RNA-seq Analysis Reveals That an ECF σ Factor, AcsS, Regulates Achromobactin Biosynthesis in Pseudomonas syringae pv. syringae B728a

Jessica W Greenwald,Tadhg P Begley,Dennis C Gross,Benjamin J Philmus,Charles J Greenwald

doi:10.1371/journal.pone.0034804

Abstract

Iron is an essential micronutrient for Pseudomonas syringae pv. syringae strain B728a and many other microorganisms; therefore, B728a has evolved methods of iron acquirement including the use of iron-chelating siderophores. In this study an extracytoplasmic function (ECF) sigma factor, AcsS, encoded within the achromobactin gene cluster is shown to be a major regulator of genes involved in the biosynthesis and secretion of this siderophore. However, production of achromobactin was not completely abrogated in the deletion mutant, implying that other regulators may be involved such as PvdS, the sigma factor that regulates pyoverdine biosynthesis. RNA-seq analysis identified 287 genes that are differentially expressed between the AcsS deletion mutant and the wild type strain. These genes are involved in iron response, secretion, extracellular polysaccharide production, and cell motility. Thus, the transcriptome analysis supports a role for AcsS in the regulation of achromobactin production and the potential activity of both AcsS and achromobactin in the plant-associated lifestyle of strain B728a.

Highlights

Iron is the fourth most common element in the Earth’s crust and, as such, has played an important role in microbial metabolism for millions of years [1]
The plant pathogenic bacterium Pseudomonas syringae pv. syringae strain B301D requires a minimum of 2 mmol/L available iron for the biosynthesis of the potent phytotoxins and major virulence factors, syringomycin and syringopeptin [5]
By examining sequence similarities to other sequenced P. syringae species at both the nucleotide and protein level, it is evident that the genes within this region were likely inherited from different microbial ancestors

Summary

Introduction

Iron is the fourth most common element in the Earth’s crust and, as such, has played an important role in microbial metabolism for millions of years [1]. In environments with physiological pH and oxygen tension, ferric iron (Fe3+) is oxidized and forms stable, insoluble ferric oxide hydrate complexes that cannot be readily utilized by microorganisms [2], [3]. The formation of these stable complexes leaves an environment with a free iron content of only 1029 to 10218 M, well below the 1026 to 1028 M required by most microorganisms [4], [3]. In addition to functioning as a co-factor for many metabolic enzymes, iron plays a predominant role in the electron transfer chain, as well as, the catalysis of numerous cellular redox reactions [3]. P. syringae utilizes high affinity iron uptake systems to procure iron, which is critical to its survival in the plant environment and expression of virulence determinants

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