A novel LuxR‐type solo of Sinorhizobium meliloti, NurR, is regulated by the chromosome replication coordinator, DnaA and activates quorum sensing

Abstract

The genome of Sinorhizobium meliloti, a model for studying plant-bacteria symbiosis, contains eight genes coding for LuxR-like proteins. Two of these, SinR and ExpR, are essential for quorum sensing (QS). Roles and regulation surrounding the others are mostly unknown. Here, we reveal the DNA recognition sequence and regulon of the LuxR-like protein SMc00877. Unlike ExpR, which uses the longchain acyl homoserine lactones (AHLs) as inducers, SMc00877 functioned independently of AHLs and was even functional in Escherichia coli. A target of SMc00877 is SinR, the major regulator of AHL production in S. meliloti. Disruption of SMc00877 decreased AHL production. A weaker production of AHLs resulted in smaller microcolonies, starting from single cells, and delayed AHL-dependent regulation. SMc00877 was expressed only in growing cells in the presence of replete nutrients. Therefore, we renamed it NurR (nutrient sensitive LuxR-like regulator). We traced this nutrient-sensitive expression to transcription control by the DNA replication initiation factor, DnaA, which is essential for growth. These results indicate that NurR has a role in modulating the threshold of QS activation according to growth. We propose growth behavior as an additional prerequisite to population density for the activation of QS in S. meliloti.

Adaptation of the Alphaproteobacterium Rhodobacter sphaeroides to stationary phase

“…To see how AHL levels matched with cerI mRNA levels and the PcerI activity profile, we used a modified S . meliloti strain that was previously shown to be capable of detection of AHLs as low as 1 nM (McIntosh et al ., ). In this bacterium, the promoter activity of the AHL synthase, sinI , responds to the presence of AHLs.…”

“…meliloti AHL indicator strain (Smais). In this strain, the AHL synthase gene, sinI , is disrupted and the promoter of sinI , previously shown to be highly sensitive to the presence of AHLs (McIntosh et al ., ), was fused to the gene coding for the fluorescent protein mCherry.…”

RNase III participates in control of quorum sensing, pigmentation and oxidative stress resistance in Rhodobacter sphaeroides

“…RNA‐seq data of the rnc − ‐mutant strain revealed an elevated cerI mRNA level under microaerobic and phototrophic conditions, where the enrichment over the wild type was highest under microaerobic conditions (Figure S6). To test whether the accumulation of cerI mRNA in R. sphaeroides has any physiological effect on AHL production, we used an S. meliloti reporter strain (McIntosh et al, 2019). This strain is characterised by a disrupted QS system unable to synthesise AHLs.…”

Design–Build–Test of Synthetic Promoters for Inducible Gene Regulation in Alphaproteobacteria

“…For the detection of AHLs, aliquots of liquid cultures were taken at regular time intervals, centrifuged to remove cells, and the supernatants were added (1:9) to cultures of Smais, an S. meliloti AHL indicator strain previously shown to be highly sensitive to the presence of AHLs . Fluorescence generated by Smais was measured after 20 h of cultivation in a 96-well plate.…”