Isolation of the rstA gene as a multicopy suppressor of YjeE, an essential ATPase of unknown function in Escherichia coli

Gradients in gene essentiality reshape antibacterial research

Essential genes encode the processes that are necessary for life. Until recently, commonly applied binary classifications left no space between essential and non-essential genes. In this review, we frame bacterial gene essentiality in the context of genetic networks. We explore how the quantitative properties of gene essentiality are influenced by the nature of the encoded process, environmental conditions and genetic background, including a strain's distinct evolutionary history. The covered topics have important consequences for antibacterials, which inhibit essential processes. We argue that the quantitative properties of essentiality can thus be used to prioritize antibacterial cellular targets and desired spectrum of activity in specific infection settings. We summarize our points with a case study on the core essential genome of the cystic fibrosis pathobiome and highlight avenues for targeted antibacterial development.

The Tyrosine-Autokinase UbK Is Required for Proper Cell Growth and Cell Morphology of Streptococcus pneumoniae

Protein phosphorylation is a key post-translational modification required for many cellular functions of the bacterial cell. Recently, we identified a new protein-kinase, named UbK, in Bacillus subtilis that belongs to a new family of protein-kinases widespread in bacteria. In this study, we analyze the function of UbK in Streptococcus pneumoniae. We show that UbK displays a tyrosine-kinase activity and autophosphorylates on a unique tyrosine in vivo. To get insights into its cellular role, we constructed a set of pneumococcal ubk mutants. Using conventional and electron microscopy, we show that the ubk deficient strain, as well as an ubk catalytic dead mutant, display both severe cell-growth and cell-morphology defects. The same defects are observed with a mutant mimicking permanent phosphorylation of UbK whereas they are not detected for a mutant mimicking defective autophosphorylation of UbK. Moreover, we find that UbK phosphorylation promotes its ability to hydrolyze ATP. These observations show that the hydrolysis of ATP by UbK serves not only for its autophosphorylation but also for a distinct purpose essential for the optimal cell growth and cell-morphogenesis of the pneumococcus. We thus propose a model in which the autophosphorylation/dephosphorylation of UbK regulates its cellular function through a negative feedback loop.

The RstAB System Impacts Virulence, Motility, Cell Morphology, Penicillin Tolerance and Production of Type II Secretion System-Dependent Factors in the Fish and Human Pathogen Photobacterium damselae subsp. damselae

The RstB histidine kinase of the two component system RstAB positively regulates the expression of damselysin (Dly), phobalysin P (PhlyP) and phobalysin C (PhlyC) cytotoxins in the fish and human pathogen Photobacterium damselae subsp. damselae, a marine bacterium of the family Vibrionaceae. However, the function of the predicted cognate response regulator RstA has not been studied so far, and the role of the RstAB system in other cell functions and phenotypes remain uninvestigated. Here, we analyzed the effect of rstA and rstB mutations in cell fitness and in diverse virulence-related features. Both rstA and rstB mutants were severely impaired in virulence for sea bream and sea bass fish. Mutants in rstA and rstB genes were impaired in hemolysis and in Dly-dependent phospholipase activity but had intact PlpV-dependent phospholipase and ColP-dependent gelatinase activities. rstA and rstB mutants grown at 0.5% NaCl exhibited impaired swimming motility, enlarged cell size and impaired ability to separate after cell division, whereas at 1% NaCl the mutants exhibited normal phenotypes. Mutation of any of the two genes also impacted tolerance to benzylpenicillin. Notably, rstA and rstB mutants showed impaired secretion of a number of type II secretion system (T2SS)-dependent proteins, which included the three major cytotoxins Dly, PhlyP and PhlyC, as well as a putative delta-endotoxin and three additional uncharacterized proteins which might constitute novel virulence factors of this pathogenic bacterium. The analysis of the T2SS-dependent secretome of P. damselae subsp. damselae also led to the identification of RstAB-independent potential virulence factors as lipoproteins, sialidases and proteases. The RstAB regulon included plasmid, chromosome I and chromosome II-encoded genes that showed a differential distribution among isolates of this subspecies. This study establishes RstAB as a major regulator of virulence and diverse cellular functions in P. damselae subsp. damselae.

Essentiality of threonylcarbamoyladenosine (t(6)A), a universal tRNA modification, in bacteria

Threonylcarbamoyladenosine (t(6)A) is a modified nucleoside universally conserved in tRNAs in all three kingdoms of life. The recently discovered genes for t(6)A synthesis, including tsaC and tsaD, are essential in model prokaryotes but not essential in yeast. These genes had been identified as antibacterial targets even before their functions were known. However, the molecular basis for this prokaryotic-specific essentiality has remained a mystery. Here, we show that t(6)A is a strong positive determinant for aminoacylation of tRNA by bacterial-type but not by eukaryotic-type isoleucyl-tRNA synthetases and might also be a determinant for the essential enzyme tRNA(Ile)-lysidine synthetase. We confirm that t(6)A is essential in Escherichia coli and a survey of genome-wide essentiality studies shows that genes for t(6)A synthesis are essential in most prokaryotes. This essentiality phenotype is not universal in Bacteria as t(6)A is dispensable in Deinococcus radiodurans, Thermus thermophilus, Synechocystis PCC6803 and Streptococcus mutans. Proteomic analysis of t(6)A(-) D. radiodurans strains revealed an induction of the proteotoxic stress response and identified genes whose translation is most affected by the absence of t(6)A in tRNAs. Thus, although t(6)A is universally conserved in tRNAs, its role in translation might vary greatly between organisms.

Patterns of evolutionary conservation of essential genes correlate with their compensability

Essential genes code for fundamental cellular functions required for the viability of an organism. For this reason, essential genes are often highly conserved across organisms. However, this is not always the case: orthologues of genes that are essential in one organism are sometimes not essential in other organisms or are absent from their genomes. This suggests that, in the course of evolution, essential genes can be rendered nonessential. How can a gene become non-essential? Here we used genetic manipulation to deplete the products of 26 different essential genes in Escherichia coli. This depletion results in a lethal phenotype, which could often be rescued by the overexpression of a non-homologous, non-essential gene, most likely through replacement of the essential function. We also show that, in a smaller number of cases, the essential genes can be fully deleted from the genome, suggesting that complete functional replacement is possible. Finally, we show that essential genes whose function can be replaced in the laboratory are more likely to be non-essential or not present in other taxa. These results are consistent with the notion that patterns of evolutionary conservation of essential genes are influenced by their compensability-that is, by how easily they can be functionally replaced, for example through increased expression of other genes.

Biosynthesis of threonylcarbamoyl adenosine (t6A), a universal tRNA nucleoside

The anticodon stem-loop (ASL) of transfer RNAs (tRNAs) drives decoding by interacting directly with the mRNA through codon/anticodon pairing. Chemically complex nucleoside modifications found in the ASL at positions 34 or 37 are known to be required for accurate decoding. Although over 100 distinct modifications have been structurally characterized in tRNAs, only a few are universally conserved, among them threonylcarbamoyl adenosine (t(6)A), found at position 37 in the anticodon loop of a subset of tRNA. Structural studies predict an important role for t(6)A in translational fidelity, and in vivo work supports this prediction. Although pioneering work in the 1970s identified the fundamental substrates for t(6)A biosynthesis, the enzymes responsible for its biosynthesis have remained an enigma. We report here the discovery that in bacteria four proteins (YgjD, YrdC, YjeE, and YeaZ) are both necessary and sufficient for t(6)A biosynthesis in vitro. Notably, YrdC and YgjD are members of universally conserved families that were ranked among the top 10 proteins of unknown function in need of functional characterization, while YeaZ and YjeE are specific to bacteria. This latter observation, coupled with the essentiality of all four proteins in bacteria, establishes this pathway as a compelling new target for antimicrobial development.

Activated by different signals, the PhoP/PhoQ two-component system differentially regulates metal uptake

The PhoP/PhoQ two-component system controls several physiological and virulence functions in Salmonella enterica. This system is activated by low Mg(2+), acidic pH, and antimicrobial peptides, but the biological consequences resulting from sensing multiple signals are presently unclear. Here, we report that the PhoP/PhoQ system regulates different Salmonella genes depending on whether the inducing signal is acidic pH or low Mg(2+). When Salmonella experiences acidic pH, the PhoP/PhoQ system promotes Fe(2+) uptake in a process that requires the response regulator RstA, activating transcription of the Fe(2+) transporter gene feoB. In contrast, the PhoP-induced RstA protein did not promote feoB expression at neutral pH with low Mg(2+). The PhoP/PhoQ system promotes the expression of the Mg(2+) transporter mgtA gene only when activated in bacteria starved for Mg(2+). This is because mgtA transcription promoted at high Mg(2+) concentrations by the acidic-pH-activated PhoP protein failed to reach the mgtA coding region due to the mgtA leader region functioning as a Mg(2+) sensor. Our results show that a single two-component regulatory system can regulate distinct sets of genes in response to different input signals.

Conserved network of proteins essential for bacterial viability

The yjeE, yeaZ, and ygjD genes are highly conserved in the genomes of eubacteria, and ygjD orthologs are also found throughout the Archaea and eukaryotes. In this study, we have constructed conditional expression strains for each of these genes in the model organism Escherichia coli K12. We show that each gene is essential for the viability of E. coli under laboratory growth conditions. Growth of the conditional strains under nonpermissive conditions results in dramatic changes in cell ultrastructure. Deliberate repression of the expression of yeaZ results in cells with highly condensed nucleoids, while repression of yjeE and ygjD expression results in at least a proportion of very enlarged cells with an unusual peripheral distribution of DNA. Each of the three conditional expression strains can be complemented by multicopy clones harboring the rstA gene, which encodes a two-component-system response regulator, strongly suggesting that these proteins are involved in the same essential cellular pathway. The results of bacterial two-hybrid experiments show that YeaZ can interact with both YjeE and YgjD but that YgjD is the preferred interaction partner. The results of in vitro experiments indicate that YeaZ mediates the proteolysis of YgjD, suggesting that YeaZ and YjeE act as regulators to control the activity of this protein. Our results are consistent with these proteins forming a link between DNA metabolism and cell division.

The ATPase activity of an 'essential' Bacillus subtilis enzyme, YdiB, is required for its cellular function and is modulated by oligomerization

Characterization of 'unknown' proteins is one of the challenges of the post-genomic era. Here, we report a study of Bacillus subtilis YdiB, which belongs to an uncharted class of bacterial P-loop ATPases. Precise deletion of the ydiB gene yielded a mutant with much reduced growth rate compared to the wild-type strain. In vitro, purified YdiB was in equilibrium among different forms, monomers, dimers and oligomers, and this equilibrium was strongly affected by salts; high concentrations of NaCl favoured the monomeric over the oligomeric form of the enzyme. Interestingly, the ATPase activity of the monomer was about three times higher than that of the oligomer, and the monomer showed a K(m) of about 60 microM for ATP and a V(max) of about 10 nmol min(-1) (mg protein)(-1) (k(cat) approximately 10 h(-1)). This low ATPase activity was shown to be specific to YdiB because mutation of an invariant lysine residue in the P-loop motif (K41A) strongly attenuated this rate. This mutant was unable to restore a normal growth phenotype when introduced into a conditional knockout strain for ydiB, showing that the ATPase activity of YdiB is required for the in vivo function of the protein. Oligomerization was also observed with the purified YjeE from Escherichia coli, a YdiB orthologue, suggesting that this property is shared by all members of this family of ATPases. Importantly, dimers of YdiB were also observed in a B. subtilis extract, or when stabilized by formaldehyde cross-linking for YjeE from E. coli, suggesting that oligomerization might regulate the function of this new class of proteins in vivo.

Induction of RpoS degradation by the two-component system regulator RstA in Salmonella enterica

Bacterial survival in diverse and changing environments relies on the accurate interplay between different regulatory pathways, which determine the design of an adequate adaptive response. The proper outcome depends on a precise gene expression profile generated from the finely tuned and concerted action of transcriptional factors of distinct regulatory hierarchies. Salmonella enterica serovar Typhimurium harbors multiple regulatory systems that are crucial for the bacterium to cope with harsh extra- and intracellular environments. In this work, we found that the expression of Salmonella RstA, a response regulator from the two-component system family, was able to downregulate the expression of three RpoS-controlled genes (narZ, spvA, and bapA). Furthermore, this downregulation was achieved by a reduction in RpoS cellular levels. The alternative sigma factor RpoS is critical for bacterial endurance under the most-stressful conditions, including stationary-phase entrance and host adaptation. Accordingly, RpoS cellular levels are tightly controlled by complex transcriptional, translational, and posttranslational mechanisms. The analysis of each regulatory step revealed that in Salmonella, RstA expression was able to promote RpoS degradation independently of the MviA-ClpXP proteolytic pathway. Additionally, we show that RstA is involved in modulating Salmonella biofilm formation. The fact that the RpoS-modulated genes affected by RstA expression have previously been demonstrated to contribute to Salmonella pathogenic traits, which include biofilm-forming capacity, suggests that under yet unknown conditions, RstA may function as a control point of RpoS-dependent pathways that govern Salmonella virulence.