Bacterial sensor evolved by decreasing complexity.

Area of Science:

• Microbiology
• Structural Biology
• Evolutionary Biology

Background:

• Bacterial receptors utilize ligand-binding domains (LBDs) to detect environmental signals, influencing cellular processes.
• Cache domains are a prevalent family of bacterial LBDs, existing as single (sCache) or double (dCache) modules, with the function of bimodular domains being unclear.

Approach:

• Identified the PacF chemoreceptor in *Pectobacterium atrosepticum* and characterized its formate-binding mechanism within the dCache domain.
• Determined high-resolution structures of formate-bound sCache and dCache domains to elucidate the molecular basis of ligand recognition.
• Investigated the evolutionary relationship between dCache and sCache domains, proposing a model for the reductive evolution of monomodular domains from bimodular ancestors.

Key Points:

• The PacF receptor recognizes formate at the membrane-distal module of its dCache domain, initiating chemoattraction.
• Formate-specific sCache domains evolved from dCache domains through the loss of the membrane-proximal module.
• Despite low sequence identity, sCache and dCache domains share a highly conserved molecular basis for formate binding.

Conclusions:

• Bacterial signal-sensing domains can evolve through simplification, challenging the paradigm of complexity increase.
• The loss of the membrane-proximal module in dCache domains may be a common evolutionary trend, as it is not typically involved in ligand binding.
• This study provides insights into the evolution and diversity of bacterial chemoreceptors and their sensing mechanisms.