Stabilization of the primary sigma factor of Staphylococcus aureus by core RNA polymerase

Abstract

The primary sigma factor (σA ) of Staphylococcus aureus, a po tential drug target, was little investigated at the structural level. Using an N-terminal histidine-tagged σA (His-σA ), here we have demonstrated that it exits as a monomer in solution, possesses multiple domains, harbors primarily α-helix and efficiently binds to a S. aureus promoter DNA in the presence of core RNA polymerase. While both N- and C-terminal ends of His σA are flexible in nature, two Trp residues in its DNA binding region are buried. Upon increasing the incubation temperature from 25º to 40ºC, ~60% of the input His-σA was cleaved by thermolysin. Aggregation of His-σA was also initiated rapidly at 45o C. From the equilibrium unfolding experiment, the Gibbs free energy of stabilization of His-σA was estimated to be +0.70 kcal mol-1. The data together suggest that primary sigma factor of S. aureus is an unstable protein. Core RNA polymer ase however stabilized σA appreciably. [BMB reports 2010; 43(3): 176-181