Proteomic Alterations Explain Phenotypic Changes in Sinorhizobium meliloti Lacking the RNA Chaperone Hfq

Abstract

The ubiquitous bacterial RNA-binding protein Hfq is involved in stress resistance and pathogenicity. In Sinorhizobium meliloti, Hfq is essential for the establishment of symbiosis with Medicago sativa and for nitrogen fixation. A proteomic analysis identifies 55 proteins with significantly affected expression in the hfq mutant; most of them are involved in cell metabolism or stress resistance. Important determinants of oxidative stress resistance, such as CysK, Gsh, Bfr, SodC, KatB, KatC, and a putative peroxiredoxine (SMc00072), are downregulated in the hfq mutant. The hfq mutant is affected for H 2 O 2 , menadione, and heat stress resistance. Part of these defects could result from the reductions of rpoE1, rpoE2, rpoE3, and rpoE4 expression levels in the hfq mutant. Some proteins required for efficient symbiosis are reduced in the hfq mutant, contributing to the drastic defect in nodulation observed in this mutant.Gene expression in bacteria is regulated by a wide diversity of mechanisms, including alternative sigma factors, transcriptional regulatory proteins, attenuation mechanisms (including riboswitches) (15), and translational and posttranslational regulations (37). The interplay of central regulatory proteins and alternative sigma factors allows the creation of complex regulatory networks modulating transcription (4).Compared to transcription regulation, the mechanisms affecting the regulation of translation are less understood. Studies dedicated to translation regulation have increased over the past few years (55,76,77). An important development has been the recognition of small regulatory RNAs (sRNAs) that have emerged as crucial actors of translation regulation. In enterobacteria, most sRNAs require Hfq to complex with their targets. Hfq is an RNA chaperone necessary for the pairing of sRNAs with mRNAs (40). Furthermore, Hfq affects translation efficiency by allowing the polyadenylation of specific mRNAs (44). Thus, Hfq is a central actor in translation regulation (72). Hfq is also able to affect transcription, directly by coupling with RNA polymerase (67) or indirectly via its action on sRNAs modulating translation of sigma factors (19,32,69).Due to its central role, hfq inactivation results in a pleiotropic phenotype in enterobacteria and Brucella abortus, including growth defects, stress susceptibility, and altered pathogenicity (56,65,76). Our accompanying study shows that loss of hfq impairs the ability of Sinorhizobium meliloti to establish a nitrogen-fixing symbiosis with its legume host, Medicago sativa. S. meliloti faces numerous stresses during the course of invading the developing root nodules and colonizing the plant cells (21,22,35,46,62). Bacterial abilities to resist and adapt to these stresses are of crucial importance for the symbiosis. Oxidative stress has been the most intensively investigated stress that S. meliloti must withstand and appears as a key factor for bacterium-plant cell interaction. To cope with oxidative stress, S. meliloti cells posses a detoxification system involvi...

The RNA Chaperone Hfq Promotes Fitness of Actinobacillus pleuropneumoniae during Porcine Pleuropneumonia

“…Brucella abortus hfq mutants exhibit poor expression of the sodC gene, which encodes a periplasmic Cu-Zn superoxide dismutase (39). Hfq mutants of Burkholderia cepacia (40), Moraxella catarrhalis (41), Neisseria meningitidis (42), Sinorhizobium meliloti (43), and uropathogenic E. coli (16) display enhanced sensitivity to methyl viologen, a redox-cycling agent which generates superoxide within bacterial cells. In contrast, hfq mutants of S. aureus (44) and Vibrio parahaemolyticus (45) exhibit higher resistance to oxidative stress induced by hydrogen peroxide and superoxide than does the parent strain.…”

Hfq Influences Multiple Transport Systems and Virulence in the Plant Pathogen Agrobacterium tumefaciens

“…In the photosynthetic alphaproteobacterium Rhodobacter sphaeroides, the Hfq protein plays an important role in protection against photooxidative stress (5,6). During the past 2 years, sev-eral studies on Hfq in the plant symbiont Sinorhizobium meliloti demonstrated the importance of the RNA chaperone in cell physiology and symbiosis (3,4,24,54,60,65).…”

A Putative Bifunctional Histidine Kinase/Phosphatase of the HWE Family Exerts Positive and Negative Control on the Sinorhizobium meliloti General Stress Response

“…RpoE2 is activated under a number of stress and starvation conditions and controls the transcription of Ͼ100 genes, including several involved in stress resistance (5)(6)(7)(8)(9)(10). Accordingly, rpoE2 mutants have been found to be more sensitive than the wild-type strain to desiccation and osmotic stress, as well as heat and oxidative stress in the stationary phase (6)(7)(8)11). RpoE2 orthologues, collectively called EcfG or ECF15 sigma factors (12), are widely distributed among Alphaproteobacteria, and several of them have been described as activated under stress or starvation conditions and to play various roles in stress resistance and/or host colonization (13)(14)(15)(16)(17)(18)(19)(20)(21)(22).…”

“…However, in the alphaproteobacterial group, the prominent role of extracytoplasmic-function sigma factors was uncovered recently with the finding that RpoE2 controls a general stress response in Sinorhizobium meliloti, the nitrogen-fixing symbiont of alfalfa (5). RpoE2 is activated under a number of stress and starvation conditions and controls the transcription of Ͼ100 genes, including several involved in stress resistance (5)(6)(7)(8)(9)(10). Accordingly, rpoE2 mutants have been found to be more sensitive than the wild-type strain to desiccation and osmotic stress, as well as heat and oxidative stress in the stationary phase (6)(7)(8)11).…”