The stringent response is a universal adaptive mechanism to protect bacteria from nutritional and environmental stresses. The role of the stringent response during lipid starvation has been studied only in Gram-negative bacteria. Here, we report that the stringent response also plays a crucial role in the adaptation of the model Gram-positive Bacillus subtilis to fatty acid starvation. B. subtilis lacking all three (p)ppGpp-synthetases (Rel , RelP and RelQ) or bearing a Rel variant that no longer synthesizes (p)ppGpp suffer extreme loss of viability on lipid starvation. Loss of viability is paralleled by perturbation of membrane integrity and function, with collapse of membrane potential as the likely cause of death. Although no increment of (p)ppGpp could be detected in lipid starved B. subtilis, we observed a substantial increase in the GTP/ATP ratio of strains incapable of synthesizing (p)ppGpp. Artificially lowering GTP with decoyinine rescued viability of such strains, confirming observations that low intracellular GTP is important for survival of nutritional stresses. Altogether, our results show that activation of the stringent response by lipid starvation is a broadly conserved response of bacteria and that a key role of (p)ppGpp is to couple biosynthetic processes that become detrimental if uncoordinated.

Although homologs of the ATP-dependent Lon protease exist in all domains of life, the relevance of this protease in archaeal physiology remains a mystery. In this study, we have constructed and phenotypically characterized deletion and conditional lon mutants in the model haloarchaeon Haloferax volcanii to elucidate the role of the unusual membrane-bound LonB protease in archaea. Hvlon could be deleted from the chromosome only when a copy of the wild type gene was provided in trans suggesting that Lon is essential for survival in this archaeon. Successful complementation of the lethal phenotype of ΔHvlon was attained by expression of the heterologous protease gene Nmlon from the haloalkaliphilic archaeon Natrialba magadii, meaning that the biological function of Lon is conserved in these organisms. Suboptimal cellular levels of Lon protein affected growth rate, cell shape, cell pigmentation, lipid composition and sensitivity to various antibiotics. The contents of bacterioruberins and some polar lipids were increased in the lon mutants suggesting that Lon is linked to maintenance of membrane lipid balance which likely affects cell viability in this archaeon. The phenotypes associated to a membrane-bound LonB protease mutant were examined for the first time providing insight on the relevance of this protease in archaeal physiology.

The bacterial flagellum is a complex and dynamic nanomachine that propels bacteria through liquids. It consists of a basal body, a hook, and a long filament. The flagellar filament is composed of thousands of copies of the protein flagellin (FliC) arranged helically and ending with a filament cap composed of an oligomer of the protein FliD. The overall structure of the filament core is preserved across bacterial species, while the outer domains exhibit high variability, and in some cases are even completely absent. Flagellar assembly is a complex and energetically costly process triggered by environmental stimuli and, accordingly, highly regulated on transcriptional, translational and post-translational levels. Apart from its role in locomotion, the filament is critically important in several other aspects of bacterial survival, reproduction and pathogenicity, such as adhesion to surfaces, secretion of virulence factors and formation of biofilms. Additionally, due to its ability to provoke potent immune responses, flagellins have a role as adjuvants in vaccine development. In this review, we summarize the latest knowledge on the structure of flagellins, capping proteins and filaments, as well as their regulation and role during the colonization and infection of the host.

BackgroundNatrialba magadii is an aerobic chemoorganotrophic member of the Euryarchaeota and is a dual extremophile requiring alkaline conditions and hypersalinity for optimal growth. The genome sequence of Nab. magadii type strain ATCC 43099 was deciphered to obtain a comprehensive insight into the genetic content of this haloarchaeon and to understand the basis of some of the cellular functions necessary for its survival.ResultsThe genome of Nab. magadii consists of four replicons with a total sequence of 4,443,643 bp and encodes 4,212 putative proteins, some of which contain peptide repeats of various lengths. Comparative genome analyses facilitated the identification of genes encoding putative proteins involved in adaptation to hypersalinity, stress response, glycosylation, and polysaccharide biosynthesis. A proton-driven ATP synthase and a variety of putative cytochromes and other proteins supporting aerobic respiration and electron transfer were encoded by one or more of Nab. magadii replicons. The genome encodes a number of putative proteases/peptidases as well as protein secretion functions. Genes encoding putative transcriptional regulators, basal transcription factors, signal perception/transduction proteins, and chemotaxis/phototaxis proteins were abundant in the genome. Pathways for the biosynthesis of thiamine, riboflavin, heme, cobalamin, coenzyme F420 and other essential co-factors were deduced by in depth sequence analyses. However, approximately 36% of Nab. magadii protein coding genes could not be assigned a function based on Blast analysis and have been annotated as encoding hypothetical or conserved hypothetical proteins. Furthermore, despite extensive comparative genomic analyses, genes necessary for survival in alkaline conditions could not be identified in Nab. magadii.ConclusionsBased on genomic analyses, Nab. magadii is predicted to be metabolically versatile and it could use different carbon and energy sources to sustain growth. Nab. magadii has the genetic potential to adapt to its milieu by intracellular accumulation of inorganic cations and/or neutral organic compounds. The identification of Nab. magadii genes involved in coenzyme biosynthesis is a necessary step toward further reconstruction of the metabolic pathways in halophilic archaea and other extremophiles. The knowledge gained from the genome sequence of this haloalkaliphilic archaeon is highly valuable in advancing the applications of extremophiles and their enzymes.