| Summary: | In bacteria, group-coordinated behavior such as biofilm formation or virulence are often mediated via cell–cell communication, a process referred to as quorum sensing (QS). The canonical QS system of Gram-negative bacteria uses <i>N</i>-acyl homoserine lactones (AHLs) as communication molecules, which are produced by LuxI-type synthases and sensed by cognate LuxR-type receptors. These receptors act as transcriptional regulators controlling the expression of specific genes. Some bacteria harbor LuxR-type receptors lacking a cognate LuxI-type synthases, designated as LuxR solos. Among many other LuxR solos, the entomopathogenic enteric bacterium <i>Photorhabdus luminescens</i> harbors a SdiA-like LuxR solo containing an AHL signal-binding domain, for which a respective signal molecule and target genes have not been identified yet. Here we performed SPR analysis to demonstrate that SdiA acts as a bidirectional regulator of transcription, tightly controlling its own expression and the adjacent <i>PluDJC_01670</i> (<i>aidA</i>) gene in <i>P. luminescens,</i> a gene supposed to be involved in the colonization of eukaryotes. Via qPCR we could further determine that in <i>sdiA</i> deletion mutant strains, <i>aidA</i> is upregulated, indicating that SdiA negatively affects expression of <i>aidA</i>. Furthermore, the Δ<i>sdiA</i> deletion mutant exhibited differences in biofilm formation and motility compared with the wild-type. Finally, using nanoDSF analysis we could identify putative binding ability of SdiA towards diverse AHLs, but also to plant-derived signals, modulating the DNA-binding capacity of SdiA, suggesting that this LuxR solo acts as an important player in interkingdom signaling between <i>P. luminescens</i> and plants.
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