The chlamydial EUO gene encodes a histone H1-specific protease

Area of Science:

• Microbiology
• Molecular Biology
• Bacterial Pathogenesis

Background:

• Chlamydia trachomatis, an obligate intracellular pathogen, causes blinding eye disease and sexually transmitted infections.
• Two eukaryotic histone H1-like proteins, Hc1 and Hc2, have been identified in Chlamydia.
• Hc1 expression correlates with chromatin condensation, while its downregulation coincides with decondensation during the early parasitic life cycle.

Purpose of the Study:

• To investigate the role of the early upstream open reading frame (EUO) gene product in Hc1 degradation and nucleoid decondensation in Chlamydia trachomatis.
• To characterize the enzymatic activity and specificity of the EUO gene product.

Main Methods:

• A fusion protein of EUO (amino acids 4-177) and glutathione S-transferase (GST) was constructed from C. trachomatis serovar Lz.
• The purified EUO-GST fusion protein was used to examine its in vitro effects on purified Hc1.
• Proteolytic activity was assessed using various substrates, including core histones and chlamydial RNA polymerase, and tested against protease inhibitors.

Main Results:

• The EUO-GST fusion protein completely digested Hc1 within 1 hour at 37°C, while GST alone showed no activity.
• EUO-GST cleaved Hc1, histone H1, and histone H5, but not core histones or chlamydial RNA polymerase alpha-subunit.
• The activity was sensitive to serine and aspartic protease inhibitors but resistant to heat; it selectively digested the DNA-binding C-terminal domain of Hc1, facilitating DNA-Hc1 complex dissociation at a 1:1 molar ratio.

Conclusions:

• The Chlamydia trachomatis EUO gene product possesses specific proteolytic activity against histone H1-like proteins.
• EUO-mediated Hc1 degradation is a key mechanism for nucleoid decondensation, enabling the transition to the early parasitic life cycle.
• This proteolytic activity represents a novel mechanism for bacterial manipulation of host or self-derived chromatin.