Pneumococcal HtrA protease mediates inhibition of competence by the CiaRH two-component signaling system

Area of Science:

• Microbiology
• Molecular Biology
• Bacterial Genetics

Background:

• Streptococcus pneumoniae competence for genetic transformation is a crucial process for its adaptation and evolution.
• The CiaRH two-component system is known to regulate competence, but its mechanism of action has remained unclear.
• Previous studies linked CiaRH to the regulation of htrA, encoding a serine protease.

Purpose of the Study:

• To elucidate the mechanism by which the CiaRH system inhibits competence development in Streptococcus pneumoniae.
• To investigate the role of the HtrA serine protease in CiaRH-mediated regulation of competence.
• To identify potential targets of HtrA in the competence regulatory pathway.

Main Methods:

• Site-directed mutagenesis of the HtrA catalytic serine residue.
• Analysis of competence levels in wild-type and mutant strains under various conditions.
• Treatment of pneumococcal cultures with recombinant HtrA (rHtrA).
• Western immunoblot analysis to assess ComD protein levels.

Main Results:

• Mutagenesis of HtrA's catalytic site restored competence in a constitutively activated CiaRH strain, indicating HtrA activity is essential for inhibition.
• Recombinant HtrA (rHtrA) reduced transformation efficiency, independent of competence-stimulating peptide (CSP) levels.
• The inhibitory effects of rHtrA and activated CiaRH were partially overcome by constitutive activation of the ComD receptor, suggesting HtrA acts upstream of ComD activation.

Conclusions:

• The HtrA serine protease is a key effector of the CiaRH two-component system in preventing competence development.
• HtrA likely acts on an unknown target protein that influences the ComDE signaling pathway, rather than directly degrading ComD or CSP.
• This study reveals a novel regulatory mechanism involving a protease modulating a two-component system to control genetic competence in Streptococcus pneumoniae.