BP participated in the design of the study. All authors read and approved

the final manuscript.”
“Background Stress response in bacteria is essential for effective adaptation to changes find more in the environment, as well as to changes in the bacterial physiological state. This response is mediated by global regulatory mechanisms that operate in an effective method of transcriptional control, with the participation of specialized RNA polymerase subunits, the alternative sigma factors [1]. Bacteria usually display two distinct responses to stress conditions: a response that controls the conditions in the cytoplasm, which is orchestrated by the alternative sigma factor σ32, and a response to the conditions in the periplasm, which is orchestrated by the alternative sigma factor σE [2]. Each response deals with the cellular ability to sense protein folding and other signals, and leads to the activation of proteins such as molecular chaperones, proteases, and regulatory factors, which play an important role in promoting homeostasis under stress conditions [3–5]. The heat shock response is a widespread phenomenon found in all living cells. In bacteria, it is controlled at the transcriptional level by the alternative sigma factor RpoH (σ32) [6–8]. In addition

to the response to high temperatures, RpoH is known to be involved in the response to pH and oxidative stress [9–11]. The selleck σ32 regulon protects many cytoplasmic molecules and processes, including transcription factors, as well as cytoplasmic membranes and inner membrane proteins [6, 8]. In E. coli, RpoH controls the expression of about 91 genes [12], including many coding for heat shock proteins, which are important for survival during stress conditions. Among these are the genes encoding chaperones, such as

GroEL, GroES, DnaK, DnaJ and GrpE and proteases, like FtsH and Lon [13]. Induction of heat shock proteins represents an important protective mechanism to cope with environmental stress, for these proteins mediate the correct folding and assembly of polypeptides. Major functions of heat shock proteins are to prevent inactivation Reverse transcriptase of cellular proteins, to reactivate once inactivated proteins, and to help degrade non-reparable denatured proteins that accumulate under stress conditions [8]. Sinorhizobium meliloti is a Gram-negative α-proteobacterium that establishes root-nodulating, nitrogen fixing, symbiosis with leguminous host plants, such as alfalfa [14–16]. Several important steps in the symbiosis process, like nodule formation and nitrogen fixation, are affected by stress conditions, which might be considered limiting factors. In the soil, variations of temperature, osmolarity, or pH, as well as nutrient starvation, are the stress conditions most frequently faced by rhizobia [17]. Commonly, bacterial genomes contain a single rpoH gene, but several α-proteobacteria have more than one rpoH homologue.