Binding of Spo0A stimulates spoIIG promoter activity in Bacillus subtilis

AbrB modulates expression and catabolite repression of a Bacillus subtilis ribose transport operon

A Bacillus subtilis ribose transport operon (rbs) was shown to be subject to AbrB-mediated control through direct AbrB-DNA binding interactions in the vicinity of the promoter. Overproduction of AbrB was shown to relieve catabolite repression of rbs during growth in the presence of poorer carbon sources such as arabinose but had much less effect when cells were grown in the presence of glucose, a rapidly metabolizable carbon source. A ccpA mutation relieved catabolite repression of rbs under all conditions tested. One of the AbrB-binding sites on the rbs promoter contains the putative site of action for the B. subtilis catabolite repressor protein CcpA, suggesting that competition for binding to this site could be at least partly responsible for modulating rbs expression during carbon-limited growth.

Krebs cycle function is required for activation of the Spo0A transcription factor in Bacillus subtilis

Expression of genes early during sporulation in Bacillus subtilis requires the activity of the transcription factor encoded by spo0A. The active, phosphorylated form of Spo0A is produced through the action of a multicomponent pathway, the phosphorelay. A mutant defective in the first three enzymes of the Krebs citric acid cycle was unable to express early sporulation genes, apparently because of a failure to activate the phosphorelay. Cells that produce an altered Spo0A protein that can be phosphorylated by an alternative pathway were not dependent on Krebs cycle function for early sporulation gene expression. These findings suggest that Krebs cycle enzymes transmit a signal to activate the phosphorelay and that B. subtilis monitors its metabolic potential before committing itself to spore formation.

The sigma factors of Bacillus subtilis

The specificity of DNA-dependent RNA polymerase for target promotes is largely due to the replaceable sigma subunit that it carries. Multiple sigma proteins, each conferring a unique promoter preference on RNA polymerase, are likely to be present in all bacteria; however, their abundance and diversity have been best characterized in Bacillus subtilis, the bacterium in which multiple sigma factors were first discovered. The 10 sigma factors thus far identified in B. subtilis directly contribute to the bacterium's ability to control gene expression. These proteins are not merely necessary for the expression of those operons whose promoters they recognize; in many instances, their appearance within the cell is sufficient to activate these operons. This review describes the discovery of each of the known B. subtilis sigma factors, their characteristics, the regulons they direct, and the complex restrictions placed on their synthesis and activities. These controls include the anticipated transcriptional regulation that modulates the expression of the sigma factor structural genes but, in the case of several of the B. subtilis sigma factors, go beyond this, adding novel posttranslational restraints on sigma factor activity. Two of the sigma factors (sigma E and sigma K) are, for example, synthesized as inactive precursor proteins. Their activities are kept in check by "pro-protein" sequences which are cleaved from the precursor molecules in response to intercellular cues. Other sigma factors (sigma B, sigma F, and sigma G) are inhibited by "anti-sigma factor" proteins that sequester them into complexes which block their ability to form RNA polymerase holoenzymes. The anti-sigma factors are, in turn, opposed by additional proteins which participate in the sigma factors' release. The devices used to control sigma factor activity in B, subtilis may prove to be as widespread as multiple sigma factors themselves, providing ways of coupling sigma factor activation to environmental or physiological signals that cannot be readily joined to other regulatory mechanisms.

Identification of a Bacillus subtilis spo0H allele that is necessary for suppression of the sporulation-defective phenotype of a spo0A mutation

A mutation in Bacillus subtilis spo0A codon 97 suppressed the sporulation defect caused by the spo0A9V mutation. The suppressor activity of the codon 97 mutation was evident only in the presence of a novel spo0H allele. Our results suggest that the spo0A gene product interacts with the sigma factor subunit of RNA polymerase.

Sequence and arrangement of genes encoding sigma factors in Clostridium acetobutylicum ATCC 824

The nucleotide sequence of a 2.7-kb region of Clostridium acetobutylicum ATCC 824 DNA containing three open reading frames was determined. They encoded homologs of three proteins of Bacillus subtilis, and the gene arrangement in both organisms was identical. The first gene, orfA, was 801-bp long; the 31-kDa (266 aa) product it encoded exhibited homology with the putative sigma E-processing enzyme. The second gene, sigE, was 708-bp long encoding a 27-kDa (235 aa) product; the third gene, sigG, was 774-bp long encoding a 30-kDa (257 aa) product. These two proteins showed high homology with sigma E and sigma G, two sporulation-specific sigma factors.

Two Lactococcus lactis genes, including lacX, cooperate to trigger an SOS response in a recA-negative background

A 4.3-kb EcoRI fragment from a Lactococcus lactis genomic library alleviates the methyl methanesulfonate, mitomycin C, and UV sensitivities of an Escherichia coli recA mutant (M. Novel, X. F. Huang, and G. Novel, FEMS Microbiol. Lett. 72:309-314, 1990). It complements recA1 and delta recA mutations but not recA13. Three proteins (with molecular masses of 20, 35, and 23 kDa) were produced from this fragment in a T7-directed system, and three corresponding genes were detected by DNA sequencing, namely, ISS1CH;lacX, which is the distal gene of the lac operon; and a third open reading frame, named lacN, which encodes 211 amino acids. Mutations produced in either lacX or in lacN resulted in the loss of the resistance to DNA-damaging agents. Thus, these two genes appeared to be involved in this activity. Introduction of pUCB214 carrying the 4.3-kb fragment into a lexA+ delta recA306 sfiA::lacZ strain resulted in UV-inducible synthesis of beta-galactosidase. A uvrA strain or a lexA (Ind-) strain containing pUCB214 did not support any DNA repair. However, a lexA (Def-) strain carrying pUCB214 could partly repair UV damage. We discuss possible targets for LacX and LacN products, and we speculate that LacX and LacN may constitute a two-component regulatory system that is able to respond to SOS signals, and then to act in the SOS response, bypassing the RecA-activated function.

Isolation and characterization of kinC, a gene that encodes a sensor kinase homologous to the sporulation sensor kinases KinA and KinB in Bacillus subtilis

Phosphorylation of the transcription factor encoded by spo0A is required for the initiation of sporulation in Bacillus subtilis. Production and accumulation of Spo0A-P is controlled by histidine protein kinases and the spo0 gene products. To identify additional genes that might be involved in the initiation of sporulation and production of Spo0A-P, we isolated genes which when present on a multicopy plasmid could suppress the sporulation defect of a spo0K mutant. kinC was one gene isolated in this way. A multicopy plasmid containing kinC completely or partially suppressed the sporulation defect caused by mutations in spo0K, kinA, spo0F, and spo0B, indicating that at least when overexpressed, KinC is capable of stimulating phosphorylation of Spo0A independently of the normal phosphorylation pathway. The predicted product of kinC is 428 amino acids long and is most similar to KinA and KinB, the histidine protein kinases involved in the initiation of sporulation. In otherwise wild-type strains, kinC null mutations caused little or no defect in sporulation under the conditions tested. However, in the absence of a functional phosphorelay (spo0F or spo0B), KinC appears to be the kinase responsible for phosphorylation of the sof-1 and rvtA11 forms of Spo0A.

Effects on Bacillus subtilis of a conditional lethal mutation in the essential GTP-binding protein Obg

The obg gene is part of the spo0B sporulation operon and codes for a GTP-binding protein which is essential for growth. A temperature-sensitive mutant in the obg gene was isolated and found to be the result of two closely linked missense mutations in the amino domain of Obg. Temperature shift experiments revealed that the mutant was able to continue cell division for 2 to 3 generations at the nonpermissive temperature. Such experiments carried out during sporulation showed that Obg was necessary for the transition from vegetative growth to stage 0 or stage II of sporulation, but sporulation subsequent to these stages was unaffected at the nonpermissive temperature. Spores of the temperature-sensitive mutant germinated normally at the nonpermissive temperature but failed to outgrow. The primary consequence of the obg mutation may be an alteration in initiation of chromosome replication.

Isolation of a Bacillus subtilis spoIIGA allele that suppresses processing-negative mutations in the Pro-sigma E gene (sigE)

sigma E, a sporulation-essential sigma factor of Bacillus subtilis, is formed by a developmentally regulated proteolysis which removes 27 to 29 amino acids from the amino terminus of an inactive precursor protein (Pro-sigma E). A mutation which facilitates the conversion of inefficiently processed Pro-sigma E variants into mature sigma E was identified and mapped to spoIIGA. The isolation of such a mutation argues that SpoIIGA is directly involved in the Pro-sigma E processing reaction.

spo0J is required for normal chromosome segregation as well as the initiation of sporulation in Bacillus subtilis

The spo0J gene of Bacillus subtilis is required for the initiation of sporulation. We show that the sporulation defect caused by null mutations in spo0J is suppressed by a null mutation in the gene located directly upstream from spo0J, soj (suppressor of spo0J). These results indicate that Soj inhibits the initiation of sporulation and that Spo0J antagonizes that inhibition. Further genetic experiments indicated that Soj ultimately affects sporulation by inhibiting the activation (phosphorylation) of the developmental transcription factor encoded by spo0A. In addition, the temperature-sensitive sporulation phenotype caused by the ftsA279 (spoIIN279) mutation was partly suppressed by the soj null mutation, indicating that FtsA might also affect the activity of Soj. Soj and Spo0J are known to be similar in sequence to a family of proteins involved in plasmid partitioning, including ParA and ParB of prophage P1, SopA and SopB of F, and IncC and KorB of RK2, spo0J was found to be required for normal chromosome partitioning as well as for sporulation. spo0J null mutants produced a significant proportion of anucleate cells during vegetative growth. The dual functions of Spo0J could provide a mechanism for regulating the initiation of sporulation in response to activity of the chromosome partition machinery.

Expression in Bacillus subtilis of the Bacillus thuringiensis cryIIIA toxin gene is not dependent on a sporulation-specific sigma factor and is increased in a spo0A mutant

Expression of the Bacillus thuringiensis cryIIIA gene encoding a Coleoptera-specific toxin is weak during vegetative growth and is activated at the onset of the stationary phase. cryIIIA'-'lacZ fusions and primer extension analysis show that the regulation of cryIIIA expression is similar in Bacillus subtilis and in B. thuringiensis. Activation of cryIIIA expression was not altered in B. subtilis mutant strains deficient for the sigma H and sigma E sporulation-specific sigma factors or for minor sigma factors such as sigma B, sigma D, or sigma L. This result and the nucleotide sequence of the -35 and -10 regions of the cryIIIA promoter suggest that cryIIIA expression might be directed by the E sigma A form of RNA polymerase. Expression of the cryIIIA'-'lacZ fusion is shut off after t2 (2 h after time zero) of sporulation in the B. subtilis wild-type strain grown on nutrient broth sporulation medium. However, no decrease in cryIIIA-directed beta-galactosidase activity occurred in sigma H, kinA, or spo0A mutant strains. Moreover, beta-galactosidase activity was higher and remained elevated after t2 in the spo0A mutant strain. beta-Galactosidase activity was weak in abrB and spo0A abrB mutant strains, suggesting that AbrB is responsible for the higher level of cryIIIA expression observed in a spo0A mutant. However, both in spo0A and spo0A abrB mutant strains, beta-galactosidase activity remained elevated after t2, suggesting that even in the absence of AbrB, cryIIIA expression is controlled through modulation of the phosphorylated form of Spo0A. When the cryIIIA gene is expressed in a B. subtilis spo0A mutant strain or in the 168 wild-type strain, large amounts of toxins are produced and accumulate to form a flat rectangular crystal characteristic of the coleopteran-specific B. thuringiensis strains.

Activation of spo0A transcription by sigma H is necessary for sporulation but not for competence in Bacillus subtilis

spo0A and spo0H are needed for the initiation of sporulation and for the development of genetic competence in Bacillus subtilis. Transcription of spo0A initiates from two promoters, Pv and Ps. Pv is active during vegetative growth and is recognized by RNA polymerase containing sigma A. Expression from Ps increases during sporulation and depends on sigma H, the spo0H gene product. A deletion mutation, spo0A delta Ps, that removes the promoter controlled by sigma H blocked sporulation but had no detectable effect on competence. These results indicate that expression of spo0A from Ps is necessary for sporulation and that the requirement for spo0H in competence development is not due to its role in expression of spo0A.

Gene expression in single cells of Bacillus subtilis: evidence that a threshold mechanism controls the initiation of sporulation

Early during endospore formation in the bacterium Bacillus subtilis, two distinct cell types are formed. The initiation of this developmental pathway requires several physiological conditions (e.g., nutrient deprivation) and is controlled by the Spo0A transcription factor. We have found that in a culture of sporulating cells, there are two subpopulations, one that has initiated the developmental program and activated the expression of early developmental genes and one in which early developmental gene expression remains uninduced. We measured the expression of developmental (spo) genes in single cells of B. subtilis by using spo-lacZ fusions. Cells containing a spo-lacZ fusion were stained with a dye that fluoresces upon hydrolysis by beta-galactosidase, and the fluorescence in individual cells was measured with a flow cytometer. For Spo+ cells, we found that the proportion of the population expressing early developmental genes correlates well with the fraction of the population that eventually produces spores. In addition, mutations that cause a decrease in the amount of activated (phosphorylated) Spo0A transcription factor cause a decrease in the size of the subpopulation expressing early developmental genes that are directly activated by Spo0A approximately P. Again, the size of the subpopulation correlates well with the fraction of cells that produce spores. These results indicate that a threshold level of activated Spo0A (Spo0A approximately P) or of a component of the phosphorylation pathway must accumulate to induce sporulation gene expression and that most of the cells that are able to induce the expression of early genes that are directly activated by Spo0A approximately P go on to produce mature spores.(ABSTRACT TRUNCATED AT 250 WORDS)

Isolation and characterization of Bacillus subtilis genomic lacZ fusions induced during partial purine starvation

Random genomic Bacillus subtilis lacZ fusions were screened in order to identify the possible existence of regulons responding to the stimuli generated by partial purine starvation. A leaky pur mutation (purL8) was isolated and used to generate the partial purine starvation conditions in the host strain used for screening. On the basis of their induction during partial purine starvation, seven genomic lacZ fusions were isolated. None of the fusions map in loci previously reported to contain purine-regulated genes. One fusion maps very close to the citB locus and may very well be a citB fusion. The fusions were divided into two types on the basis of their response to complete starvation for either ATP or GTP or both components at the same time. Except for one, type 2 fusions were induced by specific starvation for ATP and by simultaneous starvation for ATP and GTP, but not by specific GTP starvation in a gua strain or by GTP starvation induced by the addition of decoyinine. Type 1 fusions were equally well induced by all three kinds of purine starvation including GTP starvation induced by decoyinine. Further subdivisions of the fusions were obtained on the basis of their responses to the spo0A gene product. A total of five fusions showed that spo0A affected expression. One class was unable to induce lacZ expression in the absence of the spo0A gene product, whereas the other class had increased lacZ expression during partial purine starvation in a spo0A background.

Phosphorylation of Bacillus subtilis transcription factor Spo0A stimulates transcription from the spoIIG promoter by enhancing binding to weak 0A boxes

Activation of the spoIIG promoter at the onset of sporulation in Bacillus subtilis requires the regulatory protein, Spo0A, which binds to two sites in the promoter, sites 1 and 2. Phosphorylation of Spo0A is essential for the initiation of sporulation. Therefore, we examined the role of Spo0A phosphorylation in spoIIG promoter activation. Phosphorylation of Spo0A stimulated transcription from the spoIIG promoter in vitro. In DNAse I footprinting experiments with the spoIIG promoter, we found that phosphorylation of Spo0A increased its affinity for site 2 more than for site 1, which is the site to which nonphosphorylated Spo0A binds most avidly. This result could not be explained by increased cooperativity between Spo0A bound at sites 1 and 2 because the increased affinity for site 2 by phosphorylated Spo0A was also observed with a deletion derivative of the spoIIG promoter containing only site 2. We have located Spo0A-binding sequences in the spoIIG promoter by DMS protection assays and mutational analysis, and found that site 1 contains one higher-affinity binding sequence whereas site 2 contains two weaker-binding sites. Two substitutions in site 2 of the spoIIG promoter that change the sequence to be more like an optimal Spo0A-binding site were found to increase promoter activity. Moreover, phosphorylation of Spo0A was not required in vivo for activation of the spoIIG promoter containing these strong binding sites. The results suggest that the primary role for phosphorylation of Spo0A is to increase its affinity for specific sites rather than to activate an activity of Spo0A that acts on RNA polymerase at promoters.

Identification of a promoter for the crystal protein-encoding gene cryIVB from Bacillus thuringiensis subsp. israelensis

The cryIVB gene of Bacillus thuringiensis subsp. israelensis (Bti) codes for a 135-kDa insecticidal crystal protein, which is specifically toxic to dipteran larvae. We have identified a transcription start point (tsp) of cryIVB by a primer extension experiment. The promoter sequence alignment, together with the chronology of appearance of the transcript, suggested that cryIVB is transcribed by RNA polymerase containing sigma 35 (E sigma 35). This was confirmed by investigation of cryIVB transcription in several Bacillus subtilis sporulation mutants. Unlike the lepidopteran-specific crystal protein-encoding genes [cryIA(a) and cryIB], transcription of which is regulated by both sigma 35 and sigma 28, cryIVB transcription was controlled only by the sigma 35-dependent promoter at the midsporulation stage.

Alanine dehydrogenase (ald) is required for normal sporulation in Bacillus subtilis

The ski22::Tn917lac insertion mutation in Bacillus subtilis was isolated in a screen for mutations that cause a defect in sporulation but are suppressed by the presence or overexpression of the histidine protein kinase encoded by kinA (spoIIJ). The ski22::Tn917lac insertion mutation was in ald, the gene encoding alanine dehydrogenase. Alanine dehydrogenase catalyzes the deamination of alanine to pyruvate and ammonia and is needed for growth when alanine is the sole carbon or nitrogen source. The sporulation defect caused by null mutations in ald was partly relieved by the addition of pyruvate at a high concentration, indicating that the normal role of alanine dehydrogenase in sporulation might be to generate pyruvate to provide an energy source for sporulation. The spoVN::Tn917 mutation was also found to be an allele of ald. Transcription of ald was induced very early during sporulation and by the addition of exogenous alanine during growth. Expression of ald was normal in all of the regulatory mutants tested, including spo0A, spo0K, comA, sigB, and sigD mutants. The only gene in which mutations affected expression of ald was ald itself. This regulation is probably related to the metabolism of alanine.

Characterization of the gene encoding an intracellular proteinase inhibitor of Bacillus subtilis and its role in regulation of the major intracellular proteinase

The gene (ipi) for an intracellular proteinase inhibitor (BsuPI) from Bacillus subtilis was cloned and found to encode a polypeptide consisting of 119 amino acids with no cysteine residues. The deduced amino acid sequence contained the N-terminal amino acid sequence of the inhibitor, which was chemically determined previously, and showed no significant homology to any other proteinase inhibitors. Analysis of the transcription initiation site and mRNA showed that the ipi gene formed an operon with an upstream open reading frame with an unknown function. The transcriptional control of ipi gene expression was demonstrated by Northern (RNA) blot analysis, and the time course of transcriptional enhancement roughly corresponded to the results observed at the protein level. Strains in which the ipi gene was disrupted or in which BsuPI was overexpressed constitutively sporulated normally. Analysis of the time course of production of the intracellular proteinase and proteinase inhibitor in these strains suggested that BsuPI directly regulated the major intracellular proteinase (ISP-1) activity in vivo.

Phosphorylation of Spo0A activates its stimulation of in vitro transcription from the Bacillus subtilis spoIIG operon

The spoIIG operon of Bacillus subtilis codes for a sporulation-specific sigma factor, sigma E. In vivo expression of the spoIIG promoter is activated shortly after the onset of sporulation and is dependent on kinA, spo0F, spo0B and spo0A genes. The products of these genes have been shown to participate in a phosphorelay reaction in vitro, culminating in phosphorylation of the transcription factor, Spo0A. The effect of Spo0A phosphorylation on in vitro transcription from the spoIIG promoter was determined. Aliquots from phosphorelay reactions enhanced spoIIG promoter activity 10-fold in transcription assays and stimulation of transcription was dependent on Spo0A phosphorylation. Our results provide biochemical evidence that Spo0A and the phosphorelay form a signal transduction pathway which activates spoII gene expression in development.

Signal transduction in Bacillus subtilis sporulation

The initiation of sporulation in Bacillus subtilis is regulated by a signal transduction system leading to activation (by phosphorylation) of the Spo0A transcription factor. Activated Spo0A controls the expression of genes encoding different RNA polymerase sigma factors, whose synthesis and activities are related to morphological events and intercompartmental communication between the developing forespore and the mother cell.

Multisensory activation of the phosphorelay initiating sporulation in Bacillus subtilis: identification and sequence of the protein kinase of the alternate pathway

The phosphorelay is the signal-transduction system recognizing and integrating environmental signals to initiate sporulation. The major signal input to the phosphorelay is an ATP-dependent kinase, KinA, responsible for phosphorylating the SpoOF protein. Mutants lacking KinA, however, still sporulate, suggesting that other kinases can fulfil its role. In order to identify these kinases, genes for kinases were isolated by hybridization using a degenerate oligonucleotide probe designed for common regions of this class of kinases. A gene for a second kinase, KinB, was isolated which gave a sporulation negative phenotype when inactivated in a kinA background. The kinB locus was sequenced and found to be a small operon consisting of the kinB gene and another gene, kapB, transcribed from a single.sigma A.-dependent promoter. Inactivation of either kinB or kapB in a kinA strain led to severe sporulation deficiency. The kinB gene coded for a 47774 M(r) protein with the carboxyl half of this protein highly homologous to the same domain of KinA. The amino-terminal domain of KinB was hydrophobic with six recognizable membrane-spanning regions. The kapB gene coded for a moderately charged, probably soluble, protein of 14,668 M(r) with no homology to any known protein. Genetic evidence suggests that KapB is required either for the function of KinB or for its expression. Although double mutants kinA kinB cannot sporulate and assume a stage 0 phenotype, the SpoA approximately P-dependent regulation of the abrB gene is normal in these strains, suggesting that low levels of SpoA approximately P accumulate even in the absence of both kinases. This accumulation is dependent on functional spo0F and spo0B genes and its source is unknown. The KinA and KinB pathways are the only pathways capable of producing sufficient Spo0A approximately P to allow initiation and completion of sporulation under laboratory conditions.

Bacillus subtilis sporulation: regulation of gene expression and control of morphogenesis

Bacillus subtilis sporulation is an adaptive response to nutritional stress and involves the differential development of two cells. In the last 10 years or so, virtually all of the regulatory genes controlling sporulation, and many genes directing the structural and morphological changes that accompany sporulation, have been cloned and characterized. This review describes our current knowledge of the program of gene expression during sporulation and summarizes what is known about the functions of the genes that determine the specialized biochemical and morphological properties of sporulating cells. Most steps in the genetic program are controlled by transcription factors that have been characterized in vitro. Two sporulation-specific sigma factors, sigma E and sigma F, appear to segregate at septation, effectively determining the differential development of the mother cell and prespore. Later, each sigma is replaced by a second cell-specific sigma factor, sigma K in the mother cell and sigma G in the prespore. The synthesis of each sigma factor is tightly regulated at both the transcriptional and posttranslational levels. Usually this regulation involves an intercellular interaction that coordinates the developmental programmes of the two cells. At least two other transcription factors fine tune the timing and levels of expression of genes in the sigma E and sigma K regulons. The controlled synthesis of the sigma factors and other transcription factors leads to a spatially and temporally ordered program of gene expression. The gene products made during each successive stage of sporulation help to bring about a sequence of gross morphological changes and biochemical adaptations. The formation of the asymmetric spore septum, engulfment of the prespore by the mother cell, and formation of the spore core, cortex, and coat are described. The importance of these structures in the development of the resistance, dormancy, and germination properties of the spore is assessed.

Integration of multiple developmental signals in Bacillus subtilis through the Spo0A transcription factor

Multiple physiological and environmental signals are needed to initiate endospore formation in Bacillus subtilis. One key event controlling sporulation is activation of the Spo0A transcription factor. Spo0A is a member of a large family of conserved regulatory proteins whose activity is controlled by phosphorylation. We have isolated deletion mutations that remove part of the conserved amino terminus of Spo0A and make the transcription factor constitutively active, indicating that the amino terminus normally functions to keep the protein in an inactive state. Expression of an activated gene product is sufficient to activate expression of several sporulation genes in the absence of signals normally needed for initiation of sporulation. Our results indicate that nutritional, cell density, and cell-cycle signals are integrated through the phosphorylation pathway that controls activation of Spo0A.

The phosphorelay signal transduction pathway in the initiation of Bacillus subtilis sporulation

The formation of spores in Bacillus subtilis is a developmental process under genetic control. The decision to either divide or sporulate is regulated by the state of phosphorylation of the SpoOA transcription factor. Phosphorylated SpoOA (SpoOA approximately P) is both a repressor and an activator of transcription depending on the promoter it is affecting. SpoOA approximately P is the end product of the phosphorelay, a signal transduction system linking environmental information to the activation of sporulation. Activation or deinhibition of two ATP-dependent kinases, KinA and KinB, to phosphorylate the SpoOF secondary messenger initiates the phosphorelay. SpoOF approximately P is the substrate for the SpoOB protein, a phosphoprotein phosphotransferase which transfers the phosphate group to SpoOA. The SpoOA approximately P formed from this pathway orchestrates transcription events during the initial stage of spore development through direct effects on a variety of promoters and through the use of other transcription factors, termed transition state regulators, whose activity it controls. Because commitment to sporulation has serious cellular programming consequences and is not undertaken capriciously, the phosphorelay is subject to a variety of complex controls on the flow of phosphate through its components.

Sporulation in prokaryotes and lower eukaryotes

During the past year, highlights in sporulation research include the demonstration that phosphorylation of SpoOA is a critical factor in Bacillus subtilis development; the identification of C alpha proteins, adenylyl cyclase and protein kinase A genes in Dictyostelium; proof that an endogenous antisense RNA regulates gene expression in Dictyostelium; and characterization of a second type of differentiated cell in Myxococcus.

Coupling between gene expression and DNA synthesis early during development in Bacillus subtilis

Endospore formation in the bacterium Bacillus subtilis involves generation of two cell types, each with different developmental fates. Each cell type contains an active chromosome, and treatments that inhibit DNA synthesis at the beginning of development inhibit spore formation. We describe experiments demonstrating that gene expression early during sporulation is coupled to DNA synthesis. Expression of several genes that are induced early during sporulation, before the formation of two cell types, is inhibited when DNA synthesis is inhibited. Genes that are affected require the transcription factor encoded by spo0A for normal induction. Spo0A protein is normally activated early in development by a multicomponent phosphorylation pathway, or phospho-relay. Altered function mutations in spo0A that bypass the need for the phospho-relay allow early sporulation gene expression, even when DNA synthesis is inhibited. These results indicate that inhibition of DNA synthesis prevents activation of the Spo0A transcription factor by inhibiting a step in the phospho-relay. It seems likely that coupling early developmental gene expression to DNA synthesis is a general mechanism to prevent inappropriate or unnecessary gene expression.

Role of the Bacillus subtilis gsiA gene in regulation of early sporulation gene expression

The Bacillus subtilis gsiA operon was induced rapidly, but transiently, as cells entered the stationary phase in nutrient broth medium. A mutation at the gsiC locus caused sporulation to be defective and expression of gsiA to be elevated and prolonged. The sporulation defect in this strain was apparently due to persistent expression of gsiA, since a gsiA null mutation restored sporulation to wild-type levels. Detailed mapping experiments revealed that the gsiC82 mutation lies within the kinA gene, which encodes the histidine protein kinase member of a two-component regulatory system. Since mutations in this gene caused a substantial blockage in expression of spoIIA, spoIIG, and spoIID genes, it seems that accumulation of a product of the gsiA operon interferes with sporulation by blocking the completion of stage II. It apparently does so by inhibiting or counteracting the activity of KinA.

Mutational analysis of the precursor-specific region of Bacillus subtilis sigma E

sigma E is a sporulation-specific sigma factor of Bacillus subtilis that is formed from an inactive precursor protein (pro-sigma E) by the removal of 27 to 29 amino acids from the pro-sigma E amino terminus. By using oligonucleotide-directed mutagenesis, sequential deletions were constructed in the precursor-specific region of sigE and analyzed for their effect on the gene product's activity, ability to accumulate, and susceptibility to conversion into mature sigma E. The results demonstrated that the first 17 residues of the pro sequence contribute to silencing the sigma-like activity of pro-sigma E and that the amino acids between positions 12 and 17 are also important for its conversion into sigma E. Deletions that remove 21 or more codons from sigE reduce sigma E activity in cells which carry it, presumably by affecting pro-sigma E stability. A 26-codon deletion results in a gene whose product is not detectable in B. subtilis by either reporter gene activity or Western blot (immunoblot) assay. The primary structure as well as the size of the pro region of sigma E contributes to the protein's stability. The placement of additional amino acids into the pro region reduces the cell's ability to accumulate pro-sigma E. Additional sigE mutations revealed that the amino acids normally found at the putative processing site(s) of pro-sigma E are not essential to the processing reaction; however, a Glu residue upstream of these sites (position 25) was found to be important for processing. These last results suggest that the pro-sigma E processing apparatus does not recognize the actual site within pro-sigma E at which cleavage occurs but rater sequence elements that are upstream of this site.