SdiA

SdiA is a transcriptional regulator found in several members of the Enterobacteriaceae, including Escherichia coli and Salmonella enterica. It belongs to the LuxR family of quorum-sensing regulators and functions as a receptor for N-acyl-homoserine lactones (AHLs) produced by other bacteria. Unlike canonical LuxR proteins, SdiA does not encode an AHL synthase, making it an orphan or solo LuxR that responds to external signals rather than producing them.

In these organisms, SdiA has activity that is triggered by binding exogenous AHLs, allowing the cell to sense the presence of neighboring species. Upon ligand binding, SdiA modulates transcription by interacting with target promoter regions; its effect can be either activation or repression depending on the gene and condition. The regulon can vary among species and environmental contexts.

Because many Enterobacteriaceae do not produce AHLs themselves, SdiA provides a means of interspecies communication, helping bacteria adapt to mixed communities, host-associated niches, and competitive environments.

Regulon and phenotypic outcomes reported in different studies include changes in surface structures, stress responses, and metabolic pathways. The exact targets and physiological consequences are not universal and depend on the strain and conditions under which SdiA is active.

Structure and genetics: SdiA proteins have an N-terminal autoinducer-binding domain characteristic of LuxR-family regulators and a C-terminal helix-turn-helix DNA-binding domain. The sdiA gene is located on the chromosome and is co-regulated with other genes responsive to environmental cues.

Significance: SdiA-mediated detection of exogenous AHLs represents a form of interspecies quorum sensing, contributing to how Enterobacteriaceae perceive and respond to their microbial surroundings and potentially influencing virulence, colonization, and biofilm formation.