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Background   Ehrlichia chaffeensis  is a tick-transmitted rickettsial pathogen responsible for an important emerging disease, human monocytic ehrlichiosis. To date how  E. chaffeen sis and many related tick-borne rickettsial pathogens adapt and persist in vertebrate and tick hosts remain largely unknown. In recent studies, we demonstrated significant host-specific differences in protein expression in  E. chaffeensis  originating from its tick and vertebrate host cells. The adaptive response of the pathogen to different host environments entails switch of gene expression regulated at the level of transcription, possibly by altering RNA polymerase activity.    Results  In an effort to understand the molecular basis of pathogen gene expression differences, we isolated native  E. chaffeensis  RNA polymerase using a heparin-agarose purification method and developed an  in vitro  transcription system to map promoter regions of two differentially expressed genes of the p28 outer membrane protein locus,  p28-Omp14  and  p28-Omp19 . We also prepared a recombinant protein of  E. chaffeensis  σ70 homologue and used it for  in vitro  promoter analysis studies. The possible role of one or more proteins presents in  E. chaffeensis  lysates in binding to the promoter segments and on the modulation of  in vitro  transcription was also assessed.    Conclusions  Our experiments demonstrated that both the native and recombinant proteins are functional and have similar enzyme properties in driving the transcription from  E. chaffeensis  promoters. This is the first report of the functional characterization of  E. chaffeensis  RNA polymerase and  in vitro  mapping of the pathogen promoters using the enzyme. This study marks the beginning to broadly characterize the mechanisms controlling the transcription by  Anaplasmataceae  pathogens.

Isolation and characterization of Ehrlichia chaffeensis RNA polymerase and its use in evaluating p28 outer membrane protein gene promoters