The Sinorhizobium meliloti nutrient-deprivation-induced tyrosine degradation gene hmgA is controlled by a novel member of the arsR family of regulatory genes

Brucella abortus transcriptional regulator ArsR6 inhibits host pyroptosis via BAB_RS28760 by triggering the endoplasmic reticulum stress pathway

Pyroptosis, which is accompanied by inflammatory responses, is critical for pathogen clearance. However, the mechanism through which Brucella evades host pyroptosis remains unclear. The transcriptional regulator ArsR6 maintains bacterial intracellular homeostasis and possibly influences host cell death. However, whether ArsR6 acts on cellular pyroptosis is unknown. Therefore, we investigated pathogen-host interactions within macrophages infected with Brucella abortus (B. abortus), and found that ArsR6 is crucial for inhibiting host cell pyroptosis after B. abortus infection. The downstream target gene, BAB_RS28760 of ArsR6 was screened using chromatin immunoprecipitation sequencing. BAB_RS28760 belongs to the BA14K protein family and is strongly immunoreactive and induces humoral and cellular immune responses in the host during infection. Deleting ArsR6 in B. abortuspromotes pyroptosis and enhancs the intracellular survival of B. abortus. In addition, ArsR6 negatively regulated its target gene BAB_RS28760, whereas BAB_RS28760 deletion downregulated cellular pyroptosis by inhibiting endoplasmic reticulum stress and decreasing the intracellular survival of B. abortus. Our results reveal for the first time that Brucella ArsR6 reduces endoplasmic reticulum stress activation by negatively regulating its downstream target genes, thus inhibiting host cell pyroptosis. Our study provides new insights into the pathogenic mechanisms of Brucella, which can provide potential selectivity for the development of anti-Brucella therapies.

Increased acid resistance of the archaeon, Metallosphaera sedula by adaptive laboratory evolution

Extremely thermoacidophilic members of the Archaea such as the lithoautotroph, Metallosphaera sedula, are among the most acid resistant forms of life and are of great relevance in bioleaching. Here, adaptive laboratory evolution was used to enhance the acid resistance of this organism while genomics and transcriptomics were used in an effort to understand the molecular basis for this trait. Unlike the parental strain, the evolved derivative, M. sedula SARC-M1, grew well at pH of 0.90. Enargite (Cu3AsS4) bioleaching conducted at pH 1.20 demonstrated SARC-M1 leached 23.78 % more copper relative to the parental strain. Genome re-sequencing identified two mutations in SARC-M1 including a nonsynonymous mutation in Msed_0408 (an amino acid permease) and a deletion in pseudogene Msed_1517. Transcriptomic studies by RNA-seq of wild type and evolved strains at various low pH values demonstrated there was enhanced expression of genes in M. sedula SARC-M1 encoding membrane complexes and enzymes that extrude protons or that catalyze proton-consuming reactions. In addition, M. sedula SARC-M1 exhibited reduced expression of genes encoding enzymes that catalyze proton-generating reactions. These unique genomic and transcriptomic features support a model for increased acid resistance arising from enhanced control over cytoplasmic pH.

Key roles of microsymbiont amino acid metabolism in rhizobia-legume interactions

Rhizobia are bacteria in the α-proteobacterial genera Rhizobium, Sinorhizobium, Mesorhizobium, Azorhizobium and Bradyrhizobium that reduce (fix) atmospheric nitrogen in symbiotic association with a compatible host plant. In free-living and/or symbiotically associated rhizobia, amino acids may, in addition to their incorporation into proteins, serve as carbon, nitrogen or sulfur sources, signals of cellular nitrogen status and precursors of important metabolites. Depending on the rhizobia-host plant combination, microsymbiont amino acid metabolism (biosynthesis, transport and/or degradation) is often crucial to the establishment and maintenance of an effective nitrogen-fixing symbiosis and is intimately interconnected with the metabolism of the plant. This review summarizes past findings and current research directions in rhizobial amino acid metabolism and evaluates the genetic, biochemical and genome expression studies from which these are derived. Specific sections deal with the regulation of rhizobial amino acid metabolism, amino acid transport, and finally the symbiotic roles of individual amino acids in different plant-rhizobia combinations.

The tyrosine degradation gene hppD is transcriptionally activated by HpdA and repressed by HpdR in Streptomyces coelicolor, while hpdA is negatively autoregulated and repressed by HpdR

Streptomyces coelicolor produces a brown pigment on nutrient-limited agar medium (Tyr-PM) using l-tyrosine as the sole nitrogen and carbon source. The pigment production is associated with the second step of l-tyrosine catabolism catalysed by 4-hydroxyphenylpyruvate dioxygenase (HppD), which converts 4-hydroxyphenylpyruvate (4HPP) to 2, 5-dihydroxyphenylacetate (homogentisate) to provide the carbon and energy substrates for the growth of S. coelicolor on Tyr-PM. An hppD mutant did not produce brown pigment, and its normal growth was impaired on Tyr-PM. hpdA and hpdR, located close to hppD, were identified as activator and repressor genes for hppD transcription in the presence of tyrosine. hpdA, divergently transcribed from hppD, is negatively autoregulated in the absence of tyrosine, whereas it is repressed by both its own protein and HpdR in the presence of tyrosine. Electrophoretic mobility shift assays and footprinting experiments showed that HpdA and HpdR each bind to an overlapping region spanning the promoters of both hppD and hpdA, and that 4HPP, instead of tyrosine, is the specific ligand modulating the binding patterns and footprints of HpdA and HpdR on the hppD-hpdA promoter region. These results suggested that the transcription of hppD is subject to coarse and fine control by a complex regulatory system.

A highly conserved Sinorhizobium meliloti operon is induced microaerobically via the FixLJ system and by nitric oxide (NO) via NnrR

A previously generated collection of 11 Tn5-luxAB insertion mutants of Sinorhizobium meliloti harbouring lux reporter gene fusions induced under microaerobic (1% O2) conditions was further characterized and mapped on the sequenced S. meliloti genome. One highly induced gene fusion from this collection (loe-7) was found to be located in the intergenic region between sma1292, encoding a putative protease/collagenase, and a gene of unknown function (sma1294). The loe-7 fusion had been shown previously to be partially controlled by the oxygen sensor/regulator FixLJ system, but significant ( approximately 40%) Lux activity remained in a fixLJ mutant background. Therefore, a secondary Tn1721 mutagenesis of the loe-7 strain was carried out. Nine Tn1721 ('dark') insertions completely abolishing the Lux activity of the loe-7 fusion under microaerobic conditions were isolated. Surprisingly, five dark insertions mapped in denitrification genes [napA, napC, nirK--two insertions--and sma1245 encoding a NnrR-like transcriptional regulator controlling denitrification in response to nitric oxide (NO)]; Tn1721 insertions in the respiration genes fixG and fixP resulted in a reduced expression of the loe-7-lux fusion, and insertions in the regulatory genes fixJ and fixK1 resulted in low, but still detectable Lux activity. On the contrary, insertions in the norD or norQ genes resulted in constitutive Lux activity. In these mutant strains, NO would be expected to accumulate under microaerobic conditions. NO was found to be able to strongly induce the loe-7-luxAB fusion under microaerobic and aerobic conditions, but only in the presence of the functional nnrR-like gene (sma1245). These results suggest that NO, via the NnrR regulator, can serve as a signal molecule to induce the loe-7-luxAB fusion in concert with the FixLJ system.

The homogentisate pathway: a central catabolic pathway involved in the degradation of L-phenylalanine, L-tyrosine, and 3-hydroxyphenylacetate in Pseudomonas putida

Pseudomonas putida metabolizes Phe and Tyr through a peripheral pathway involving hydroxylation of Phe to Tyr (PhhAB), conversion of Tyr into 4-hydroxyphenylpyruvate (TyrB), and formation of homogentisate (Hpd) as the central intermediate. Homogentisate is then catabolized by a central catabolic pathway that involves three enzymes, homogentisate dioxygenase (HmgA), fumarylacetoacetate hydrolase (HmgB), and maleylacetoacetate isomerase (HmgC), finally yielding fumarate and acetoacetate. Whereas the phh, tyr, and hpd genes are not linked in the P. putida genome, the hmgABC genes appear to form a single transcriptional unit. Gel retardation assays and lacZ translational fusion experiments have shown that hmgR encodes a specific repressor that controls the inducible expression of the divergently transcribed hmgABC catabolic genes, and homogentisate is the inducer molecule. Footprinting analysis revealed that HmgR protects a region in the Phmg promoter that spans a 17-bp palindromic motif and an external direct repetition from position -16 to position 29 with respect to the transcription start site. The HmgR protein is thus the first IclR-type regulator that acts as a repressor of an aromatic catabolic pathway. We engineered a broad-host-range mobilizable catabolic cassette harboring the hmgABC, hpd, and tyrB genes that allows heterologous bacteria to use Tyr as a unique carbon and energy source. Remarkably, we show here that the catabolism of 3-hydroxyphenylacetate in P. putida U funnels also into the homogentisate central pathway, revealing that the hmg cluster is a key catabolic trait for biodegradation of a small number of aromatic compounds.