Possible role for the essential GTP-binding protein Obg in regulating the initiation of sporulation in Bacillus subtilis

Experimental evolution of gene essentiality in bacteria

Essential gene products carry out fundamental cellular activities in interaction with other components. However, the lack of essential gene mutants and appropriate methodologies to link essential gene functions with their partners poses significant challenges. Here, we have generated deletion mutants in 32 genes previously identified as essential, with 23 mutants showing extremely slow growth in the SK36 strain of Streptococcus sanguinis . Whole-genome sequencing of 243 independently evolved populations of these mutants has identified >1000 spontaneous suppressor mutations in experimental evolution, many of which are new gene and pathway relationships, such as F1Fo-ATPase/V1V1-ATPase/TrkA1-H1 gene pathway. Patterns of spontaneous mutations occurring in essential gene mutants differed from those found in wildtype. While gene duplications occurred rarely, substitutions, deletions, and insertions were prevalent in evolved populations. These essential gene deletion mutants and spontaneous mutations fixed in the mutant populations during evolution established a foundation for understanding gene essentiality and the interaction of essential genes in networks.

Characterization of a Riboflavin-Producing Mutant of Bacillus subtilis Isolated by Droplet-Based Microfluidics Screening

Bacillus subtilis is one of the commonly used industrial strains for riboflavin production. High-throughput screening is useful in biotechnology, but there are still an insufficient number of articles focusing on improving the riboflavin production of B. subtilis by this powerful tool. With droplet-based microfluidics technology, single cells can be encapsulated in droplets. The screening can be carried out by detecting the fluorescence intensity of secreted riboflavin. Thus, an efficient and high-throughput screening method suitable for riboflavin production strain improvement could be established. In this study, droplet-based microfluidics screening was applied, and a more competitive riboflavin producer U3 was selected from the random mutation library of strain S1. The riboflavin production and biomass of U3 were higher than that of S1 in flask fermentation. In addition, the results of fed-batch fermentation showed that the riboflavin production of U3 was 24.3 g/L, an 18% increase compared with the parent strain S1 (20.6 g/L), and the yield (g riboflavin/100 g glucose) increased by 19%, from 7.3 (S1) to 8.7 (U3). Two mutations of U3 (sinR^G89R and icd^D28E) were identified through whole genome sequencing and comparison. Then they were introduced into BS168DR (parent of S1) for further analysis, which also caused riboflavin production to increase. This paper provides protocols for screening riboflavin-producing B. subtilis with droplet-based microfluidics technology and reveals mutations in riboflavin overproduction strains.

Deletion analyses reveal insights into the domain specific activities of an essential GTPase CgtA in Vibrio cholerae

CgtA is an essential bacterial GTPase protein involved in multiple cellular activities. In the presence of 50S ribosome, its GTPase activity increases significantly. Through sequential deletions of CgtA protein of Vibrio cholerae (CgtA_vc) we found that its N terminal Obg domain is essential for ribosome binding and augmenting the ribosome mediated GTPase activity. Strategic deletions of the three glycine rich loops of Obg domain revealed that loop 1 of Obg domain is involved in anchoring the protein into the 50S, whereas, loop 2 & loop 3 are involved in conveying the effect of interaction of the Obg domain with the 50S to the GTPase domain through an interdomain linker, followed by GTP hydrolysis. On the other hand, the non-conserved C-terminal domain (CTD) is not directly involved in ribosome binding but shows negative impact on GTPase activity.

Complementation of the Mycoplasma synoviae MS-H vaccine strain with wild-type obg influencing its growth characteristics

The temperature-sensitive (ts+) Mycoplasma synoviae vaccine strain MS-H harbors a non-synonymous mutation which results in Glycine to Arginine substitution at position 123 in the highly conserved glycine-rich motif of Obg-fold in the GTP-binding protein Obg. In-silico analysis of the wild-type and mutant Obgs of M. synoviae has indicated that this amino acid substitution affects structure of the protein, potentially leading to abrogation of Obg function in vivo. Present study was conducted to develop the first expression vector for M. synoviae and to investigate the potential effect(s) of complementation of MS-H vaccine with the wild-type obg from 86079/7NS, the parent strain of MS-H. An oriC vector, pKS-VOTL, harboring the 86079/7NS obg gene, downstream of the variable lipoprotein haemagglutinin (vlhA) gene promoter, also cloned from 86079/7NS, was used to transform MS-H. The plasmid was localised at the chromosomal oriC locus of MS-H without any detectable integration at the chromosomal obg locus. Analysis of the MS-H transformants revealed abundant obg transcripts as well as Obg protein, when compared to the MS-H transformed with a similar vector, pMAS-LoriC, lacking obg coding sequence. The MS-H transformants complemented with wild-type Obg maintained their original temperature-sensitivity phenotype (consistent with MS-H vaccine) but, when compared to the MS-H transformed with pMAS-LoriC, had significantly higher (p < 0.05) growth rate and viability at the permissive (33°C) and non-permissive temperature (39.5°C), respectively. Analysis of Obg expression in MS-H and its wild-type parent strain revealed comparatively lower levels of Obg in MS-H. These results indicate that not only the mutation in Obg, but also the level of Obg expression, can confer functional abnormalities in the bacterial host. Furthermore, with the construction of first expression vector for M. synoviae, this study has set foundation for the development of recombinant vaccine(s) based on MS-H.

Slowdown of growth controls cellular differentiation

How can changes in growth rate affect the regulatory networks behavior and the outcomes of cellular differentiation? We address this question by focusing on starvation response in sporulating Bacillus subtilis We show that the activity of sporulation master regulator Spo0A increases with decreasing cellular growth rate. Using a mathematical model of the phosphorelay-the network controlling Spo0A-we predict that this increase in Spo0A activity can be explained by the phosphorelay protein accumulation and lengthening of the period between chromosomal replication events caused by growth slowdown. As a result, only cells growing slower than a certain rate reach threshold Spo0A activity necessary for sporulation. This growth threshold model accurately predicts cell fates and explains the distribution of sporulation deferral times. We confirm our predictions experimentally and show that the concentration rather than activity of phosphorelay proteins is affected by the growth slowdown. We conclude that sensing the growth rates enables cells to indirectly detect starvation without the need for evaluating specific stress signals.

Two conserved amino acids of juxtaposed domains of a ribosomal maturation protein CgtA sustain its optimal GTPase activity

CgtA is a highly conserved ribosome binding protein involved in ribosome biogenesis and associated with stringent response. It is a 55 KDa GTPase protein consisting of GTPase, Obg and C-terminal domains. The function of the latter two domains was not clear and despite the importance, the mode of action of CgtA is still largely unknown. Knocking out of CgtA gene is lethal and mutations lead to growth, sporulation and developmental defects in bacteria. It was found that a growth defect and pinhole size colony morphology of Bacillus subtilis was associated with a Gly92Asp point mutation on the Obg domain of its CgtA protein, instead of its GTPase domain. CgtA is an important and essential protein of the deadly diarrhea causing bacteria Vibrio cholerae and in order to investigate the mode of action of the V. cholerae CgtA we have utilized this information. We measured the GTPase activity of V. cholerae CgtA (CgtAvc) protein in the presence of purified ribosome. Our results showed 5-fold increased GTP hydrolysis activity compared to its intrinsic activity. Then we explored the GTPase activity of the mutated CgtAvc (Gly98Asp) located at the Obg domain, which reduced the GTP hydrolysis rate to half. The double point mutations (Gly98Asp, and Tyr194Gly) encompassing another conserved residue, Tyr194, located at the diagonally opposite position in the GTPase domain largely restored (about 82%) the reduced GTPase activity, revealing a fine-tuned inter-domain movement readily associated with the GTPase activity of CgtA and thus maintaining the proper functioning of the CgtA protein.

Functional characterization of Arabidopsis HsfA6a as a heat-shock transcription factor under high salinity and dehydration conditions

Although heat-shock transcription factors are well characterized in the heat stress-related pathway, they are poorly understood in other stress responses. Here, we functionally characterized AtHsfA6a in the presence of exogenous abscisic acid (ABA) and under high salinity and dehydration conditions. AtHsfA6a expression under normal conditions is very low, but was highly induced by exogenous ABA, NaCl and drought. Unexpectedly, the levels of AtHsfA6a transcript were not significantly altered under heat and cold stresses. Electrophoretic mobility shift assays and transient transactivation assays indicated that AtHsfA6a is transcriptionally regulated by ABA-responsive element binding factor/ABA-responsive element binding protein, which are key regulators of the ABA signalling pathway. Additionally, fractionation and protoplast transient assays showed that AtHsfA6a was in cytoplasm and nucleus simultaneously; however, under conditions of high salinity the majority of AtHsfA6A was in the nucleus. Furthermore, at both seed germination and seedlings stage, plants overexpressing AtHsfA6a were hypersensitive to ABA and exhibited enhanced tolerance against salt and drought stresses. Finally, the microarray and qRT-PCR analyses revealed that many stress-responsive genes were up-regulated in the plants overexpressing AtHsfA6a. Taken together, the data strongly suggest that AtHsfA6a acts as a transcriptional activator of stress-responsive genes via the ABA-dependent signalling pathway.

Enhanced antibacterial activity of silver nanoparticles/halloysite nanotubes/graphene nanocomposites with sandwich-like structure

A sandwich-like antibacterial reagent (Ag/HNTs/rGO) was constructed through the direct growth of silver nanoparticles on the surface graphene-based HNTs nanosheets. Herein, various nanomaterials were combined by adhesion effect of DOPA after self-polymerization. Ag/HNTs/rGO posses enhanced antibacterial ability against E. coli and S. aureus compared with individual silver nanoparticles, rGO nanosheets or their nanocomposites.

Mutations in GTP binding protein Obg of Mycoplasma synoviae vaccine strain MS-H: implications in temperature-sensitivity phenotype

Mycoplasma synoviae strain MS-H, developed by chemical mutagenesis of the Australian field strain 86079/7NS, is a live temperature-sensitive (ts (+)) vaccine used for control of M. synoviae infection in poultry worldwide. Genetic basis of temperature sensitivity and attenuation of MS-H has not been revealed thus far. Comparison of the complete genome sequence of MS-H, its parent strain 86079/7NS and two non-temperature sensitive (ts (-)) reisolates of MS-H revealed a mutation in a highly conserved domain of GTP binding protein Obg of MS-H, with reversion in ts (-) MS-H reisolates. Nucleotide change from G to A at position 369 of the obg gene resulted in an alteration of glycine to arginine at position 123 in Obg fold. Further analysis of the complete obg gene sequence in several MS-H reisolates revealed that a Gly123Arg substitution was associated with alteration in temperature sensitivity phenotype of MS-H. A second mutation, C to T at position 629, in obg gene was found in some of the MS-H reisolates and appeared to suppress the effects of the Gly123Arg substitution. In silico analysis of point mutations revealed that Gly123Arg has highly destabilizing effect on the MS-H Obg structure that can potentially abolish its biological functions in vivo especially at non-permissive temperature. Findings of this study implicate Obg alteration (Gly123Arg) as one of the possible causes of MS-H attenuation/temperature sensitivity and warrant further investigations into exploring the role of Obg-like proteins, an evolutionarily conserved protein from human to bacteria, in the biology of mycoplasmas.

Inhibition of infection and transmission of HIV-1 and lack of significant impact on the vaginal commensal lactobacilli by carbohydrate-binding agents

OBJECTIVES

A selection of carbohydrate-binding agents (CBAs) with different glycan specificities were evaluated for their inhibitory effect against HIV infection and transmission, and their interaction with vaginal commensal bacteria.

METHODS

Several assays were used for the antiviral evaluation: (i) cell-free virus infection of human CD4+ T lymphocyte C8166 cells; (ii) syncytium formation in co-cultures of persistently HIV-1-infected HUT-78/HIV-1 and non-infected CD4+ SupT1 cells; (iii) DC-SIGN-directed capture of HIV-1 particles; and (iv) transmission of DC-SIGN-captured HIV-1 particles to uninfected CD4+ C8166 cells. CBAs were also examined for their interaction with vaginal commensal lactobacilli using several viability, proliferation and adhesion assays.

RESULTS

The CBAs showed efficient inhibitory activity in the nanomolar to low-micromolar range against four events that play a crucial role in HIV-1 infection and transmission: cell-free virus infection, fusion between HIV-1-infected and non-infected cells, HIV-1 capture by DC-SIGN and transmission of DC-SIGN-captured virus to T cells. As candidate microbicides should not interfere with the normal human microbiota, we examined the effect of CBAs against Lactobacillus strains, including a variety of vaginal strains, a gastrointestinal strain and several non-human isolates. None of the CBAs included in our studies inhibited the growth of these bacteria in several media, affected their viability or had any significant impact on their adhesion to HeLa cell monolayers.

CONCLUSIONS

The CBAs in this study were inhibitory to HIV-1 in several in vitro infection and transmission models, and may therefore qualify as potential microbicide candidates. The lack of significant impact on commensal vaginal lactobacilli is an important property of these CBAs in view of their potential microbicidal use.

Bacterial Obg proteins: GTPases at the nexus of protein and DNA synthesis

Obg proteins (also known as ObgE, YhbZ and CgtA) are conserved P-loop GTPases, essential for growth in bacteria. Like other GTPases, Obg proteins cycle between a GTP-bound ON and a GDP-bound OFF state, thereby controlling cellular processes. Interestingly, the in vitro biochemical properties of Obg proteins suggest that they act as sensors for the cellular GDP/GTP pools and adjust their activity according to the cellular energy status. Obg proteins have been attributed a host of cellular functions, including roles in essential cellular processes (DNA replication, ribosome maturation) and roles in different stress adaptation pathways (stringent response, sporulation, general stress response). This review summarizes the current knowledge on Obg activity and function. Furthermore, we present a model that integrates the different functions of Obg by assigning it a fundamental role in cellular physiology, at the hub of protein and DNA synthesis. In particular, we believe that Obg proteins might provide a connection between different global pathways in order to fine-tune cellular processes in response to a given energy status.

GTPases involved in bacterial ribosome maturation

The ribosome is an RNA- and protein-based macromolecule having multiple functional domains to facilitate protein synthesis, and it is synthesized through multiple steps including transcription, stepwise cleavages of the primary transcript, modifications of ribosomal proteins and RNAs and assemblies of ribosomal proteins with rRNAs. This process requires dozens of trans-acting factors including GTP- and ATP-binding proteins to overcome several energy-consuming steps. Despite accumulation of genetic, biochemical and structural data, the entire process of bacterial ribosome synthesis remains elusive. Here, we review GTPases involved in bacterial ribosome maturation.

The Escherichia coli GTPase ObgE modulates hydroxyl radical levels in response to DNA replication fork arrest

Obg proteins are universally conserved GTP-binding proteins that are essential for viability in bacteria. Homologs in different organisms are involved in various cellular processes, including DNA replication. The goal of this study was to analyse the structure-function relationship of Escherichia coli ObgE with regard to DNA replication in general and sensitivity to stalled replication forks in particular. Defined C-terminal chromosomal deletion mutants of obgE were constructed and tested for sensitivity to the replication inhibitor hydroxyurea. The ObgE C-terminal domain was shown to be dispensable for normal growth of E.coli. However, a region within this domain is involved in the cellular response to replication fork stress. In addition, a mutant obgE over-expression library was constructed by error-prone PCR and screened for increased hydroxyurea sensitivity. ObgE proteins with substitutions L159Q, G163V, P168V, G216A or R237C, located within distinct domains of ObgE, display dominant-negative effects leading to hydroxyurea hypersensitivity when over-expressed. These effects are abolished in strains with a single deletion of the iron transporter TonB or combined deletions the toxin/antitoxin modules RelBE/MazEF, strains both of which have been shown to be involved in a pathway that stimulates hydroxyl radical formation following hydroxyurea treatment. Moreover, the observed dominant-negative effects are lost in the presence of the hydroxyl radical scavenger thiourea. Together, these results indicate involvement of hydroxyl radical toxicity in ObgE-mediated protection against replication fork stress.

The universally conserved prokaryotic GTPases

Members of the large superclass of P-loop GTPases share a core domain with a conserved three-dimensional structure. In eukaryotes, these proteins are implicated in various crucial cellular processes, including translation, membrane trafficking, cell cycle progression, and membrane signaling. As targets of mutation and toxins, GTPases are involved in the pathogenesis of cancer and infectious diseases. In prokaryotes also, it is hard to overestimate the importance of GTPases in cell physiology. Numerous papers have shed new light on the role of bacterial GTPases in cell cycle regulation, ribosome assembly, the stress response, and other cellular processes. Moreover, bacterial GTPases have been identified as high-potential drug targets. A key paper published over 2 decades ago stated that, "It may never again be possible to capture [GTPases] in a family portrait" (H. R. Bourne, D. A. Sanders, and F. McCormick, Nature 348:125-132, 1990) and indeed, the last 20 years have seen a tremendous increase in publications on the subject. Sequence analysis identified 13 bacterial GTPases that are conserved in at least 75% of all bacterial species. We here provide an overview of these 13 protein subfamilies, covering their cellular functions as well as cellular localization and expression levels, three-dimensional structures, biochemical properties, and gene organization. Conserved roles in eukaryotic homologs will be discussed as well. A comprehensive overview summarizing current knowledge on prokaryotic GTPases will aid in further elucidating the function of these important proteins.

Biochemical and physiological characterization of the GTP-binding protein Obg of Mycobacterium tuberculosis

BACKGROUND

Obg is a highly conserved GTP-binding protein that has homologues in bacteria, archaea and eukaryotes. In bacteria, Obg proteins are essential for growth, and they participate in spore formation, stress adaptation, ribosome assembly and chromosomal partitioning. This study was undertaken to investigate the biochemical and physiological characteristics of Obg in Mycobacterium tuberculosis, which causes tuberculosis in humans.

RESULTS

We overexpressed M. tuberculosis Obg in Escherichia coli and then purified the protein. This protein binds to, hydrolyzes and is phosphorylated with GTP. An anti-Obg antiserum, raised against the purified Obg, detects a 55 kDa protein in immunoblots of M. tuberculosis extracts. Immunoblotting also discloses that cultured M. tuberculosis cells contain increased amounts of Obg in the late log phase and in the stationary phase. Obg is also associated with ribosomes in M. tuberculosis, and it is distributed to all three ribosomal fractions (30 S, 50 S and 70 S). Finally, yeast two-hybrid analysis reveals that Obg interacts with the stress protein UsfX, indicating that M. tuberculosis Obg, like other bacterial Obgs, is a stress related protein.

CONCLUSIONS

Although its GTP-hydrolyzing and phosphorylating activities resemble those of other bacterial Obg homologues, M. tuberculosis Obg differs from them in these respects: (a) preferential association with the bacterial membrane; (b) association with all three ribosomal subunits, and (c) binding to the stress protein UsfX, rather than to RelA. Generation of mutant alleles of Obg of M. tuberculosis, and their characterization in vivo, may provide additional insights regarding its role in this important human pathogen.

Cloning and characterization of engA, a GTP-binding protein from Mycobacterium tuberculosis H(37)Rv

Guanine nucleotides are key signaling molecules and many members of the G-protein family bind and hydrolyze nucleotides, particularly GTP, and regulate intracellular level of GTP and GDP. EngA is one of the members of these universally conserved GTPases. Amino acid sequence alignment of EngA of Mycobacterium tuberculosis H(37)Rv with other homologous bacterial proteins have shown that EngA of M. tuberculosis H(37)Rv has significant homology with EngA of other bacteria. EngA protein has shown GTP-binding and GTP hydrolysis activities as intrinsic biochemical properties of protein and this serves as a base to further investigate the physiological significance of this protein in the pathogenesis mechanism of M. tuberculosis H(37)Rv. In this paper for the first time EngA GTP-binding protein of M. tuberculosis H(37)Rv was functionally characterized for its GTPase and GTP-hydrolyzing activity. GTPases such as era, obg, lepA, and FtsZ are vital for growth and development and specifically cellular functions of bacteria, in view of these observations it can be concluded that EngA GTPase can be further utilized for the study of its functional role in the pathogenesis of M. tuberculosis H(37)Rv.

Molecular modeling study for interaction between Bacillus subtilis Obg and Nucleotides

The bacterial Obg proteins (Spo0B-associated GTP-binding protein) belong to the subfamily of P-loop GTPase proteins that contain two equally and highly conserved domains, a C-terminal GTP binding domain and an N-terminal glycine-rich domain which is referred as the “Obg fold” and now it is considered as one of the new targets for antibacterial drug. When the Obg protein is associated with GTP, it becomes activated, because conformation of Obg fold changes due to the structural changes of GTPase switch elements in GTP binding site. In order to investigate the effects and structural changes in GTP bound to Obg and GTPase switch elements for activation, four different molecular dynamics (MD) simulations were performed with/without the three different nucleotides (GTP, GDP, and GDP + Pi) using the Bacillus subtilis Obg (BsObg) structure. The protein structures generated from the four different systems were compared using their representative structures. The pattern of C_α-C_α distance plot and angle between the two Obg fold domains of simulated apo form and each system (GTP, GDP, and GDP+Pi) were significantly different in the GTP-bound system from the others. The switch 2 element was significantly changed in GTP-bound system. Also root-mean-square fluctuation (RMSF) analysis revealed that the flexibility of the switch 2 element region was much higher than the others. This was caused by the characteristic binding mode of the nucleotides. When GTP was bound to Obg, its γ-phosphate oxygen was found to interact with the key residue (D212) of the switch 2 element, on the contrary there was no such interaction found in other systems. Based on the results, we were able to predict the possible binding conformation of the activated form of Obg with L13, which is essential for the assembly with ribosome.

Arabidopsis OBG-like GTPase (AtOBGL) is localized in chloroplasts and has an essential function in embryo development

OBG-like GTPases, a subfamily of P-loop GTPases, have divers and important functions in bacteria, including initiation of sporulation, DNA replication, and protein translation. Homologs of the Bacillus subtilis spo0B GTP-binding protein (OBG) can be found in plants and algae but their specific function in these organisms has not yet been elucidated. Here, it is shown that AT5G18570 encodes an Arabidopsis thaliana OBG-like protein (AtOBGL) that is localized in chloroplasts. In contrast to the bacterial members of this protein family, AtOBGL and other OBG-like proteins from green algae and plants possess an additional N-terminal domain, indicating functional adaptation. Disruption of the gene locus of ATOBGL by TDNA insertion resulted in an embryo-lethal phenotype and light microscopy using Normarski optics revealed that embryo maturation in the atobgl mutant is arrested at the late globular stage before development of a green embryo. Expression of 35S::ATOBGL within the atobgl mutant background could rescue the mutant phenotype, confirming that embryo-lethality is caused by the loss of AtOBGL. Together, the data show that the bacterial-derived OBG-like GTPases have retained an essential role in chloroplasts of plants and algae. They furthermore corroborate the significance of chloroplast functions for embryo development - an important stage within the Arabidopsis lifecycle.

Role of GTPases in bacterial ribosome assembly

The assembly of the ribosome, a complex molecular machine composed of RNA and protein, is a poorly understood process. Recent work has demonstrated that GTPases are likely to play key roles in the assembly of ribosomes in bacteria and eukaryotes. This review highlights several bacterial ribosome assembly GTPases (RA-GTPases) and discusses possible functions for these proteins in the biogenesis of individual ribosomal subunits and subunit joining. RA-GTPases appear to link various aspects of the cell cycle and metabolism with translation. How these RA-GTPases may coordinate these connections are discussed.

Biochemical characterization of Arabidopsis developmentally regulated G-proteins (DRGs)

Developmentally regulated G-proteins (DRGs) are a highly conserved family of GTP-binding proteins found in archaea, plants, fungi and animals, indicating important roles in fundamental pathways. Their function is poorly understood, but they have been implicated in cell division, proliferation, and growth, as well as several medical conditions. Individual subfamilies within the G-protein superfamily possess unique nucleotide binding and hydrolysis rates that are intrinsic to their cellular function, and so characterization of these rates for a particular G-protein may provide insight into its cellular activity. We have produced recombinant active DRG protein using a bacterial expression system and refolding, and performed biochemical characterization of their GTP binding and hydrolysis. We show that recombinant Arabidopsis thaliana atDRG1 and atDRG2a are able to bind GDP and GTP. We also show that DRGs can hydrolyze GTP in vitro without the assistance of GTPase-activating proteins and guanine exchange factors. The atDRG proteins hydrolyze GTP at a relatively slow rate (0.94x10(-3)min(-1) for DRG1 and 1.36x10(-3)min(-1) for DRG2) that is consistent with their nearest characterized relatives, the Obg subfamily. The ability of DRGs to bind nucleotide substrates without assistance, their slow rate of GTP hydrolysis, heat stress activation and domain conservation suggest a possible role as a chaperone in ribosome assembly in response to stress as it has been suggested for the Obg proteins, a different but related G-protein subfamily.

The growth-promoting and stress response activities of the Bacillus subtilis GTP binding protein Obg are separable by mutation

Bacillus subtilis Obg is a ribosome-associating GTP binding protein that is needed for growth, sporulation, and induction of the bacterium's general stress regulon (GSR). It is unclear whether the roles of Obg in sporulation and stress responsiveness are direct or a secondary effect of its growth-promoting functions. The present work addresses this question by an analysis of two obg alleles whose phenotypes argue for direct roles for Obg in each process. The first allele [obg(G92D)] encodes a missense change in the protein's highly conserved "obg fold" region. This mutation impairs cell growth and the ability of Obg to associate with ribosomes but fails to block sporulation or the induction of the GSR. The second obg mutation [obg(Delta22)] replaces the 22-amino-acid carboxy-terminal sequence of Obg with an alternative 26-amino-acid sequence. This Obg variant cofractionates with ribosomes and allows normal growth but blocks sporulation and impairs the induction of the GSR. Additional experiments revealed that the block on sporulation occurs early, preventing the activation of the essential sporulation transcription factor Spo0A, while inhibition of the GSR appears to involve a failure of the protein cascade that normally activates the GSR to effectively catalyze the reactions needed to activate the GSR transcription factor (sigma(B)).

Functional analysis of the essential GTP-binding-protein-coding gene cgtA of Vibrio cholerae

The cgtA gene, coding for the conserved G protein CgtA, is essential in bacteria. In contrast to a previous report, here we show by using genetic analysis that cgtA is essential in Vibrio cholerae even in a Delta relA background. Depletion of CgtA affected the growth of V. cholerae and rendered the cells highly sensitive to the replication inhibitor hydroxyurea. Overexpression of V. cholerae CgtA caused distinct elongation of Escherichia coli cells. Deletion analysis indicated that the C-terminal end of CgtA plays a critical role in its proper function.

Restricting conformational flexibility of the switch II region creates a dominant-inhibitory phenotype in Obg GTPase Nog1

Nog1 is a conserved eukaryotic GTPase of the Obg family involved in the biogenesis of 60S ribosomal subunits. Here we report the unique dominant-inhibitory properties of a point mutation in the switch II region of mouse Nog1; this mutation is predicted to restrict conformational mobility of the GTP-binding domain. We show that although the mutation does not significantly affect GTP binding, ectopic expression of the mutant in mouse cells disrupts productive assembly of pre-60S subunits and arrests cell proliferation. The mutant impairs processing of multiple pre-rRNA intermediates, resulting in the degradation of the newly synthesized 5.8S/28S rRNA precursors. Sedimentation analysis of nucleolar preribosomes indicates that defective Nog1 function inhibits the conversion of 32S pre-rRNA-containing complexes to a smaller form, resulting in a drastic accumulation of enlarged pre-60S particles in the nucleolus. These results suggest that conformational changes in the switch II element of Nog1 have a critical importance for the dissociation of preribosome-bound factors during intranucleolar maturation and thereby strongly influence the overall efficiency of the assembly process.

G-protein control of the ribosome-associated stress response protein SpoT

The bacterial response to stress is controlled by two proteins, RelA and SpoT. RelA generates the alarmone (p)ppGpp under amino acid starvation, whereas SpoT is responsible for (p)ppGpp hydrolysis and for synthesis of (p)ppGpp under a variety of cellular stress conditions. It is widely accepted that RelA is associated with translating ribosomes. The cellular location of SpoT, however, has been controversial. SpoT physically interacts with the ribosome-associated GTPase CgtA, and we show here that, under an optimized salt condition, SpoT is also associated with a pre-50S particle. Analysis of spoT and cgtA mutants and strains overexpressing CgtA suggests that the ribosome associations of SpoT and CgtA are mutually independent. The steady-state level of (p)ppGpp is increased in a cgtA mutant, but the accumulation of (p)ppGpp during amino acid starvation is not affected, providing strong evidence that CgtA regulates the (p)ppGpp level during exponential growth but not during the stringent response. We show that CgtA is not associated with pre-50S particles during amino acid starvation, indicating that under these conditions in which (p)ppGpp accumulates, CgtA is not bound either to the pre-50S particle or to SpoT. We propose that, in addition to its role as a 50S assembly factor, CgtA promotes SpoT (p)ppGpp degradation activity on the ribosome and that the loss of CgtA from the ribosome is necessary for maximal (p)ppGpp accumulation under stress conditions. Intriguingly, we found that in the absence of spoT and relA, cgtA is still an essential gene in Escherichia coli.

Chromosome segregation control by Escherichia coli ObgE GTPase

Escherichia coli cells depleted of the conserved GTPase, ObgE, show early chromosome-partitioning defects and accumulate replicated chromosomes in which the terminus regions are colocalized. Cells lacking ObgE continue to initiate replication, with a normal ratio of the origin to terminus. Localization of the SeqA DNA binding protein, normally seen as punctate foci, however, was disturbed. Depletion of ObgE also results in cell filamentation, with polyploid DNA content. Depletion of ObgE did not cause lethality, and cells recovered fully after expression of ObgE was restored. We propose a model in which ObgE is required to license chromosome segregation and subsequent cell cycle events.

Regulation of the stringent response is the essential function of the conserved bacterial G protein CgtA in Vibrio cholerae

The gene encoding the conserved bacterial G protein CgtA (Obg) is essential for viability in every organism in which it has been studied. CgtA has been reported to be involved in several diverse bacterial functions, including ribosome assembly, DNA repair, sporulation, and morphological development. However, none of these functions have been identified as essential. Here we show that depletion of CgtA in Vibrio cholerae causes global changes in gene expression that are consistent with induction of a classical low nutrient stress response or "stringent" response. We show that depletion of CgtA leads to increased ppGpp levels that correlate with induction of the global stress response and cessation of growth. The enzyme RelA is responsible for synthesis of the alarmone ppGpp during the stringent response. We show that CgtA is no longer essential in a relA deletion mutant and thus conclude that the essentiality of CgtA is directly linked to its ability to affect ppGpp levels. The enzyme SpoT degrades ppGpp, and here we show that SpoT is essential in a RelA+ CgtA+ background but not in a relA deletion mutant. We also confirmed that CgtA interacts with SpoT in a two-hybrid assay. We suggest that the essential function of CgtA is as a repressor of the stringent response that acts by regulating SpoT activity to maintain low ppGpp levels when bacteria are growing in a nutrient-rich environment.

The Escherichia coli GTPase CgtAE is involved in late steps of large ribosome assembly

The bacterial ribosome is an extremely complicated macromolecular complex the in vivo biogenesis of which is poorly understood. Although several bona fide assembly factors have been identified, their precise functions and temporal relationships are not clearly defined. Here we describe the involvement of an Escherichia coli GTPase, CgtA(E), in late steps of large ribosomal subunit biogenesis. CgtA(E) belongs to the Obg/CgtA GTPase subfamily, whose highly conserved members are predominantly involved in ribosome function. Mutations in CgtA(E) cause both polysome and rRNA processing defects; small- and large-subunit precursor rRNAs accumulate in a cgtA(E) mutant. In this study we apply a new semiquantitative proteomic approach to show that CgtA(E) is required for optimal incorporation of certain late-assembly ribosomal proteins into the large ribosomal subunit. Moreover, we demonstrate the interaction with the 50S ribosomal subunits of specific nonribosomal proteins (including heretofore uncharacterized proteins) and define possible temporal relationships between these proteins and CgtA(E). We also show that purified CgtA(E) associates with purified ribosomal particles in the GTP-bound form. Finally, CgtA(E) cofractionates with the mature 50S but not with intermediate particles accumulated in other large ribosome assembly mutants.

The Vibrio harveyi GTPase CgtAV is essential and is associated with the 50S ribosomal subunit

It was previously reported that unlike the other obg/cgtA GTPases, the Vibrio harveyi cgtAV is not essential. Here we show that cgtAV was not disrupted in these studies and is, in fact, essential for viability. Depletion of CgtAV did not result in cell elongation. CgtAV is associated with the large ribosomal particle. In light of our results, we predict that the V. harveyi CgtAV protein plays a similar essential role to that seen for Obg/CgtA proteins in other bacteria.

Conserved P-loop GTPases of unknown function in bacteria: an emerging and vital ensemble in bacterial physiology

Establishing the roles of conserved gene products in bacteria is of fundamental importance to our understanding of the core protein complement necessary to sustain cellular life. P-loop GTPases and related ATPases represent an abundant and remarkable group of proteins in bacteria that, in many cases, have evaded characterization. Here, efforts aimed at understanding the cellular function of a group of 8 conserved, poorly characterized genes encoding P-loop GTPases, era, obg, trmE, yjeQ, engA, yihA, hflX, ychF, and a related ATPase, yjeE, are reviewed in considerable detail. While concrete cellular roles remain elusive for all of these genes and considerable pleiotropy has plagued their study, experiments to date have frequently implicated the ribosome. In the case of era, obg, yjeQ, and engA, the evidence is most consistent with roles in ribosome biogenesis, though the prediction is necessarily putative. While the protein encoded in trmE clearly has a catalytic function in tRNA modification, the participation of its GTPase domain remains obscure, as do the functions of the remaining proteins. A full understanding of the cellular functions of all of these important proteins remains the goal of ongoing studies of cellular phenotype and protein biochemistry.

DNA replication defect in the Escherichia coli cgtA(ts) mutant arising from reduced DnaA levels

In Escherichia coli and other bacteria, the ribosome-associated CgtA GTP-binding protein plays a critical role in many basic cellular processes, including the control of DNA replication and/or segregation. However, the mechanism of this control is largely unknown. Here we report that ectopic expression of the dnaA gene partially restored both early growth in liquid medium and DNA synthesis defects of the cgtA(ts) mutant. Amounts of DnaA protein in the cgtA(ts) mutant incubated at elevated (42 degrees C) temperature were significantly lower relative to wild-type bacteria. Both level of dnaA mRNA and transcriptional activity of the dnaA promoter-lacZ fusion were decreased in the CgtA-deficient cells. The effects of ectopic expression of dnaA were specific as analogous expression of another gene coding for a replication regulator, seqA, had no significant changes in growth and DNA synthesis in the cgtA mutant. Thus, it appears that the DNA replication defect in this mutant is a consequence of reduced DnaA levels.

Biochemical properties of the Vibrio harveyi CgtAV GTPase

Bacteria encode a number of relatively poorly characterized GTPases, including the essential, ribosome-associated Obg/CgtA proteins. In contrast to Ras-like proteins, it appears that the Obg/CgtA proteins bind guanine nucleotides with modest affinity and hydrolyze GTP relatively slowly. We show here that the Vibrio harveyi CgtA(V) exchanges guanine nucleotides rapidly and has a modest affinity for nucleotides, suggesting that these features are a universal property of the Obg/CgtA family. Interestingly, CgtA(V) possesses a significantly more rapid GTP hydrolysis rate than is typical of other family members, perhaps reflecting the diversity and specificity of bacterial ecological niches.

The essential GTPase RbgA (YlqF) is required for 50S ribosome assembly inBacillus subtilis

In this paper the essential GTPase YlqF is shown to participate in the biogenesis of the 50S ribosomal subunit in Bacillus subtilis. Cells depleted of YlqF displayed gene expression profiles and nucleoid morphologies that were consistent with a function for YlqF in translation. In addition, YlqF is evolutionarily linked to two eukaryotic GTPases, Nog2p and Nug1p, that are involved in the biogenesis and the nuclear export of the 60S ribosomal subunit. Analysis of ribosomes from cells depleted of YlqF demonstrated that the formation of 70S ribosomes was greatly reduced and the large subunit sedimented at 45S. Cells grown with varying depleted levels of YlqF, yielding doubling times ranging from 38 min to 150 min, all displayed the 45S intermediate. Purified YlqF-His(6) protein associates with the 45S intermediate, but not the mature 50S subunit in vitro. Analysis of proteins from the 45S intermediate indicated that ribosomal protein L16, which is added late during in vitro Escherichia coli 50S ribosome biogenesis, was missing from the 45S intermediate. These results support a model in which YlqF participates in the formation of active 70S ribosomes in the cell by functioning in a late step of 50S subunit biogenesis. Based on these results we propose to rename the ylqF gene rbgA (ribosome biogenesis GTPase A).

The GTP binding protein Obg homolog ObgE is involved in ribosome maturation

Obg proteins belong to a subfamily of GTP binding proteins, which are highly conserved from bacteria to human. Mutations of obgE genes cause pleiotropic defects in various species but the function remained unclear. Here we examine the function of ObgE, the Obg homolog in Escherichia coli. The growth rate correlates with the amount of ObgE in cells. Co-fractionation experiments further suggest that ObgE binds to 30S and 50S ribosomal subunits, but not to 70S ribosome. Pull-down assays suggest that ObgE associates with several specific ribosomal proteins of 30S and 50S subunits, as well as RNA helicase CsdA. Purified ObgE cosediments with 16S and 23S ribosomal RNAs in vitro in the presence of GTP. Finally, mutation of ObgE affects pre-16Sr-RNA processing, ribosomal protein levels, and ribosomal protein modification, thereby significantly reducing 70S ribosome levels. This evidence implicates that ObgE functions in ribosomal biogenesis, presumably through the binding to rRNAs and/or rRNA-ribosomal protein complexes, perhaps as an rRNA/ribosomal protein folding chaperone or scaffold protein.

A bacterial G protein-mediated response to replication arrest

To define factors in E. coli promoting survival to replication fork stress, we isolated insertion mutants sensitive to replication inhibitors. One insertion caused partial loss of the universally conserved GTPase, obgE/yhbZ gene. Although obgE is essential for growth, our insertion allele supported viability until challenged with various replication inhibitors. A mutation designed to negate the GTPase activity of the protein produced similar phenotypes, but was genetically dominant. Synergistic genetic interactions with recA and recB suggested that chromosome breaks and regressed forks accumulate in obgE mutants. Mutants in obgE also exhibited asynchronous overreplication during normal growth, as revealed by flow cytometry. ObgE overexpression caused SeqA foci, normally localized to replication forks, to spread extensively within the cell. We propose that ObgE defines a pathway analogous to the replication checkpoint response of eukaryotes and acts in a complementary way to the RecA-dependent SOS response to promote bacterial cell survival to replication fork arrest.

Guanine nucleotides stabilize the binding of Bacillus subtilis Obg to ribosomes

Obg is a GTP-binding protein of Bacillus subtilis with essential, but undefined roles in the bacterium's growth, sporulation, and stress responses. Obg orthologs are widely conserved among both bacteria and eukaryotes. Gel filtration and affinity blot assays have suggested that Obg may be ribosome-associated. In the current work, we continue an examination of the putative Obg:ribosome interaction. Velocity centrifugation analyses of crude B. subtilis extracts or purified Obg:ribosome mixtures suggest that Obg is initially ribosome-bound, but can separate from ribosomes during sedimentation in the absence of added nucleotides. Addition of either GTP, GDP or ATP to the gradient prolonged the Obg:ribosome association, while inclusion of a nonhydrolyzable GTP analog (5-guanylyl-imidodiphosphate) preserved it. The data strengthen the notion that Obg is a ribosome-associated protein, demonstrate that Obg's association with ribosomes is stabilized by GTP, and indicate that the ribosome-bound Obg can likely hydrolyze GTP and be released as a consequence.

The Escherichia coli GTPase CgtAE cofractionates with the 50S ribosomal subunit and interacts with SpoT, a ppGpp synthetase/hydrolase

CgtA(E)/Obg(E)/YhbZ is an Escherichia coli guanine nucleotide binding protein of the Obg/GTP1 subfamily whose members have been implicated in a number of cellular functions including GTP-GDP sensing, sporulation initiation, and translation. Here we describe a kinetic analysis of CgtA(E) with guanine nucleotides and show that its properties are similar to those of the Caulobacter crescentus homolog CgtA(C). CgtA(E) binds both GTP and GDP with moderate affinity, shows high guanine nucleotide exchange rate constants for both nucleotides, and has a relatively low GTP hydrolysis rate. We show that CgtA(E) is associated predominantly with the 50S ribosomal subunit. Interestingly, CgtA(E) copurifies with SpoT, a ribosome-associated ppGpp hydrolase/synthetase involved in the stress response. The interaction between CgtA(E) and SpoT was confirmed by reciprocal coprecipitation experiments and by two-hybrid assays. These studies raise the possibility that the ribosome-associated CgtA(E) is involved in the SpoT-mediated stress response.

Crystal structure of the GTP-binding protein Obg from Thermus thermophilus HB8

Obg comprises a unique family of high-molecular mass GTPases conserved from bacteria to eukaryotes. Bacterial Obg is essential for cellular growth, sporulation, and differentiation. Here, we report the crystal structure of the full-length form of Obg from Thermus thermophilus HB8 at 2.07 A resolution, in the nucleotide-free state. It reveals a three-domain arrangement, composed of the N-terminal domain, the guanine nucleotide-binding domain (G domain), and the C-terminal domain. The N-terminal and G domains have the Obg fold and the Ras-like fold, respectively. These global folds are similar to those of the recently published structure of the C-terminal domain-truncated form of Obg from Bacillus subtilis. On the other hand, the C-terminal domain of Obg was found to have a novel fold (the OCT fold). A comparison of the T.thermophilus and B.subtilis nucleotide-free Obg structures revealed significant conformational changes in the switch-I and switch-II regions of the G domain. Notably, the N-terminal domain is rotated drastically, by almost 180 degrees, around the G domain axis. In the T.thermophilus Obg crystal, the nucleotide-binding site of the G domain interacts with the C-terminal domain of the adjacent molecule. These data suggest a possible domain rearrangement of Obg, and a potential role of the C-terminal domain in the regulation of the nucleotide-binding state.

The Caulobacter crescentus CgtAC protein cosediments with the free 50S ribosomal subunit

The Obg family of GTPases is widely conserved and predicted to play an as-yet-unknown role in translation. Recent reports provide circumstantial evidence that both eukaryotic and prokaryotic Obg proteins are associated with the large ribosomal subunit. Here we provide direct evidence that the Caulobacter crescentus CgtA(C) protein is associated with the free large (50S) ribosomal subunit but not with 70S monosomes or with translating ribosomes. In contrast to the Bacillus subtilis and Escherichia coli proteins, CgtA(C) does not fractionate in a large complex by gel filtration, indicating a moderately weak association with the 50S subunit. Moreover, binding of CgtA(C) to the 50S particle is sensitive to salt concentration and buffer composition but not guanine nucleotide occupancy of CgtA(C). Assays of epitope-tagged wild-type and mutant variants of CgtA(C) indicate that the C terminus of CgtA(C) is critical for 50S association. Interestingly, the addition of a C-terminal epitope tag also affected the ability of various cgtA(C) alleles to function in vivo. Depletion of CgtA(C) led to perturbations in the polysome profile, raising the possibility that CgtA(C) is involved in ribosome assembly or stability.

Survey for g-proteins in the prokaryotic genomes: prediction of functional roles based on classification

The members of the family of G-proteins are characterized by their ability to bind and hydrolyze guanosine triphosphate (GTP) to guanosine diphosphate (GDP). Despite a common biochemical function of GTP hydrolysis shared among the members of the family of G-proteins, they are associated with diverse biological roles. The current work describes the identification and detailed analysis of the putative G-proteins encoded in the completely sequenced prokaryotic genomes. Inferences on the biological roles of these G-proteins have been obtained by their classification into known functional subfamilies. We have identified 497 G-proteins in 42 genomes. Seven small GTP-binding protein homologues have been identified in prokaryotes with at least two of the diagnostic sequence motifs of G-proteins conserved. The translation factors have the largest representation (234 sequences) and are found to be ubiquitous, which is consistent with their critical role in protein synthesis. The GTP_OBG subfamily comprises of 79 sequences in our dataset. A total of 177 sequences belong to the subfamily of GTPase of unknown function and 154 of these could be associated with domains of known functions such as cell cycle regulation and t-RNA modification. The large GTP-binding proteins and the alpha-subunit of heterotrimeric G-proteins are not detected in the genomes of the prokaryotes surveyed.

The NOG1 GTP-binding protein is required for biogenesis of the 60 S ribosomal subunit

NOG1 is a nucleolar GTP-binding protein present in eukaryotes ranging from trypanosomes to humans. In this report we demonstrate that NOG1 is functionally linked to ribosome biogenesis. In sucrose density gradients Trypanosoma brucei NOG1 co-sediments with 60 S ribosomal subunits but not with monosomes. 60 S precursor RNAs are co-precipitated with NOG1. Together with the nucleolar localization of NOG1, these data indicate that NOG1 is associated with a precursor particle to the 60 S subunit. Disruption of NOG1 function through RNA interference led to a dramatic decrease in the levels of free 60 S particles and the appearance of an atypical rRNA intermediate in which ITS2 was not cleaved. Overexpression of mutant nog1 with a defect in its GTP binding motif on a wild type background caused a modest defect in 60 S biogenesis and a relative decrease in processing of the large subunit rRNAs. In contrast to the mutant protein, neither the N-terminal half of NOG1, which contains the GTP binding motifs, nor the C-terminal half of NOG1 associated with pre-ribosomal particles, although both localized to the nucleolus.

Involvement of the cgtA gene function in stimulation of DNA repair in Escherichia coli and Vibrio harveyi

CgtA is a member of the Obg/Gtp1 subfamily of small GTP-binding proteins. CgtA homologues have been found in various prokaryotic and eukaryotic organisms, ranging from bacteria to humans. Nevertheless, despite the fact that cgtA is an essential gene in most bacterial species, its function in the regulation of cellular processes is largely unknown. Here it has been demonstrated that in two bacterial species, Escherichia coli and Vibrio harveyi, the cgtA gene product enhances survival of cells after UV irradiation. Expression of the cgtA gene was found to be enhanced after UV irradiation of both E. coli and V. harveyi. Moderate overexpression of cgtA resulted in higher UV resistance of E. coli wild-type and dnaQ strains, but not in uvrA, uvrB, umuC and recA mutant hosts. Overexpression of the E. coli recA gene in the V. harveyi cgtA mutant, which is very sensitive to UV light, restored the level of survival of UV-irradiated cells to the levels observed for wild-type bacteria. Moreover, the basal level of the RecA protein was lower in a temperature-sensitive cgtA mutant of E. coli than in the cgtA(+) strain, and contrary to wild-type bacteria, no significant increase in recA gene expression was observed after UV irradiation of this cgtA mutant. Finally, stimulation of uvrB gene transcription under these conditions was impaired in the V. harveyi cgtA mutant. All these results strongly suggest that the cgtA gene product is involved in DNA repair processes, most probably by stimulation of recA gene expression and resultant activation of RecA-dependent DNA repair pathways.

Role of the cgtA gene function in DNA replication of extrachromosomal elements in Escherichia coli

The cgtA gene codes for a common GTP-binding protein whose homologues were found in all prokaryotic and eukaryotic organisms investigated so far. Although cgtA is an essential gene in most bacterial species, its precise functions in the regulation of cellular processes are largely unknown. In Escherichia coli, dysfunction or overexpression of the cgtA gene causes problems in various chromosomal functions, like synchronization of DNA replication initiation and partitioning of daughter chromosomes after a replication round. It is not know how the cgtA gene product regulates these processes. Here we investigated effects of cgtA dysfunction on replication of plasmid and phage replicons. We found that replication of some plasmids (e.g., ColE1-like) is not affected in the cgtA mutant. On the other hand, dysfunction of the cgtA gene caused a strong inhibition of lambda plasmid DNA replication. Bacteriophage lambda development was severely impaired in the cgtA mutant. Replication of other plasmid replicons (derivatives of F, R1, R6K, and RK2) was influenced by the cgtA mutation moderately. It seems that DNA synthesis per se is not affected by CgtA, and that this protein might control replication initiation indirectly, by regulation of function(s) or production of one or more replication factors. In fact, we found that level of the host-encoded replication protein DnaA is significantly decreased in the cgtA mutant. This indicates that CgtA is involved in the regulation of dnaA gene expression.

Structural and biochemical analysis of the Obg GTP binding protein

The Obg nucleotide binding protein family has been implicated in stress response, chromosome partitioning, replication initiation, mycelium development, and sporulation. Obg proteins are among a large group of GTP binding proteins conserved from bacteria to man. Members of the family contain two equally and highly conserved domains, a C-terminal GTP binding domain and an N-terminal glycine-rich domain. Structural analysis of Bacillus subtilis Obg revealed respective domain architectures and how they are coupled through the putative switch elements of the C-terminal GTPase domain in apo and nucleotide-bound configurations. Biochemical analysis of bacterial and human Obg proteins combined with the structural observation of the ppGpp nucleotide within the Obg active sight suggest a potential role for ppGpp modulation of Obg function in B. subtilis.

Six GTP-binding proteins of the Era/Obg family are essential for cell growth in Bacillus subtilis

GTP-binding proteins are found in all domains of life and are involved in various essential cellular processes. With the recent explosion of available genome sequence data, a widely distributed bacterial subfamily of GTP-binding proteins was discovered, represented by the Escherichia coli Era and the Bacillus subtilis Obg proteins. Although only a limited number of the GTP-binding proteins belonging to the subfamily have been experimentally characterized, and their function remains unknown, the available data suggests that many of them are essential to bacterial growth. When the complete genomic sequence of B. subtilis was surveyed for genes encoding GTP-binding proteins of the Era/Obg family, nine such genes were identified. As a first step in elucidating the functional networks of those nine GTP-binding proteins, data presented here indicates that six of them are essential for B. subtilis viability. Additionally, it is shown that the six essential proteins are able to specifically bind GTP and GDP in vitro. Experimental depletion of the essential GTP-binding proteins was examined in the context of cell morphology and chromosome replication, and it was found that two proteins, Bex and YqeH, appeared to participate in the regulation of initiation of chromosome replication. Collectively, these results suggest that members of the GTP-binding Era/Obg family are important proteins with precise, yet still not fully understood, roles in bacterial growth and viability.