Genetic diversity and virulence of phytopathogenic Burkholderia glumae strains isolated from rice cultivars in valleys of the high jungle of Peru

Burkholderia glumae causes bacterial leaf blight in rice, and its global spread has been exacerbated by climate change. To understand the genetic diversity and virulence of B. glumae strains isolated from rice cultivars in Peru, 47 isolates were obtained from infected rice fields, all belonging to B. glumae, and confirmed by recA and toxB sequences. The BOX-PCR typing group 38 genomic profiles, and these turn into 7 Variable Number Tandem Repeats (VNTR) haplotypes. There was no correlation between clustering and geographical origin. Nineteen strains were selected for phenotypic characterization and virulence, using both the maceration level of the onion bulb proxy and inoculation of seeds of two rice cultivars. Several strains produced pigments other than toxoflavin, which correlated with onion bulb maceration. In terms of virulence at the seed level, all strains produced inhibition at the root and coleoptile level, but the severity of symptoms varied significantly between strains, revealing significant differences in pathogenicity. There is no correlation between maceration and virulence scores, probably reflecting different virulence mechanisms depending on the host infection stage. This is the first study to evaluate the VNTR diversity and virulence of Peruvian strains of B. glumae in two commercial cultivars.

Toxoflavin analog D43 exerts antiproliferative effects on breast cancer by inducing ROS-mediated apoptosis and DNA damage

Triple-negative breast cancer (TNBC) is regarded as the deadliest subtype of breast cancer because of its high heterogeneity, aggressiveness, and limited treatment options. Toxoflavin has been reported to possess antitumor activity. In this study, a series of toxoflavin analogs were synthesized, among which D43 displayed a significant dose-dependent inhibitory effect on the proliferation of TNBC cells (MDA-MB-231 and HCC1806). Additionally, D43 inhibited DNA synthesis in TNBC cells, leading to cell cycle arrest at the G2/M phase. Furthermore, D43 consistently promoted intracellular ROS generation, induced DNA damage, and resulted in apoptosis in TNBC cells. These effects could be reversed by N-acetylcysteine. Moreover, D43 significantly inhibited the growth of breast cancer patient-derived organoids and xenografts with a favorable biosafety profile. In conclusion, D43 is a potent anticancer agent that elicits significant antiproliferation, oxidative stress, apoptosis, and DNA damage effects in TNBC cells, and D43 holds promise as a potential candidate for the treatment of TNBC.

Genome re-seqeunce and analysis of Burkholderia glumae strain AU6208 and evidence of toxoflavin: A potential bacterial toxin

Burkholderia glumae, the primary causative agent of bacterial panicle blight in rice, has been reported as an opportunistic pathogen in patients with chronic infections. This study aimed to re-sequence the clinical isolate B. glumae strain AU6208 and comparatively analyze its genome using B. glumae strain BGR1 from rice plant as the reference. Re-sequencing results revealed that the genome of strain AU6208 comprised 96 contigs corresponding to a 6.1 Mbp genome of the strain AU6208, with 5322 coding sequences and 68.2 % GC content; this is much larger compared to the genome previously sequenced by us and described by Seo et al (2015), which was reported to be 4.1 Mbp comprising >1200 contigs, 4361 coding sequences, and 67.31 % GC content. Moreover, this updated genome shares >80 % identity to the 7.2 Mbp genome of BGR1, which encodes 6491 coding sequences and has 68.3 % GC content. Further computational analysis revealed that the strain AU6208 encodes several bacteriocin biosynthesis genes, antibiotic, as well as virulent genes such as toxoflavin genes, which included 425 specialty genes and 12 toxoflavin genes. Upon further characterization, 12 toxoflavins (ToxA, B, C, D, E, F, G, H, I, J, TofI, and TofR) were found in AU6208 with 70-100 % sequence, family, and domain similarity with that of BGR1. Upon comparison with BGR1, the structural characterizations of selected toxoflavin genes (ToxB, ToxC, ToxG, H, and TofI) revealed variations in 2D and 3D structures such as differences in α-helix, β-sheets, loops, physiological properties of proteins, RMSD values, etc. These variations may play significant role in different mode of action in different hosts thereby indicating that in addition to their respective hosts, toxoflavins could also contribute to exploit other hosts across the kingdom. In addition to understanding the epidemiology of strain AU6208, this updated genomics data will also unfold the pathogenicity of bacteria in diversity of various hosts and anti-virulence.

Loci Identification of a N-acyl Homoserine Lactone Type Quorum Sensing System and a New LysR-type Transcriptional Regulator Associated with Antimicrobial Activity and Swarming in Burkholderia Gladioli UAPS07070

A random transposition mutant library of B. gladioli UAPS07070 was analyzed for searching mutants with impaired microbial antagonism. Three derivates showed diminished antimicrobial activity against a sensitive strain. The mutated loci showed high similarity to the quorum sensing genes of the AHL-synthase and its regulator. Another mutant was affected in a gene coding for a LysrR-type transcriptional regulator. The production of toxoflavin, the most well known antimicrobial-molecule and a major virulence factor of plant-pathogenic B. glumae and B. gladioli was explored. The absence of a yellowish pigment related to toxoflavin and the undetectable transcription of toxA in the mutants indicated the participation of the QS system and of the LysR-type transcriptional regulator in the regulation of toxoflavin. Additionally, those genes were found to be related to the swarming phenotype. Lettuce inoculated with the AHL synthase and the lysR mutants showed less severe symptoms. We present evidence of the participation of both, the quorum sensing and for the first time, of a LysR-type transcriptional regulator in antibiosis and swarming phenotype in a strain of B. gladioli

Efficient selenocysteine-dependent reduction of toxoflavin by mammalian thioredoxin reductase

BACKGROUND

Toxoflavin (1,6-dimethylpyrimido[5,4-e][1,2,4]triazine-5,7-dione; xanthothricin) is a well-known natural toxin of the pyrimidinetriazinedione type that redox cycles with oxygen under reducing conditions. In mammalian systems, toxoflavin is an inhibitor of Wnt signaling as well as of SIRT1 and SIRT2 activities, but other molecular targets in mammalian cells have been scarcely studied. Interestingly, in a library of nearly 400,000 compounds (PubChem assay ID 588456), toxoflavin was identified as one out of only 56 potential substrates of the mammalian selenoprotein thioredoxin reductase 1 (TrxR1, TXNRD1). This activity was here examined in further detail.

METHODS

Kinetic parameters in interactions of toxoflavin with rat or human TrxR isoenzymes were determined and compared with those of juglone (5-Hydroxy-1,4-naphthoquinone; walnut toxin) and 9,10-phenanthrene quinone. Selenocysteine dependence was examined using Sec-to-Cys and Sec-to-Ser substituted variants of recombinant rat TrxR1.

RESULTS

Toxoflavin was confirmed as an efficient substrate for TrxR. Rat and human cytosolic TrxR1 supported NADPH-dependent redox cycling coupled to toxoflavin reduction, accompanied by H₂O₂ production under aerobic conditions. Apparent kinetic parameters for the initial rates of reduction showed that rat TrxR1 displayed higher apparent turnover (k_cat = 1700 ± 330 min^-1) than human TrxR1 (k_cat = 1100 ± 82 min^-1) but also a higher Km (K_m = 24 ± 4.3 μM for human TrxR1 versus K_m = 54 ± 18 μM for rat TrxR1). Human TrxR2 (TXNRD2) was less efficient in reduction of toxoflavin (K_m = 280 ± 110 μM and k_cat = 740 ± 240 min^-1). The activity was absolutely dependent upon selenocysteine (Sec). Toxoflavin was also a subversive substrate indirectly inhibiting reduction of other substrates of TrxR1.

CONCLUSIONS

Our results identify toxoflavin as an efficient redox cycling substrate of mammalian TrxR enzymes, in a strict Sec-dependent manner.

GENERAL SIGNIFICANCE

Тhe interactions of toxoflavin with mammalian TrxR isoenzymes can help to explain parts of the molecular mechanisms giving rise to the well-known toxicity as well as pro-oxidant properties of this toxin.

A simple and sensitive biosensor strain for detecting toxoflavin using β-galactosidase activity

In this study, we developed a simple and sensitive biosensor for the determination of toxoflavin (which is toxic to various plants, fungi, animals, and bacteria) in natural samples based on β-galactosidase activity. The proposed toxoflavin detection method for toxin-producing bacteria or toxin-contaminated foods is simple and cost effective. Burkholderia glumae, a species known to cause rice grain rot and wilt in various field crops, produces toxoflavin under the control of a LysR-type transcriptional regulator ToxR and its ligand toxoflavin. As the expression of toxoflavin biosynthetic genes requires toxoflavin as a co-activator of ToxR, a novel biosensor stain was constructed based on lacZ reporter gene integration into the first gene of the toxoflavin biosynthesis operon, toxABCDE of B. glumae. The biosensor was composed of a sensor strain (COK71), substrates (X-gal or ONPG), and culture medium, without any complex preparation process. We demonstrated that the biosensor strain is highly specific to toxoflavin, and can quantify relative amounts of toxoflavin compared with known concentrations of toxoflavin. The proposed method was reliable and simple; samples containing 50-500 nM of toxoflavin could be analyzed. More importantly, the proposed biosensor strain could identify toxoflavin-producing bacteria in real samples. The excellent performance of this biosensor is useful for diagnostic purposes, such as detecting toxoflavin-contaminated foods and environmental samples.