Bacteria use small molecules to assess the density and identity of nearby organisms and formulate a response. This process, called quorum sensing (QS), commonly regulates bioluminescence, biofilm formation, and virulence. Vibrio harveyi have three described QS circuits. Each involves the synthesis of a molecule that regulates phosphorylation of its cognate receptor kinase. Each receptor exchanges phosphate with a common phosphorelay protein, LuxU, which ultimately regulates bioluminescence. Here, we show that another small molecule, nitric oxide (NO), participates in QS through LuxU. V. harveyi display a NO concentration-dependent increase in bioluminescence that is regulated by an hnoX gene. We demonstrate that H-NOX is a NO sensor and NO/H-NOX regulates phosphorylation of a kinase that transfers phosphate to LuxU. This study reveals the discovery of a fourth QS pathway in V. harveyi and suggests that bacteria use QS to integrate not only the density of bacteria but also other diverse information about their environment into decisions about gene expression.

Cell signaling plays an important role in the survival of bacterial colonies. They use small molecules to coordinate gene expression in a cell density dependent manner. This process, known as quorum sensing, helps bacteria regulate diverse functions such as bioluminescence, biofilm formation and virulence. In Vibrio harveyi, a bioluminescent marine bacterium, four parallel quorum-sensing systems have been identified to regulate light production. We have previously reported that nitric oxide (NO), through the H-NOX/HqsK quorum sensing pathway contributes to light production in V. harveyi through the LuxU/LuxO/LuxR quorum sensing pathway. In this study, we show that nitric oxide (NO) also regulates flagellar production and enhances biofilm formation. Our data suggest that V. harveyi is capable of switching between lifestyles to be able to adapt to changes in the environment.

Group behavior of the human pathogen Vibrio cholerae, including biofilm formation and virulence factor secretion, is mediated by a process known as quorum sensing. Quorum sensing is a way by which bacteria coordinate gene expression in response to population density through the production, secretion, and detection of small molecules called autoinducers. Four autoinducer-mediated receptor histidine kinases have been implicated in quorum sensing through the phosphotransfer protein LuxU: CqsS, LuxP/Q, CqsR, and VpsS (Vc1445). Of these receptor kinases, VpsS is predicted to be cytosolic, and its cognate autoinducer is currently unknown. In this study, we demonstrate that the nitric oxide-bound complex of a member of the recently discovered family of nitric oxide-responsive hemoproteins called NosP (VcNosP is encoded by Vc1444; this gene product is also known as VpsV) inhibits the autophosphorylation activity of VpsS and thus phosphate flow to LuxU. Therefore, we propose that VpsS contributes to the regulation of quorum sensing in a nitric-oxide-dependent manner through its interaction with NosP.

Ascochyta lentis causes ascochyta blight in lentil (Lens culinaris Medik.) and yield loss can be as high as 50%. With careful agronomic management practices, fungicide use and advances in breeding resistant lentil varieties, disease severity and impact to farmers have been largely controlled. However, evidence from major lentil producing countries, Canada and Australia, suggests that A. lentis isolates can change their virulence profile and level of aggressiveness over time and under different selection pressures. In this paper, we describe the first genome assembly for A. lentis for the Australian isolate Al4, through the integration of data from Illumina and PacBio SMRT sequencing. The Al4 reference genome assembly is almost 42 Mb in size and encodes 11,638 predicted genes. The Al4 genome comprises 21 full-length and gapless chromosomal contigs and two partial chromosome contigs each with one telomere. We predicted 31 secondary metabolite clusters, and 38 putative protein effectors, many of which were classified as having unknown function. Comparison of A. lentis genome features with the recently published reference assembly for closely related Ascochyta rabiei, shows that genome synteny between these species is highly conserved. However, there are several translocations and inversions of genome sequence. The location of secondary metabolite clusters near transposable element and repeat-rich genomic regions was common for A. lentis as has been reported for other fungal plant pathogens.

bThe ResD response regulator activates transcription of diverse genes in Bacillus subtilis in response to oxygen limitation. ResD regulon genes that are the most highly induced during nitrate respiration include the nitrite reductase operon (nasDEF) and the flavohemoglobin gene (hmp), whose products function in nitric oxide (NO) metabolism. Transcription of these genes is also under the negative control of the NO-sensitive NsrR repressor. Recent studies showed that the NsrR regulon contains genes with no apparent relevance to NO metabolism and that the ResD response regulator and NsrR coordinately regulate transcription. To determine whether these genes are direct targets of NsrR and ResD, we used chromatin affinity precipitation coupled with tiling chip (ChAP-chip) and ChAP followed by quantitative PCR (ChAP-qPCR) analyses. The study showed that ResD and NsrR directly control transcription of the ykuNOP operon in the Fur regulon. ResD functions as an activator at the nasD and hmp promoters, whereas it functions at the ykuN promoter as an antirepressor of Fur and a corepressor for NsrR. This mechanism likely participates in fine-tuning of transcript levels in response to different sources of stress, such as oxygen limitation, iron limitation, and exposure to NO. Bacillus subtilis undergoes either nitrate respiration or fermentation to generate ATP when oxygen becomes limited (reviewed in reference 1). Growth under oxygen-limited conditions, particularly via nitrate respiration, requires the ResD-ResE twocomponent regulatory system (2, 3). During nitrate respiration in B. subtilis, unlike the case with denitrifiers, nitrite is reduced to ammonium instead of nitric oxide (NO). However, NO is generated at low concentrations from nitrite as a by-product of nitrate respiration in B. subtilis (4), as it is in Escherichia coli (5). Since accumulation of NO is cytotoxic, B. subtilis uses flavohemoglobin (Hmp) (6) and nitrite reductase (NasDEF) (7) to reduce NO levels by conversion of NO to nitrate (or N 2 O under anaerobic conditions) (8-10) and by metabolism of nitrite to ammonium (7), respectively. NsrR, a member of the Rrf2 family, is known to control transcription of genes involved in NO detoxification in both Gram-positive and Gram-negative bacteria (reviewed in references 11 and 12). B. subtilis NsrR represses transcription of the nasD operon and hmp under anaerobic fermentative conditions (4). Transcription of these genes is dependent on the ResD response regulator and the ResE sensor kinase (6, 7). NsrR binds to the Ϫ35 region of the nasD promoter, resulting in disruption of the RNA polymerase (RNAP)-ResD-DNA complex (13). When NO is present endogenously via nitrate respiration or exogenously, NsrR-dependent repression of nasD and hmp is relieved. This derepression is attributed to the release of NsrR from the nasD promoter by direct interaction of NO with iron in the [4Fe-4S] cluster of NsrR (13,14). More genes controlled by NsrR were identified by transcriptome analysis, which was validated by transcriptional ...