expression. We have identified a new subunit of RNA polymerase present in the high-A+T Firmicutes phylum of Gram-positive bacteria and have named it epsilon. Previously epsilon had been identified as a small protein (omega(1)) that copurified with RNA polymerase. We have solved the structure of epsilon by X-ray crystallography and show that it is not an omega subunit. Rather, epsilon bears remarkable similarity to the Gp2 family of phage proteins involved in the inhibition of host cell transcription following infection. Deletion of epsilon shows no phenotype and has no effect on the transcriptional profile of the cell. Determination of the location of epsilon within the assembly of RNA polymerase core by single-particle analysis suggests that it binds toward the downstream side of the DNA binding cleft. Due to the structural similarity of epsilon with Gp2 and the fact they bind similar regions of RNA polymerase, we hypothesize that epsilon may serve a role in protection from phage infection.