Abstract
Signal transduction systems and ABC transporters often contribute jointly to adaptive bacterial responses to environmental changes. In Bacillus subtilis, three such pairs are involved in responses to antibiotics: BceRSAB, YvcPQRS and YxdJKLM. They are characterized by a histidine kinase belonging to the intramembrane sensing kinase family and by a translocator possessing an unusually large extracytoplasmic loop. It was established here using a phylogenomic approach that systems of this kind are specific but widespread in Firmicutes, where they originated. The present phylogenetic analyses brought to light a highly dynamic evolutionary history involving numerous horizontal gene transfers, duplications and lost events, leading to a great variety of Bce-like repertories in members of this bacterial phylum. Based on these phylogenetic analyses, it was proposed to subdivide the Bce-like modules into six well-defined subfamilies. Functional studies were performed on members of subfamily IV comprising BceRSAB from B. subtilis, the expression of which was found to require the signal transduction system as well as the ABC transporter itself. The present results suggest, for the members of this subfamily, the occurrence of interactions between one component of each partner, the kinase and the corresponding translocator. At functional and/or structural levels, bacitracin dependent expression of bceAB and bacitracin resistance processes require the presence of the BceB translocator loop. Some other members of subfamily IV were also found to participate in bacitracin resistance processes. Taken together our study suggests that this regulatory mechanism might constitute an important common antibiotic resistance mechanism in Firmicutes. [Supplemental material is available online at http://www.genome.org.]
Highlights
Survival of microorganisms in their natural habitat depends on their ability to cope with fast environmental changes by controlling parameters such as ionic strength and osmotic pressure, making use of the nutriments available and resisting any toxic compounds present in the environment
In the simplest systems of this kind, which are known as twocomponent systems, a two-step phosphorus transfer process is effected by a histidine protein kinase (HK) and a response regulator protein (RR)
Among the 579 BceS homologues collected, 212 showed the same architectural features as B. subtilis BceS, i.e. exactly two transmembrane segments (TMS) separated by a loop consisting of 12 amino acids or less
Summary
Survival of microorganisms in their natural habitat depends on their ability to cope with fast environmental changes by controlling parameters such as ionic strength and osmotic pressure, making use of the nutriments available and resisting any toxic compounds present in the environment. Microorganisms have developed sophisticated signal transduction systems whereby extracellular stimuli are detected by membrane-integrated sensors [1]. The second group is composed of kinases in which 2 to 20 transmembrane segments are connected by very short linkers These kinases are able to detect membrane or membrane-associated stimuli and have been called intra-membrane sensing kinases. Once a stimulus has been sensed, the kinase autophosphorylates a conserved histidine residue present in its transmitter domain. The phosphoryl group is subsequently transferred to a conserved aspartate residue in the regulator receiver domain, which controls the expression of target genes [3]. There exist other, more complex regulatory systems involving the activation of a four-step phosphorylation cascade via extra receiver and transmitter domains [4]
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