A region in the Bacillus subtilis transcription factor Spo0A that is important for spoIIG promoter activation

The regulatory network controlling spore formation in Clostridium difficile

Clostridium difficile, a Gram-positive, anaerobic, spore-forming bacterium, is a major cause of nosocomial infections such as antibiotic-associated diarrhea. Spores are the vector of its transmission and persistence in the environment. Despite the importance of spores in the infectious cycle of C. difficile, little was known until recently about the control of spore development in this enteropathogen. In this review, we describe recent advances in our understanding of the regulatory network controlling C. difficile sporulation. The comparison with the model organism Bacillus subtilis highlights major differences in the signaling pathways between the forespore and the mother cell and a weaker connection between morphogenesis and gene expression. Indeed, the activation of the SigE regulon in the mother cell is partially independent of SigF although the forespore protein SpoIIR, itself partially independent of SigF, is essential for pro-SigE processing. Furthermore, SigG activity is not strictly dependent on SigE. Finally, SigG is dispensable for SigK activation in agreement with the absence of a pro-SigK sequence. The excision of the C. difficile skin element is also involved in the regulation of SigK activity. The C. difficile sporulation process might be a simpler, more ancestral version of the program characterized for B. subtilis.

An A257V mutation in the bacillus subtilis response regulator Spo0A prevents regulated expression of promoters with low-consensus binding sites

In Bacillus species, the master regulator of sporulation is Spo0A. Spo0A functions by both activating and repressing transcription initiation from target promoters that contain 0A boxes, the binding sites for Spo0A. Several classes of spo0A mutants have been isolated, and the molecular basis for their phenotypes has been determined. However, the molecular basis of the Spo0A(A257V) substitution, representative of an unusual phenotypic class, is not understood. Spo0A(A257V) is unusual in that it abolishes sporulation; in vivo, it fails to activate transcription from key stage II promoters yet retains the ability to repress the abrB promoter. To determine how Spo0A(A257V) retains the ability to repress but not stimulate transcription, we performed a series of in vitro and in vivo assays. We found unexpectedly that the mutant protein both stimulated transcription from the spoIIG promoter and repressed transcription from the abrB promoter, albeit twofold less than the wild type. A DNA binding analysis of Spo0A(A257V) showed that the mutant protein was less able to tolerate alterations in the sequence and arrangement of its DNA binding sites than the wild-type protein. In addition, we found that Spo0A(A257V) could stimulate transcription of a mutant spoIIG promoter in vivo in which low-consensus binding sites were replaced by high-consensus binding sites. We conclude that Spo0A(A257V) is able to bind to and regulate the expression of only genes whose promoters contain high-consensus binding sites and that this effect is sufficient to explain the observed sporulation defect.

Promoter activation by repositioning of RNA polymerase

Spo0A, a classical two-component-type response regulator in Bacillus subtilis, binds to a specific DNA sequence found in many promoters to repress or activate the transcription of over 100 genes. On the spoIIG promoter, one of the Spo0A binding sites, centered at position -40, overlaps a consensus -35 element that may also interact with region 4 of the sigma A (sigma(A)) subunit of RNA polymerase. Molecular modeling corroborated by genetic evidence led us to propose that the binding of Spo0A to this site repositions sigma(A) region 4 on the promoter. Therefore, we used a chemical nuclease, p-bromoacetamidobenzyl-EDTA-Fe, that was covalently tethered to a single cysteine in region 4 of sigma(A) to map the position of sigma(A) on the promoter. The results indicated that in the absence of Spo0A, sigma(A) region 4 of the RNA polymerase was located near the -35 element sequence centered at position -40. However, in the presence of Spo0A, sigma(A) region 4 was displaced downstream from the -35 element by 4 bp. These and other results support the model in which the binding of Spo0A to the spoIIG promoter stimulates promoter utilization by repositioning prebound RNA polymerase and stabilizing the repositioned RNA polymerase-promoter complex at a new position that aligns sigma(A) region 2 with the -10 region sequences of the promoter, thus facilitating open complex formation.

Bacillus subtilis RNA Polymerase Recruits the Transcription Factor Spo0A∼P to Stabilize a Closed Complex during Transcription Initiation

The Bacillus subtilis response regulator Spo0A approximately P activates transcription from the spoIIG promoter by stimulating a rate-limiting transition between the initial interaction of RNA polymerase with the promoter and initiation of RNA synthesis. Previous work showed that Spo0A exerts its effect on RNA polymerase prior to the formation of an open complex in which the DNA strands at the initiation site have been separated. To isolate the effect of Spo0A approximately P on events prior to DNA strand separation at spoIIG we studied RNA polymerase binding to DNA fragments that were truncated to contain only promoter sequences 5' to the -10 element by electrophoretic mobility shift assays. RNA polymerase bound to these fragments readily though highly reversibly, and polymerase-promoter complexes recruited Spo0A approximately P. Sequence-independent interactions between the RNA polymerase and the DNA upstream of the core promoter were important for RNA polymerase binding and essential for Spo0A approximately P recruitment, while sequence-specific Spo0A approximately P-DNA interactions positioned and stabilized RNA polymerase binding to the DNA. Spo0A approximately P decreased the dissociation rate of the complexes formed with truncated promoter templates which could contribute to the means by which Spo0A approximately P stimulates spoIIG expression.

Phosphorylation and functional analysis of the sporulation initiation factor Spo0A from Clostridium botulinum

The initiation of sporulation in aerobic Bacillus species is regulated by the phosphorelay consisting of several sensor histidine kinases, the Spo0F response regulator, the Spo0B phosphotransferase and the Spo0A transcription factor that upon phosphorylation represses genes for growth and activates the developmental process. Clostridium species lack both Spo0F and Spo0B and the identities of the sensor histidine kinases are unknown. The amino acid sequence of Spo0A is highly conserved in Clostridium botulinum relative to Bacillus subtilis but the cloned C. botulinum Spo0A was unable to complement a spo0A mutant of B. subtilis for sporulation. However, it was able to repress the abrB gene of B. subtilis. Active site mutations in Spo0A still repressed, indicating this activity was independent of phosphorylation. An orphan sensor histidine kinase of C. botulinum appeared to normally phosphorylate C. botulinum Spo0A and expression of this kinase in combination with C. botulinum Spo0A in B. subtilis was lethal, suggesting phosphorylation of C. botulinum Spo0A repressed essential growth genes as a prerequisite to sporulation but could not compensate for this effect by inducing sporulation. A chimera Spo0A consisting of a B. subtilis Spo0A response regulator domain fused to a C. botulinum DNA-binding domain was capable of restoring sporulation to a spo0A mutant of B. subtilis albeit at less than wild-type levels. The data suggest that induction of sporulation requires interactions of both domains of Spo0A with other conserved proteins and despite the high conservation of the amino acid sequence of C. botulinum Spo0A, some of these interactions have been lost.

A gene encoding a holin-like protein involved in spore morphogenesis and spore germination in Bacillus subtilis

We report here studies of expression and functional analysis of a Bacillus subtilis gene, ywcE, which codes for a product with features of a holin. Primer extension analysis of ywcE transcription revealed that a single transcript accumulated from the onset of sporulation onwards, produced from a sigma(A)-type promoter bearing the TG dinucleotide motif of "extended" -10 promoters. No primer extension product was detected in vivo during growth. However, specific runoff products were produced in vitro from the ywcE promoter by purified sigma(A)-containing RNA polymerase (Esigma(A)), and the in vivo and in vitro transcription start sites were identical. These results suggested that utilization of the ywcE promoter by Esigma(A) during growth was subjected to repression. Studies with a lacZ fusion revealed that the transition-state regulator AbrB repressed the transcription of ywcE during growth. This repression was reversed at the onset of sporulation in a Spo0A-dependent manner, but Spo0A did not appear to contribute otherwise to ywcE transcription. We found ywcE to be required for proper spore morphogenesis. Spores of the ywcE mutant showed a reduced outer coat which lacked the characteristic striated pattern, and the outer coat failed to attach to the underlying inner coat. The mutant spores also accumulated reduced levels of dipicolinic acid. ywcE was also found to be important for spore germination.

Alpha-helix E of Spo0A is required for sigmaA- but not for sigmaH-dependent promoter activation in Bacillus subtilis

At the onset of endospore formation in Bacillus subtilis, the DNA binding protein Spo0A activates transcription from two types of promoters. The first type includes the spoIIG and spoIIE promoters, which are used by sigma(A)-RNA polymerase, whereas the second type includes the spoIIA promoter, which is used by RNA polymerase containing the secondary sigma factor sigma(H). Previous genetic analyses have identified specific amino acids in alpha-helix E of Spo0A that are important for activation of Spo0A-dependent, sigma(A)-dependent promoters. However, these amino acids are not required for activation of the sigma(H)-dependent spoIIA promoter. We now report the effects of additional single-amino-acid substitutions and the effects of deletions in alpha-helix E. The effects of alanine substitutions revealed one new position (239) in Spo0A that appears to be specifically required for activation of the sigma(A)-dependent promoters. Based on the effects of a deletion mutation, we suggest that alpha-helix E in Spo0A is not directly involved in interaction with sigma(H)-RNA polymerase.

The Bacillus subtilis response regulator Spo0A stimulates sigmaA-dependent transcription prior to the major energetic barrier

At the spoIIG promoter phosphorylated Spo0A (Spo0A approximately P) binds 0A boxes overlapping the -35 element, interacting with RNA polymerase to facilitate open complex formation. We have compared in vitro transcription from a series of heteroduplex templates containing denatured regions within the promoters. Transcription from heteroduplex templates with 12, 8, or 6 base pairs denatured was independent of Spo0A approximately P, but heteroduplexes with 4 or 2 base pairs denatured required Spo0A approximately P for maximal levels of transcription. Investigation of the thermal dependence of transcription suggested that strand separation was the primary thermodynamic barrier to transcription initiation but indicated that Spo0A approximately P does not reduce this energetic barrier. Kinetic assays revealed that Spo0A approximately P stimulated both the rate of formation of initiated complexes as well as increasing the number of complexes capable of initiating transcription. These results imply that Spo0A approximately P stimulates transcription at least in part by stabilizing the RNA polymerase-spoIIG complex until contacts between RNA polymerase and the -10 element induce strand separation.

Surfaces of Spo0A and RNA polymerase sigma factor A that interact at the spoIIG promoter in Bacillus subtilis

In Bacillus subtilis, the DNA binding protein Spo0A activates transcription from two classes of promoters, those used by RNA polymerase containing the primary sigma factor, sigma(A) (e.g., spoIIG), and those used by RNA polymerase containing the secondary sigma factor, sigma(H) (e.g., spoIIA). Several single amino acid substitutions in region 4 of sigma(A) define positions in sigma(A) that are specifically required for Spo0A-dependent promoter activation. Similarly, several single amino acid substitutions in Spo0A define positions in Spo0A that are required for sigma(A)-dependent promoter activation but not for other functions of Spo0A. It is unknown whether these amino acids in Spo0A interact directly with those in region 4 of sigma(A) or whether they interact with another subunit of RNA polymerase to effect promoter activation. Here we report the identification of a new amino acid in region 4 of sigma(A), arginine at position 355 (R355), that is involved in Spo0A-dependent promoter activation. To further investigate the role of R355, we used the coordinates of Spo0A and sigma region 4, each in complex with DNA, to build a model for the interaction of sigma(A) and Spo0A at the spoIIG promoter. We tested the model by examining the effects of amino acid substitutions in the putative interacting surfaces of these molecules. As predicted by the model, we found genetic evidence for interaction of R355 of sigma(A) with glutamine at position 221 of Spo0A. These results appear to define the surfaces of Spo0A and sigma(A) that directly interact during activation of the spoIIG promoter.

A mutation in GerE that affects cotC promoter activation in Bacillus subtilis

The DNA-binding protein GerE acts as both a repressor and an activator of transcription of genes transcribed by sigma(K)-RNA polymerase (RNA-P) during the later stages of endospore formation in Bacillus subtilis. GerE represses transcription from the sigK promoter, and activates transcription from other promoters, including cotC and cotX. Two different regions of GerE (AR1 and AR2) are required for activation of cotC and cotX, respectively. We used a genetic screen to seek mutations that would define additional regions of GerE required for promoter activation. We found that a substitution of proline for leucine at position 12 of GerE (L12P) decreased cotC promoter activity but did not interfere with GerE-dependent repression of the sigK promoter or with activation of the cotX promoter in vivo. We also found that the L12P substitution had no effect on binding to cotC in vitro. However, the L12P-substituted GerE failed to stimulate cotC transcription in vitro, whereas it stimulated transcription from PcotX. The crystal structure of GerE suggests that L12 is not exposed on the surface of the molecule. Therefore, we propose that the L12P substitution reduces the flexibility of the N-terminal arm, preventing an interaction of AR1 with RNA-P that is essential for activation of the cotC promoter.

Two regions of GerE required for promoter activation in Bacillus subtilis

GerE from Bacillus subtilis is the smallest member of the LuxR-FixJ family of transcription activators. Its 74-amino-acid sequence is similar over its entire length to the DNA binding domain of this protein family, including a putative helix-turn-helix (HTH) motif. In this report, we sought to define regions of GerE involved in promoter activation. We examined the effects of single alanine substitutions at 19 positions that were predicted by the crystal structure of GerE to be located on its surface. A single substitution of alanine for the phenylalanine at position 6 of GerE (F6A) resulted in decreased transcription in vivo and in vitro from the GerE-dependent cotC promoter. However, the F6A substitution had little effect on transcription from the GerE-dependent cotX promoter. In contrast, a single alanine substitution for the leucine at position 67 (L67A) reduced transcription from the cotX promoter, but not from the cotC promoter. The results of DNase I protection assays and in vitro transcription reactions lead us to suggest that the F6A and L67A substitutions define two regions of GerE, activation region 1 (AR1) and AR2, that are required for activation of the cotC and cotX promoters, respectively. A comparison of our results with those from studies of MalT and BvgA indicated that other members of the LuxR-FixJ family may use more than one surface to interact with RNA polymerase during promoter activation.

Lessons and questions from the structure of the Spo0A activation domain

The carboxy-terminal domain of Spo0A in Bacillus subtilis is one of the few response regulator activation domains for which the structure is known. Here, we discuss some of the mutational data and biological roles of Spo0A in light of its structure.

Mutational analysis of conserved residues in the putative DNA-binding domain of the response regulator Spo0A of Bacillus subtilis

The Spo0A protein of Bacillus subtilis is a DNA-binding protein that is required for the expression of genes involved in the initiation of sporulation. Spo0A binds directly to and both activates and represses transcription from the promoters of several genes required during the onset of endospore formation. The C-terminal 113 residues are known to contain the DNA-binding activity of Spo0A. Previous studies identified a region of the C-terminal half of Spo0A that is highly conserved among species of endospore-forming Bacillus and Clostridium and which encodes a putative helix-turn-helix DNA-binding domain. To test the functional significance of this region and determine if this motif is involved in DNA binding, we changed three conserved residues, S210, E213, and R214, to Gly and/or Ala by site-directed mutagenesis. We then isolated and analyzed the five substitution-containing Spo0A proteins for DNA binding and sporulation-specific gene activation. The S210A Spo0A mutant exhibited no change from wild-type binding, although it was defective in spoIIA and spoIIE promoter activation. In contrast, both the E213G and E213A Spo0A variants showed decreased binding and completely abolished transcriptional activation of spoIIA and spoIIE, while the R214G and R214A variants completely abolished both DNA binding and transcriptional activation. These data suggest that these conserved residues are important for transcriptional activation and that the E213 residue is involved in DNA binding.

Suppression of temperature-sensitive sporulation mutation in the Bacillus subtilis sigA gene by rpoB mutation

We isolated a temperature-sensitive sporulation defective mutant of the sigA gene, encoding a major sigma factor, sigma(A) protein, in Bacillus subtilis, and designated it as sigA21. The sigA21 mutation caused a single-amino acid substitution, E314K, in region 4 of the sigma(A) protein. In this mutant, expression of the spoIIG gene, whose transcription depends on both sigma(A) and the phosphorylated Spo0A protein, Spo0A approximately P, a major transcription factor during early stages of sporulation, was greatly reduced at 43 degrees C. To obtain further information on the mechanism of sigma(A) function during the early spore development, we isolated a spontaneous sporulation-proficient suppressor mutant at 43 degrees C. This extragenic suppressor mutation was mapped within the rpoB gene, encoding the beta subunit of RNA polymerase, and was found to have a single-amino acid substitution, A863G. In this mutant, the expression of the spoIIG is partially restored at 43 degrees C.

The trans-activation domain of the sporulation response regulator Spo0A revealed by X-ray crystallography

Sporulation in Bacillus involves the induction of scores of genes in a temporally and spatially co-ordinated programme of cell development. Its initiation is under the control of an expanded two-component signal transduction system termed a phosphorelay. The master control element in the decision to sporulate is the response regulator, Spo0A, which comprises a receiver or phosphoacceptor domain and an effector or transcription activation domain. The receiver domain of Spo0A shares sequence similarity with numerous response regulators, and its structure has been determined in phosphorylated and unphosphorylated forms. However, the effector domain (C-Spo0A) has no detectable sequence similarity to any other protein, and this lack of structural information is an obstacle to understanding how DNA binding and transcription activation are controlled by phosphorylation in Spo0A. Here, we report the crystal structure of C-Spo0A from Bacillus stearothermophilus revealing a single alpha-helical domain comprising six alpha-helices in an unprecedented fold. The structure contains a helix-turn-helix as part of a three alpha-helical bundle reminiscent of the catabolite gene activator protein (CAP), suggesting a mechanism for DNA binding. The residues implicated in forming the sigmaA-activating region clearly cluster in a flexible segment of the polypeptide on the opposite side of the structure from that predicted to interact with DNA. The structural results are discussed in the context of the rich array of existing mutational data.

Identification of a second region of the Spo0A response regulator of Bacillus subtilis required for transcription activation

Deletion of the 10 C-terminal amino acids of the Bacillus subtilis response regulator Spo0A or valine substitution at D258 and L260 resulted in a sporulation-negative phenotype and loss of in vivo activation of the spoIIG and spoIIA operon promoters. Repression of the abrB promoter was not affected by the mutations. In combination with the previously characterized mutation (A257V), the results identify amino acids at positions 257, 258, and 260 as being required for transcription activation by Spo0A.

A role for Asp75 in domain interactions in the Bacillus subtilis response regulator Spo0A

Spo0A is a two-domain response regulator required for sporulation initiation in Bacillus subtilis. Studies on response regulators have focused on the activity of each domain, but very little is known about the mechanism by which the regulatory domain inhibits the activator domain. In this study, we created a single amino acid substitution in the regulatory domain, D75S, which resulted in a dramatic decrease in sporulation in vivo. In vitro studies with the purified Spo0AD75S protein demonstrated that phosphorylation and DNA binding were comparable with wild type Spo0A. However, the mutant was unable to stimulate transcription by final sigma(A)-RNA polymerase from the Spo0A-dependent spoIIG operon promoter. We suggest that the amino acid Asp(75) and/or the region within which it resides, the alpha3-beta4 loop, are involved in the inhibitory interaction between the regulatory and activator domains of Spo0A.

Phylogeny and functional conservation of sigma(E) in endospore-forming bacteria

Conservation of the sporulation processes between Bacillus spp. and Clostridium spp. was investigated through evolutionary and complementation analyses of sigma(E). Alignment of partial predicted sigma(E) amino acid sequences from three Bacillus spp., Paenibacillus polymyxa and five Clostridium spp. revealed that amino acid residues previously reported to be involved in promoter utilization (M124, E119 and N120) and strand opening (C117) are conserved among all these species. Phylogenetic analyses of various sigma factor sequences from endospore-forming bacteria revealed that homologues of sigma(E), sigma(K) and sigma(G) clustered together regardless of genus, suggesting a common origin of sporulation sigma factors. The functional equivalence between Clostridium acetobutylicum sigma(E) and Bacillus subtilis sigma(E) was investigated by complementing a non-polar B. subtilis sigma(E) null mutant with the spoIIG operon from either B. subtilis (spoIIG(Bs)) or C. acetobutylicum (spoIIG(Ca)). Single-copy integration of spoIIG(Bs) into the amyE locus of the sigma(E) null mutant completely restored the wild-type sporulation phenotype, while spoIIG(Ca) only partially restored sporulation. Maximal expression of spoIIG(Ca)-lacZ occurred approximately 12 h later than maximal expression of spoIIG(Bs)-lacZ. Differences in temporal expression patterns for spoIIG(Ca) and spoIIG(Bs) in the B. subtilis background may at least partially explain the observed sporulation complementation phenotypes. This study suggests a common phylogenetic ancestor for sigma(E) in Bacillus spp. and Clostridium spp., although regulation of sigma(E) expression may differ in these two genera.

A new mutation in spo0A with intragenic suppressors in the effector domain

Spo0A is a two domain response regulator, a key protein in the initiation of sporulation of Bacillus subtilis. This protein controls a number of changes in gene expression that occur during the transition from stationary phase to the onset of sporulation. The phosphorylated form of Spo0A influences the transcription of a specific set of genes. In addition to others, it represses abrB and activates spoIIA and spoIIE transcription. Although the N-terminal phosphoacceptor domain is well characterised, there is limited information on the C-terminal, DNA-binding domain. Comparisons of Spo0A homologues from a number of Bacillus and Clostridium species show that the C-terminal domain contains three highly conserved regions. In this study, we have investigated the influence of spo0A mutations mapping within the C-terminal domain on transcription from the abrB, spoIIA and spoIIE promoters using lacZ fusions. Our results indicate that described mutations can be part of signalling between N- and C-terminal domains of the protein. Also, the increased expression observed from the spoIIE promoter in some Spo0A mutants might result from a stabilising function of these mutations on the transcriptional apparatus utilising sigma(A).

The Spo0A sof mutations reveal regions of the regulatory domain that interact with a sensor kinase and RNA polymerase

Spo0A is a two-domain response regulator required for the initiation of sporulation in Bacillus subtilis. Spo0A is activated by phosphorylation of its regulatory domain by a multicomponent phosphorelay. To define the role of the regulatory domain in the activation of Spo0A, we have characterized four of the sof mutations in vitro. The sof mutations were identified previously as suppressors of the sporulation-negative phenotype resulting from a deletion of the gene for one of the phosphorelay components, spo0F. Like wild-type Spo0A, the transcription stimulation properties of all of the Sof proteins were dependent upon phosphorylation. Sof mutants from two classes were improved substrates for direct phosphorylation by the KinA sensor kinase, providing an explanation for their suppression properties. Two other Sof proteins showed a phosphorylation-dependent enhancement of the stability of the Sof approximately P-RNA polymerase-DNA complex. One of these mutants, Sof114, increased the stability of the Sof114 approximately P-RNAP-DNA complex without increasing its own affinity for the spoIIG promoter. A comparison of the location of the sof mutations with mutations in CheY suggests that phosphorylation of Spo0A results in the exposure of a region in the regulatory domain that interacts with RNA polymerase, thereby contributing to the signal transduction mechanism.

A region in Bacillus subtilis sigmaH required for Spo0A-dependent promoter activity

Spo0A activates transcription in Bacillus subtilis from promoters that are used by two types of RNA polymerase, RNA polymerase containing the primary sigma factor, sigmaA, and RNA polymerase containing a secondary sigma factor, known as sigmaH. The region of sigmaA near positions 356 to 359 is required for Spo0A-dependent promoter activation, possibly because Spo0A interacts with this region of sigmaA at these promoters. To determine if the amino acids in the corresponding region of sigmaH are also important in Spo0A-dependent promoter activation, we examined the effects of single alanine substitutions at 10 positions in sigmaH (201 to 210). Two alanine substitutions in sigmaH, at glutamine 201 (Q201A) and at arginine 205 (R205A), significantly decreased activity from the Spo0A-dependent, sigmaH-dependent promoter spoIIA but did not affect expression from the sigmaH-dependent, Spo0A-independent promoters citGp2 and spoVG. Therefore, promoter activation by Spo0A requires homologous regions in sigmaA and sigmaH. A mutant form of Spo0A, S231F, that suppresses the sporulation defect caused by several amino acid substitutions in sigmaA did not suppress the sporulation defects caused by the Q201A and R205A substitutions in sigmaH. This result and others indicate that different surfaces of Spo0A probably interact with sigmaA and sigmaH RNA polymerases.

Spo0A mutants of Bacillus subtilis with sigma factor-specific defects in transcription activation

The transcription factor Spo0A of Bacillus subtilis has the unique ability to activate transcription from promoters that require different forms of RNA polymerase holoenzyme. One class of Spo0A-activated promoter, which includes spoIIEp, is recognized by RNA polymerase associated with the primary sigma factor, sigma A (sigmaA); the second, which includes spoIIAp, is recognized by RNA polymerase associated with an early-sporulation sigma factor, sigma H (sigmaH). Evidence suggests that Spo0A probably interacts directly with RNA polymerase to activate transcription from these promoters. To identify residues of Spo0A that may be involved in transcriptional activation, we used PCR mutagenesis of the entire spo0A gene and designed a screen using two distinguishable reporter fusions, spoIIE-gus and spoIIA-lacZ. Here we report the identification and characterization of five mutants of Spo0A that are specifically defective in activation of sigmaA-dependent promoters while maintaining activation of sigmaH-dependent promoters. These five mutants identify a 14-amino-acid segment of Spo0A, from residue 227 to residue 240, that is required for transcriptional activation of sigmaA-dependent promoters. This region may define a surface or domain of Spo0A that makes direct contacts with sigmaA-associated holoenzyme.