In this study, plasmid pBBad22K was modulated to be able to coexpress the subunits of Xanthomonas campestris pv. campestris (Xcc) core RNA polymerase (RNAP) in an Escherichia coli host. The subunit-encoding genes of Xcc core RNAP were PCR-amplified respectively to convert into gene cassettes in which an intact subunit-encoding gene and a ribosome binding site (RBS) preceding the gene were contained, and were then cloned one by one into pBBad22K. In addition, a hexahistidine tag (His-tag) was introduced into the C-terminus of alpha subunit-encoded rpoA during PCR for facilitating purification of Xcc core RNAP. The resultant vectors, pBC-CBA and pBC-CBAZ, were used for overproduction of Xcc core RNAP lacking or containing omega, respectively. The assembly of Xcc core RNAP subunits that were coexpressed from these vectors was demonstrated after purification via two-step column chromatography. The yield of Xcc core RNAP containing omega had a 67% increase compared with that of one lacking omega, indicating that omega promotes the assembly of Xcc core RNAP. In addition, Xcc core RNAP lacking omega showed a 13-fold decrease in enzymatic activity in comparison with that containing omega. Promoter-specific transcription assays by recombinant Xcc core RNAP reconstituted with external added sigma factor showed that the absence of omega debilitates the transcriptional activity of Xcc RNAP. Our results demonstrated that omega is not only capable of strengthening the stability, but is also required for the maintenance of enzymatic activity of Xcc RNAP. (C) 2009 Elsevier Inc. All rights reserved.