CarD stabilizes mycobacterial open complexes via a two-tiered kinetic mechanism

Area of Science:

• Molecular Biology
• Microbiology
• Biochemistry

Background:

• CarD is a crucial transcriptional regulator in mycobacteria.
• Its precise biological function and mechanism of action remain largely undefined.
• CarD interacts directly with RNA polymerase (RNAP) holoenzyme promoter complexes.

Purpose of the Study:

• To investigate the role of Mycobacterium tuberculosis CarD in the energetics of RNAP-promoter complex formation.
• To elucidate the mechanism by which CarD influences open complex (RPo) formation and stability.
• To characterize the differential interactions of CarD with RNAP from different bacterial species.

Main Methods:

• Utilized a fluorescent reporter system to quantify RNAP open complex (RPo) formation in real-time.
• Analyzed the kinetics and stability of RPo formation on the Mycobacterium tuberculosis rrnAP3 promoter.
• Investigated the effects of CarD concentration and specific CarD mutants on RPo dynamics.
• Compared the RPo formation efficiency between Mycobacterium bovis RNAP and Escherichia coli RNAP.

Main Results:

• Mycobacterium tuberculosis CarD significantly impacts the energetics of RNAP-bound complexes on the rrnAP3 promoter.
• CarD stabilizes the otherwise unstable open complex (RPo) formed by Mycobacterium bovis RNAP.
• A two-tiered, concentration-dependent mechanism for CarD action was proposed, involving differential affinities for open and closed complexes.
• CarD accelerates RPo opening and slows bubble collapse at saturating concentrations.
• Mutational analysis identified key residues (W85, K90, R25) critical for CarD's function in gene regulation.
• Escherichia coli RNAP efficiently forms RPo on rrnAP3, unlike M. bovis RNAP, indicating polymerase-specific differences.

Conclusions:

• CarD acts as a critical stabilizer of RNAP open complexes, particularly for Mycobacterium bovis RNAP.
• The findings support a concentration-dependent, two-tiered mechanism for CarD's regulatory function.
• Key residues in CarD are essential for its in vivo gene regulatory activity.
• Significant differences exist in transcriptional machinery between bacterial genera, exemplified by M. bovis and E. coli RNAP interactions with promoters.