pH74

pH74 is a protein found in the bacterium Bacillus subtilis. It belongs to the family of small metal‑binding proteins with a Cys2His2 zinc‑finger motif. The protein was first identified in 2004 during a screen for genes up‑regulated during acid stress. Subsequent Northern blot analysis showed that pH74 mRNA levels rise when growth medium is adjusted to pH 4.5, suggesting a role in acid resistance.

The pH74 protein is 72 amino acids long and has a predicted molecular weight of 8.2 kDa. Crystallographic studies at 1.8 Å resolution revealed a compact fold consisting of two β‑strands and a short α‑helix. The zinc‑binding cysteine residues are positioned in a canonical Cys2His2 framework, coordinating a single zinc ion. Despite its small size, the protein forms stable dimers in solution, as demonstrated by analytical ultracentrifugation.

Functional assays indicate that pH74 confers protection to Escherichia coli when expressed under control of the native Bacillus subtilis promoter. Mutagenesis of the cysteine residues abolishes this protective effect, supporting a metal‑binding dependent mechanism. The exact biochemical activity remains unknown; however, isotope‑labeling experiments suggest that pH74 may act as a chaperone for other acid‑sensitive proteins.

Industrial applications of pH74 are still speculative. Its acid‑stable nature has made it a candidate for use in protease inhibitors designed for low‑pH detergents. Further research is required to clarify its physiological role and potential utility in biotechnology.