Escherichia coli RNA polymerase subunit omega and its N-terminal domain bind full-length beta' to facilitate incorporation into the alpha2beta subassembly.

The omega subunit of Escherichia coli RNA polymerase, consisting of 90 amino acids, is present in stoichiometric amounts per molecule of core RNA polymerase (alpha2betabeta'). The presence of omega is necessary to restore denatured RNA polymerase in vitro to its fully functional form, and, in an omega-less strain of E. coli, GroEL appears to substitute for omega in the maturation of RNA polymerase. The X-ray structure of Thermus aquaticus core RNA polymerase suggests that two regions of omega latch on to beta' at its N-terminus and C-terminus. We show here that omega binds only the intact beta' subunit and not the beta' N-terminal domain or beta' C-terminal domain, implying that omega binding requires both these regions of beta'. We further show that omega can prevent the aggregation of beta' during its renaturation in vitro and that a V8-protease-resistant 52-amino-acid-long N-terminal domain of omega is sufficient for binding and renaturation of beta'. CD and functional assays show that this N-terminal fragment retains the structure of native omega and is able to enhance the reconstitution of core RNA polymerase. Reconstitution of core RNA polymerase from its individual subunits proceeds according to the steps alpha + alpha --> alpha2 + beta --> alpha2beta + beta' --> alpha2betabeta'. It is shown here that omega participates during the last stage of enzyme assembly when beta' associates with the alpha2beta subassembly.