Crystallization and preliminary characterization of a novel haem-binding protein of Streptomyces reticuli

Streptomyces reticuli is a soil-growing Gram-positive bacteria that has been shown to secrete a novel haem-binding protein known as HbpS. Sequence analysis reveals that homologues of HbpS are found in a wide variety of bacteria, including different Actinobacteria and the Gram-negative Vibrio cholera and Klebsiella pneumoniae. The in vivo production of HbpS is greatly increased when S. reticuli is cultured in the presence of the natural antibiotic haemin (Fe³⁺ oxidized form of haem). Mutational analysis demonstrated that HbpS significantly increases the resistance of S. reticuli to toxic concentrations of haemin. Previous data show that the presence of the newly identified two-component sensor system SenS-SenR also considerably enhances the resistance of S. reticuli to haemin and the redox-cycling compound plumbagin, suggesting a role in the sensing of redox changes. Specific interaction between HbpS and SenS-SenR, which regulates the expression of the catalase-peroxidase CpeB, as well as HbpS, has been demonstrated in vitro. HbpS has been recombinantly overexpressed, purified and crystallized in space group P2₁3, with a cell edge of 152.5 Å. Diffraction data were recorded to a maximal resolution of 2.25 Å and phases were obtained using the SAD method from crystals briefly soaked in high concentrations of sodium bromide.