Structural characterization of Mycobacterium tuberculosis RNA polymerase binding protein A (RbpA) and its interactions with sigma factors

The RNA polymerase binding protein A (RbpA) is a 13 kDa protein, encoded by the gene Rv2050, that was shown to be essential for the growth and survival of the important human pathogen Mycobacterium tuberculosis. Although is not clear yet why RbpA is essential in M. tuberculosis, significant progress has been made in the characterization of the protein. For instance, it was shown that RbpA binds to the β-subunit of the RNA polymerase (RNAP) and activates transcription. Interestingly, it was reported that RbpA can enhance the transcription activity of the RNAP containing the primary σ-subunit σ[superscript A] but does not have any detectable effect if the RNAP is associated with the alternative σ-subunit σ[superscript F]. Moreover, it was also shown that RbpA might influence the response of M. tuberculosis to the current frontline anti-tuberculosis drug rifampicin. The research project described in this thesis contributes to the ongoing efforts to characterize RbpA by providing the structure of the protein and identifying the principle σ-subunit σ[superscript A], and the principle-like σ-subunit σ[superscript B], as interaction partners. The solution structure of RbpA reveals the presence of a central structured region and highly dynamic N- and C- termini. Both termini are involved in the formation of a tight complex with the σ-subunit but only the C-terminal region appears to be essential for this interaction. The finding that RbpA also binds to the RNAP σ-subunit suggests new possibilities for the mechanism of action used by RbpA to activate transcription. Furthermore, preliminary data obtained using a ΔRv2050 conditional mutant strain of M. tuberculosis suggest that the interaction with the σ-subunit is essential for the functionality of RbpA.