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...

There exist commonalities between symbiotic Sinorhizobium meliloti and pathogenic Brucella bacteria in terms of extensive gene synteny and the requirements for intracellular survival in their respective hosts. The RNA chaperone Hfq is essential for virulence for several bacterial groups, including Brucella; however, its role in S. meliloti has not been investigated. Our studies of an S. meliloti loss-of-function hfq mutant have revealed that Hfq plays a key role in the establishment of the symbiosis between S. meliloti and its host Medicago sativa. S. meliloti Hfq is involved in controlling the population density under a free-living state and affects the growth parameters and nodulation. An hfq mutant poorly colonizes the infection threads that are necessary for the bacteria to invade the developing nodule. An hfq mutant is severely impaired in its ability to invade plant cells within the nodule, which leads to the formation of small, ineffective nodules unable to fix nitrogen. In culture, the hfq mutant did not accumulate transcripts of nifA, which encodes a key regulator necessary for nitrogen fixation. Hfq may be involved in regulation of several proteins relevant to hfq mutant phenotypes. The crucial role of Hfq in symbiosis suggests that small regulatory RNAs are important for its interactions with its plant host.The alpha subgroup of proteobacteria includes members such as Sinorhizobium meliloti, which establishes a symbiosis with certain leguminous plants, as well as members, such as Brucella abortus, that are highly pathogenic to animals and humans. Despite the very different outcomes of the chronic intracellular infections established by S. meliloti and B. abortus in their respective hosts, parallels can be drawn because in both cases the bacteria need to survive within acidic membrane compartments for a prolonged time after endocytosis (50). Comparisons of genomic sequences between brucellae and rhizobia have revealed similarities. For example, extensive gene synteny exists between chromosome I of Brucella and the genome of Mesorhizobium loti, while chromosome II shares regions of gene synteny with the pSym megaplasmids of S. meliloti. In addition, the transport and metabolic capabilities of Brucella have similarities to those of the plant symbiont (46). Genomic comparisons suggest that these two types of bacteria share a complex evolutionary history and that Brucella evolved from soil/plant-associated ancestral bacteria of the Rhizobium/ Agrobacterium group (46).The parallel between these two bacterial groups is well illustrated by the inner membrane protein BacA, which is critical for the maintenance of the chronic intracellular infections that underlie these two very diverse host-bacterial relationships (20, 35). S. meliloti bacA mutants lyse soon after being endocytosed into the plant cytoplasm (20), while B. abortus bacA mutants are defective in intracellular replication in macrophages and are unable to establish a chronic infection in BALB/c mice (35). BacA is required for the transport of certai...