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The abuse of antibiotics and the consequent increase of drug-resistant bacteria constitute a serious threat to human health, and new antibiotics are urgently needed. Research shows that antimicrobial peptides produced by natural organisms are potential substitutes for antibiotics. Based on Deinagkistrodonacutus (known as five-pacer viper) genome bioinformatics analysis, we discovered a new cathelicidin antibacterial peptide which was called FP-CATH. Circular dichromatic analysis showed a typical helical structure. FP-CATH showed broad-spectrum antibacterial activity. It has antibacterial activity to Gram-negative bacteria and Gram-positive bacteria including methicillin-resistant Staphylococcus aureus (MRSA). The results of transmission electron microscopy (TEM) and scanning electron microscopy (SEM) showed that FP-CATH could cause the change of bacterial cell integrity, having a destructive effect on Gram-negative bacteria and inducing Gram-positive bacterial surface formation of vesicular structure. FP-CATH could bind to LPS and showed strong binding ability to bacterial DNA. In vivo, FP-CATH can improve the survival rate of nematodes in bacterial invasion experiments, and has a certain protective effect on nematodes. To sum up, FP-CATH is likely to play a role in multiple mechanisms of antibacterial action by impacting bacterial cell integrity and binding to bacterial biomolecules. It is hoped that the study of FP-CATH antibacterial mechanisms will prove useful for development of novel antibiotics.

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7-Hydroxytropolone (7-HT) is a unique iron scavenger synthesized by Pseudomonas donghuensis HYS that has various biological activities in addition to functioning as a siderophore. P. donghuensis HYS is more pathogenic than P. aeruginosa toward Caenorhabditis elegans, an observation that is closely linked to the biosynthesis of 7-HT. The nonfluorescent siderophore (nfs) gene cluster is responsible for the orderly biosynthesis of 7-HT and represents a competitive advantage that contributes to the increased survival of P. donghuensis HYS; however, the regulatory mechanisms of 7-HT biosynthesis remain unclear. This study is the first to propose that the ECF σ factor has a regulatory effect on 7-HT biosynthesis. In total, 20 ECF σ factors were identified through genome-wide scanning, and their responses to extracellular ferrous ions were characterized. We found that SigW was both significantly upregulated under high-iron conditions and repressed by an adjacent anti-σ factor. RNA-Seq results suggest that the SigW/RsiW system is involved in iron metabolism and 7-HT biosynthesis. Combined with the siderophore phenotype, we also found that SigW could inhibit siderophore synthesis, and this inhibition can be relieved by RsiW. EMSA assays proved that SigW, when highly expressed, can directly bind to the promoter region of five operons of the nfs cluster to inhibit the transcription of the corresponding genes and consequently suppress 7-HT biosynthesis. In addition, SigW not only directly negatively regulates structural genes related to 7-HT synthesis but also inhibits the transcription of regulatory proteins, including of the Gac/Rsm cascade system. Taken together, our results highlight that the biosynthesis of 7-HT is negatively regulated by SigW and that the SigW/RsiW system is involved in mechanisms for the regulation of iron homeostasis in P. donghuensis HYS. As a result of this work, we identified a novel mechanism for the global negative regulation of 7-HT biosynthesis, complementing our understanding of the function of ECF σ factors in Pseudomonas.