OUP accepted manuscript

RNA-seq analysis in simulated microgravity unveils down-regulation of the beta-rhizobial siderophore phymabactin

Exploiting the symbiotic interaction between crops and nitrogen-fixing bacteria is a simple and ecological method to promote plant growth in prospective extraterrestrial human outposts. In this study, we performed an RNA-seq analysis to investigate the adaptation of the legume symbiont Paraburkholderia phymatum STM815T to simulated microgravity (s0-g) at the transcriptome level. The results revealed a drastic effect on gene expression, with roughly 23% of P. phymatum genes being differentially regulated in s0-g. Among those, 951 genes were upregulated and 858 downregulated in the cells grown in s0-g compared to terrestrial gravity (1 g). Several genes involved in posttranslational modification, protein turnover or chaperones encoding were upregulated in s0-g, while those involved in translation, ribosomal structure and biosynthesis, motility or inorganic ions transport were downregulated. Specifically, the whole phm gene cluster, previously bioinformatically predicted to be involved in the production of a hypothetical malleobactin-like siderophore, phymabactin, was 20-fold downregulated in microgravity. By constructing a mutant strain (ΔphmJK) we confirmed that the phm gene cluster codes for the only siderophore secreted by P. phymatum as assessed by the complete lack of iron chelating activity of the P. phymatum ΔphmJK mutant on chrome azurol S (CAS) agar plates. These results not only provide a deeper understanding of the physiology of symbiotic organisms exposed to space-like conditions, but also increase our knowledge of iron acquisition mechanisms in rhizobia.

Identification of a system for hydroxamate xenosiderophore-mediated iron transport in Burkholderia cenocepacia

One of the mechanisms employed by the opportunistic pathogen Burkholderia cenocepacia to acquire the essential element iron is the production and release of two ferric iron chelating compounds (siderophores), ornibactin and pyochelin. Here we show that B. cenocepacia is also able to take advantage of a range of siderophores produced by other bacteria and fungi ('xenosiderophores') that chelate iron exclusively by means of hydroxamate groups. These include the tris-hydroxamate siderophores ferrioxamine B, ferrichrome, ferricrocin and triacetylfusarinine C, the bis-hydroxamates alcaligin and rhodotorulic acid, and the monohydroxamate siderophore cepabactin. We also show that of the 24 TonB-dependent transporters encoded by the B. cenocepacia genome, two (FhuA and FeuA) are involved in the uptake of hydroxamate xenosiderophores, with FhuA serving as the exclusive transporter of iron-loaded ferrioxamine B, triacetylfusarinine C, alcaligin and rhodotorulic acid, while both FhuA and FeuA are able to translocate ferrichrome-type siderophores across the outer membrane. Finally, we identified FhuB, a putative cytoplasmic membrane-anchored ferric-siderophore reductase, as being obligatory for utilization of all of the tested bis- and tris-hydroxamate xenosiderophores apart from alcaligin.

CysB Is a Key Regulator of the Antifungal Activity of Burkholderia pyrrocinia JK-SH007

Burkholderia pyrrocinia JK-SH007 can effectively control poplar canker caused by pathogenic fungi. Its antifungal mechanism remains to be explored. Here, we characterized the functional role of CysB in B. pyrrocinia JK-SH007. This protein was shown to be responsible for the synthesis of cysteine and the siderophore ornibactin, as well as the antifungal activity of B. pyrrocinia JK-SH007. We found that deletion of the cysB gene reduced the antifungal activity and production of the siderophore ornibactin in B. pyrrocinia JK-SH007. However, supplementation with cysteine largely restored these two abilities in the mutant. Further global transcriptome analysis demonstrated that the amino acid metabolic pathway was significantly affected and that some sRNAs were significantly upregulated and targeted the iron-sulfur metabolic pathway by TargetRNA2 prediction. Therefore, we suggest that, in B. pyrrocinia JK-SH007, CysB can regulate the expression of genes related to Fe-S clusters in the iron-sulfur metabolic pathway to affect the antifungal activity of B. pyrrocinia JK-SH007. These findings provide new insights into the various biological functions regulated by CysB in B. pyrrocinia JK-SH007 and the relationship between iron-sulfur metabolic pathways and fungal inhibitory substances. Additionally, they lay the foundation for further investigation of the main antagonistic substances of B. pyrrocinia JK-SH007.

The Global Regulator MftR Controls Virulence and Siderophore Production in Burkholderia thailandensis

Bacterial pathogens face iron limitation in a host environment. To overcome this challenge, they produce siderophores, small iron-chelating molecules.