YqfS from Bacillus subtilis is a spore protein and a new functional member of the type IV apurinic/apyrimidinic-endonuclease family

Bacillus subtilis stress-associated mutagenesis and developmental DNA repair

SUMMARYThe metabolic conditions that prevail during bacterial growth have evolved with the faithful operation of repair systems that recognize and eliminate DNA lesions caused by intracellular and exogenous agents. This idea is supported by the low rate of spontaneous mutations (10-9) that occur in replicating cells, maintaining genome integrity. In contrast, when growth and/or replication cease, bacteria frequently process DNA lesions in an error-prone manner. DNA repairs provide cells with the tools needed for maintaining homeostasis during stressful conditions and depend on the developmental context in which repair events occur. Thus, different physiological scenarios can be anticipated. In nutritionally stressed bacteria, different components of the base excision repair pathway may process damaged DNA in an error-prone approach, promoting genetic variability. Interestingly, suppressing the mismatch repair machinery and activating specific DNA glycosylases promote stationary-phase mutations. Current evidence also suggests that in resting cells, coupling repair processes to actively transcribed genes may promote multiple genetic transactions that are advantageous for stressed cells. DNA repair during sporulation is of interest as a model to understand how transcriptional processes influence the formation of mutations in conditions where replication is halted. Current reports indicate that transcriptional coupling repair-dependent and -independent processes operate in differentiating cells to process spontaneous and induced DNA damage and that error-prone synthesis of DNA is involved in these events. These and other noncanonical ways of DNA repair that contribute to mutagenesis, survival, and evolution are reviewed in this manuscript.

DNA Repair in Staphylococcus aureus

Staphylococcus aureus is a common cause of both superficial and invasive infections of humans and animals. Despite a potent host response and apparently appropriate antibiotic therapy, staphylococcal infections frequently become chronic or recurrent, demonstrating a remarkable ability of S. aureus to withstand the hostile host environment. There is growing evidence that staphylococcal DNA repair makes important contributions to the survival of the pathogen in host tissues, as well as promoting the emergence of mutants that resist host defenses and antibiotics. While much of what we know about DNA repair in S. aureus is inferred from studies with model organisms, the roles of specific repair mechanisms in infection are becoming clear and differences with Bacillus subtilis and Escherichia coli have been identified. Furthermore, there is growing interest in staphylococcal DNA repair as a target for novel therapeutics that sensitize the pathogen to host defenses and antibiotics. In this review, we discuss what is known about staphylococcal DNA repair and its role in infection, examine how repair in S. aureus is similar to, or differs from, repair in well-characterized model organisms, and assess the potential of staphylococcal DNA repair as a novel therapeutic target.

Identification of a Novel Cobamide Remodeling Enzyme in the Beneficial Human Gut Bacterium Akkermansia muciniphila

Cobamides, comprising the vitamin B₁₂ family of cobalt-containing cofactors, are required for metabolism in all domains of life, including most bacteria. Cobamides have structural variability in the lower ligand, and selectivity for particular cobamides has been observed in most organisms studied to date.

The beneficial human gut bacterium Akkermansia muciniphila provides metabolites to other members of the gut microbiota by breaking down host mucin, but most of its other metabolic functions have not been investigated. A. muciniphila strain Muc^T is known to use cobamides, the vitamin B₁₂ family of cofactors with structural diversity in the lower ligand. However, A. muciniphila Muc^T is unable to synthesize cobamides de novo, and the specific forms that can be used by A. muciniphila have not been examined. We found that the levels of growth of A. muciniphila Muc^T were nearly identical with each of seven cobamides tested, in contrast to nearly all bacteria that had been studied previously. Unexpectedly, this promiscuity is due to cobamide remodeling—the removal and replacement of the lower ligand—despite the absence of the canonical remodeling enzyme CbiZ in A. muciniphila. We identified a novel enzyme, CbiR, that is capable of initiating the remodeling process by hydrolyzing the phosphoribosyl bond in the nucleotide loop of cobamides. CbiR does not share similarity with other cobamide remodeling enzymes or B₁₂-binding domains and is instead a member of the apurinic/apyrimidinic (AP) endonuclease 2 enzyme superfamily. We speculate that CbiR enables bacteria to repurpose cobamides that they cannot otherwise use in order to grow under cobamide-requiring conditions; this function was confirmed by heterologous expression of cbiR in Escherichia coli. Homologs of CbiR are found in over 200 microbial taxa across 22 phyla, suggesting that many bacteria may use CbiR to gain access to the diverse cobamides present in their environment.

LC-MS/MS proteomic analysis of starved Bacillus subtilis cells overexpressing ribonucleotide reductase (nrdEF): implications in stress-associated mutagenesis

The non-appropriate conditions faced by nutritionally stressed bacteria propitiate error-prone repair events underlying stationary-phase- or stress-associated mutagenesis (SPM). The genetic and molecular mechanisms involved in SPM have been deeply studied but the biochemical aspects of this process have so far been less explored. Previous evidence showed that under conditions of nutritional stress, non-dividing cells of strain B. subtilis YB955 overexpressing ribonucleotide reductase (RNR) exhibited a strong propensity to generate true reversions in the hisC952 (amber), metB5 (ochre) and leuC425 (missense) mutant alleles. To further advance our knowledge on the metabolic conditions underlying this hypermutagenic phenotype, a high-throughput LC-MS/MS proteomic analysis was performed in non-dividing cells of an amino acid-starved strain, deficient for NrdR, the RNR repressor. Compared with the parental strain, the level of 57 proteins was found to increase and of 80 decreases in the NrdR-deficient strain. The proteomic analysis revealed an altered content in proteins associated with the stringent response, nucleotide metabolism, DNA repair, and cell signaling in amino acid-starved cells of the ∆nrdR strain. Overall, our results revealed that amino acid-starved cells of strain B. subtilis ∆nrdR that escape from growth-limiting conditions exhibit a complex proteomic pattern reminiscent of a disturbed metabolism. Future experiments aimed to understand the consequences of disrupting the cell signaling pathways unveiled in this study, will advance our knowledge on the genetic adaptations deployed by bacteria to escape from growth-limiting environments.

Identification of a conserved 5'-dRP lyase activity in bacterial DNA repair ligase D and its potential role in base excision repair

Bacillus subtilis is one of the bacterial members provided with a nonhomologous end joining (NHEJ) system constituted by the DNA-binding Ku homodimer that recruits the ATP-dependent DNA Ligase D (BsuLigD) to the double-stranded DNA breaks (DSBs) ends. BsuLigD has inherent polymerization and ligase activities that allow it to fill the short gaps that can arise after realignment of the broken ends and to seal the resulting nicks, contributing to genome stability during the stationary phase and germination of spores. Here we show that BsuLigD also has an intrinsic 5'-2-deoxyribose-5-phosphate (dRP) lyase activity located at the N-terminal ligase domain that in coordination with the polymerization and ligase activities allows efficient repairing of 2'-deoxyuridine-containing DNA in an in vitro reconstituted Base Excision Repair (BER) reaction. The requirement of a polymerization, a dRP removal and a final sealing step in BER, together with the joint participation of BsuLigD with the spore specific AP endonuclease in conferring spore resistance to ultrahigh vacuum desiccation suggest that BsuLigD could actively participate in this pathway. We demonstrate the presence of the dRP lyase activity also in the homolog protein from the distantly related bacterium Pseudomonas aeruginosa, allowing us to expand our results to other bacterial LigDs.

Efficient processing of abasic sites by bacterial nonhomologous end-joining Ku proteins

Intracellular reactive oxygen species as well as the exposure to harsh environmental conditions can cause, in the single chromosome of Bacillus subtilis spores, the formation of apurinic/apyrimidinic (AP) sites and strand breaks whose repair during outgrowth is crucial to guarantee cell viability. Whereas double-stranded breaks are mended by the nonhomologous end joining (NHEJ) system composed of an ATP-dependent DNA Ligase D (LigD) and the DNA-end-binding protein Ku, repair of AP sites would rely on an AP endonuclease or an AP-lyase, a polymerase and a ligase. Here we show that B. subtilis Ku (BsuKu), along with its pivotal role in allowing joining of two broken ends by B. subtilis LigD (BsuLigD), is endowed with an AP/deoxyribose 5'-phosphate (5'-dRP)-lyase activity that can act on ssDNA, nicked molecules and DNA molecules without ends, suggesting a potential role in BER during spore outgrowth. Coordination with BsuLigD makes possible the efficient joining of DNA ends with near terminal abasic sites. The role of this new enzymatic activity of Ku and its potential importance in the NHEJ pathway is discussed. The presence of an AP-lyase activity also in the homolog protein from the distantly related bacterium Pseudomonas aeruginosa allows us to expand our results to other bacterial Ku proteins.

Error-prone processing of apurinic/apyrimidinic (AP) sites by PolX underlies a novel mechanism that promotes adaptive mutagenesis in Bacillus subtilis

In growing cells, apurinic/apyrimidinic (AP) sites generated spontaneously or resulting from the enzymatic elimination of oxidized bases must be processed by AP endonucleases before they compromise cell integrity. Here, we investigated how AP sites and the processing of these noncoding lesions by the AP endonucleases Nfo, ExoA, and Nth contribute to the production of mutations (hisC952, metB5, and leuC427) in starved cells of the Bacillus subtilis YB955 strain. Interestingly, cells from this strain that were deficient for Nfo, ExoA, and Nth accumulated a greater amount of AP sites in the stationary phase than during exponential growth. Moreover, under growth-limiting conditions, the triple nfo exoA nth knockout strain significantly increased the amounts of adaptive his, met, and leu revertants produced by the B. subtilis YB955 parental strain. Of note, the number of stationary-phase-associated reversions in the his, met, and leu alleles produced by the nfo exoA nth strain was significantly decreased following disruption of polX. In contrast, during growth, the reversion rates in the three alleles tested were significantly increased in cells of the nfo exoA nth knockout strain deficient for polymerase X (PolX). Therefore, we postulate that adaptive mutations in B. subtilis can be generated through a novel mechanism mediated by error-prone processing of AP sites accumulated in the stationary phase by the PolX DNA polymerase.

Uncoupling of the apyrimidinic/apurinic endonucleolytic and 3'→5' exonucleolytic activities of the Nfo protein of Mycoplasma pneumoniae through mutation of specific amino acid residues

The DNA recombination and repair machineries of Mycoplasma pneumoniae and Mycoplasma genitalium were predicted to consist of a set of ~11 proteins. The function of one of these proteins was inferred from its homology with proteins belonging to the Endo IV enzyme family. The members of this family function in the repair of apyrimidinic/apurinic (AP) sites in DNA. As such activity may be crucial in the mycoplasmal life cycle, we set out to study the Endo IV-like proteins encoded by M. pneumoniae and M. genitalium. Both proteins, termed NfoMpn and NfoMge, respectively, were assessed for their ability to interact with damaged and undamaged DNA. In the absence of divalent cations, both proteins exhibited specific cleavage of AP sites. Surprisingly, the proteins also recognized and cleaved cholesteryl-bound deoxyribose moieties in DNA, showing that these Nfo proteins may also function in repair of large DNA adducts. In the presence of Mg(2+), NfoMpn and NfoMge also showed 3'→5' exonucleolytic activity. By introduction of 13 single point mutations at highly conserved positions within NfoMpn, two major types of mutants could be distinguished: (i) mutants that showed no, or limited, AP cleavage activity in the presence of EDTA, but displayed significant levels of AP cleavage activity in the presence of Mg(2+); these mutants displayed no, or very low, exonucleolytic activity; and (ii) mutants that only demonstrated marginal levels of AP site cleavage activity in the presence of Mg(2+) and did not show exonucleolytic activity. Together, these results indicated that the AP endonucleolytic activity of the NfoMpn protein can be uncoupled from its 3'→5' exonucleolytic activity.

DNA repair and genome maintenance in Bacillus subtilis

From microbes to multicellular eukaryotic organisms, all cells contain pathways responsible for genome maintenance. DNA replication allows for the faithful duplication of the genome, whereas DNA repair pathways preserve DNA integrity in response to damage originating from endogenous and exogenous sources. The basic pathways important for DNA replication and repair are often conserved throughout biology. In bacteria, high-fidelity repair is balanced with low-fidelity repair and mutagenesis. Such a balance is important for maintaining viability while providing an opportunity for the advantageous selection of mutations when faced with a changing environment. Over the last decade, studies of DNA repair pathways in bacteria have demonstrated considerable differences between Gram-positive and Gram-negative organisms. Here we review and discuss the DNA repair, genome maintenance, and DNA damage checkpoint pathways of the Gram-positive bacterium Bacillus subtilis. We present their molecular mechanisms and compare the functions and regulation of several pathways with known information on other organisms. We also discuss DNA repair during different growth phases and the developmental program of sporulation. In summary, we present a review of the function, regulation, and molecular mechanisms of DNA repair and mutagenesis in Gram-positive bacteria, with a strong emphasis on B. subtilis.

The role of the PHP domain associated with DNA polymerase X from Thermus thermophilus HB8 in base excision repair

Base excision repair (BER) is one of the most commonly used DNA repair pathways involved in genome stability. X-family DNA polymerases (PolXs) play critical roles in BER, especially in filling single-nucleotide gaps. In addition to a polymerase core domain, bacterial PolXs have a polymerase and histidinol phosphatase (PHP) domain with phosphoesterase activity which is also required for BER. However, the role of the PHP domain of PolX in bacterial BER remains unresolved. We found that the PHP domain of Thermus thermophilus HB8 PolX (ttPolX) functions as two types of phosphoesterase in BER, including a 3'-phosphatase and an apurinic/apyrimidinic (AP) endonuclease. Experiments using T. thermophilus HB8 cell lysates revealed that the majority of the 3'-phosphatase and AP endonuclease activities are attributable to the another phosphoesterase in T. thermophilus HB8, endonuclease IV (ttEndoIV). However, ttPolX possesses significant 3'-phosphatase activity in ΔttendoIV cell lysate, indicating possible complementation. Our experiments also reveal that there are only two enzymes that display the 3'-phosphatase activity in the T. thermophilus HB8 cell, ttPolX and ttEndoIV. Furthermore, phenotypic analysis of ΔttpolX, ΔttendoIV, and ΔttpolX/ΔttendoIV using hydrogen peroxide and sodium nitrite supports the hypothesis that ttPolX functions as a backup for ttEndoIV in BER.

Alternative excision repair of ultraviolet B- and C-induced DNA damage in dormant and developing spores of Bacillus subtilis

The nucleotide excision repair (NER) and spore photoproduct lyase DNA repair pathways are major determinants of Bacillus subtilis spore resistance to UV radiation. We report here that a putative ultraviolet (UV) damage endonuclease encoded by ywjD confers protection to developing and dormant spores of B. subtilis against UV DNA damage. In agreement with its predicted function, a His(6)-YwjD recombinant protein catalyzed the specific incision of UV-irradiated DNA in vitro. The maximum expression of a reporter gene fusion to the ywjD opening reading frame occurred late in sporulation, and this maximal expression was dependent on the forespore-specific RNA polymerase sigma factor, σ(G). Although the absence of YwjD and/or UvrA, an essential protein of the NER pathway, sensitized developing spores to UV-C, this effect was lower when these cells were treated with UV-B. In contrast, UV-B but not UV-C radiation dramatically decreased the survival of dormant spores deficient in both YwjD and UvrA. The distinct range of lesions generated by UV-C and UV-B and the different DNA photochemistry in developing and dormant spores may cause these differences. We postulate that in addition to the UvrABC repair system, developing and dormant spores of B. subtilis also rely on an alternative excision repair pathway involving YwjD to deal with the deleterious effects of various UV photoproducts.

Roles of endonuclease V, uracil-DNA glycosylase, and mismatch repair in Bacillus subtilis DNA base-deamination-induced mutagenesis

The disruption of ung, the unique uracil-DNA-glycosylase-encoding gene in Bacillus subtilis, slightly increased the spontaneous mutation frequency to rifampin resistance (Rif(r)), suggesting that additional repair pathways counteract the mutagenic effects of uracil in this microorganism. An alternative excision repair pathway is involved in this process, as the loss of YwqL, a putative endonuclease V homolog, significantly increased the mutation frequency of the ung null mutant, suggesting that Ung and YwqL both reduce the mutagenic effects of base deamination. Consistent with this notion, sodium bisulfite (SB) increased the Rif(r) mutation frequency of the single ung and double ung ywqL strains, and the absence of Ung and/or YwqL decreased the ability of B. subtilis to eliminate uracil from DNA. Interestingly, the Rif(r) mutation frequency of single ung and mutSL (mismatch repair [MMR] system) mutants was dramatically increased in a ung knockout strain that was also deficient in MutSL, suggesting that the MMR pathway also counteracts the mutagenic effects of uracil. Since the mutation frequency of the ung mutSL strain was significantly increased by SB, in addition to Ung, the mutagenic effects promoted by base deamination in growing B. subtilis cells are prevented not only by YwqL but also by MMR. Importantly, in nondividing cells of B. subtilis, the accumulations of mutations in three chromosomal alleles were significantly diminished following the disruption of ung and ywqL. Thus, under conditions of nutritional stress, the processing of deaminated bases in B. subtilis may normally occur in an error-prone manner to promote adaptive mutagenesis.

Role of the Nfo and ExoA apurinic/apyrimidinic endonucleases in radiation resistance and radiation-induced mutagenesis of Bacillus subtilis spores

The roles of DNA repair by apurinic/apyrimidinic (AP) endonucleases alone, and together with DNA protection by α/β-type small acid-soluble spore proteins (SASP), in Bacillus subtilis spore resistance to different types of radiation have been studied. Spores lacking both AP endonucleases (Nfo and ExoA) and major SASP were significantly more sensitive to 254-nm UV-C, environmental UV (>280 nm), X-ray exposure, and high-energy charged (HZE)-particle bombardment and had elevated mutation frequencies compared to those of wild-type spores and spores lacking only one or both AP endonucleases or major SASP. These findings further implicate AP endonucleases and α/β-type SASP in repair and protection, respectively, of spore DNA against effects of UV and ionizing radiation.

Intrinsic apurinic/apyrimidinic (AP) endonuclease activity enables Bacillus subtilis DNA polymerase X to recognize, incise, and further repair abasic sites

The N-glycosidic bond can be hydrolyzed spontaneously or by glycosylases during removal of damaged bases by the base excision repair pathway, leading to the formation of highly mutagenic apurinic/apyrimidinic (AP) sites. Organisms encode for evolutionarily conserved repair machinery, including specific AP endonucleases that cleave the DNA backbone 5' to the AP site to prime further DNA repair synthesis. We report on the DNA polymerase X from the bacterium Bacillus subtilis (PolX(Bs)) that, along with polymerization and 3'-5'-exonuclease activities, possesses an intrinsic AP-endonuclease activity. Both, AP-endonuclease and 3'-5'-exonuclease activities are genetically linked and governed by the same metal ligands located at the C-terminal polymerase and histidinol phosphatase domain of the polymerase. The different catalytic functions of PolX(Bs) enable it to perform recognition and incision at an AP site and further restoration (repair) of the original nucleotide in a standalone AP-endonuclease-independent way.

Management of oxidative stress in Bacillus

The spore-forming bacterium and model prokaryotic genetic system, Bacillus subtilis, is extremely useful in the study of oxidative stress management through proteomic and genome-wide transcriptomic analyses, as well as through detailed structural studies of the regulatory factors that govern the oxidative stress response. The factors that sense oxidants and induce expression of protective activities include the PerR and OhrR proteins, which show acute discrimination for their peroxide stimuli, whereas the general stress control factor, the RNA polymerase sigma(B) subunit and the thiol-based sensor Spx, govern the protective response to oxidants under multiple stress conditions. Some specific and some redundant protective mechanisms are mobilized at different stages of the Bacillus developmental cycle to deal with vulnerable cells in stationary-phase conditions and during spore germination and outgrowth. An important unknown is the nature and influence of the low-molecular-weight thiols that mediate the buffering of the redox environment.

African swine fever virus AP endonuclease is a redox-sensitive enzyme that repairs alkylating and oxidative damage to DNA

African swine fever virus (ASFV) encodes an AP endonuclease (pE296R) which is essential for virus growth in swine macrophages. We show here that the DNA repair functions of pE296R (AP endonucleolytic, 3′ → 5′ exonuclease, 3′-diesterase and nucleotide incision repair (NIR) activities) and DNA binding are inhibited by reducing agents. Protein pE296R contains one intramolecular disulfide bond, whose disruption by reducing agents might perturb the interaction of the viral AP endonuclease with the DNA substrate. The characterization of the 3′ → 5′ exonuclease and 3′-repair diesterase activities of pE296R indicates that it has strong preference for mispaired and oxidative base lesions at the 3′-termini of single-strand breaks. Finally, the viral protein protects against DNA damaging agents in both prokaryotic and eukaryotic cells, emphasizing its importance in vivo. The biochemical and genetic properties of ASFV AP endonuclease are consistent with the repair of DNA damage generated by the genotoxic intracellular environment of the host macrophage.

Editing of misaligned 3'-termini by an intrinsic 3'-5' exonuclease activity residing in the PHP domain of a family X DNA polymerase

Bacillus subtilis gene yshC encodes a family X DNA polymerase (PolX_Bs), whose biochemical features suggest that it plays a role during DNA repair processes. Here, we show that, in addition to the polymerization activity, PolX_Bs possesses an intrinsic 3′–5′ exonuclease activity specialized in resecting unannealed 3′-termini in a gapped DNA substrate. Biochemical analysis of a PolX_Bs deletion mutant lacking the C-terminal polymerase histidinol phosphatase (PHP) domain, present in most of the bacterial/archaeal PolXs, as well as of this separately expressed protein region, allow us to state that the 3′–5′ exonuclease activity of PolX_Bs resides in its PHP domain. Furthermore, site-directed mutagenesis of PolX_Bs His339 and His341 residues, evolutionary conserved in the PHP superfamily members, demonstrated that the predicted metal binding site is directly involved in catalysis of the exonucleolytic reaction. The implications of the unannealed 3′-termini resection by the 3′–5′ exonuclease activity of PolX_Bs in the DNA repair context are discussed.

Role of the Nfo (YqfS) and ExoA apurinic/apyrimidinic endonucleases in protecting Bacillus subtilis spores from DNA damage

The Bacillus subtilis enzymes ExoA and Nfo (originally termed YqfS) are endonucleases that can repair apurinic/apyrimidinic (AP) sites and strand breaks in DNA. We have analyzed how the lack of ExoA and Nfo affects the resistance of growing cells and dormant spores of B. subtilis to a variety of treatments, some of which generate AP sites and DNA strand breaks. The lack of ExoA and Nfo sensitized spores (termed alpha-beta-) lacking the majority of their DNA-protective alpha/beta-type small, acid-soluble spore proteins (SASP) to wet heat. However, the lack of these enzymes had no effect on the wet-heat resistance of spores that retained alpha/beta-type SASP. The lack of either ExoA or Nfo sensitized wild-type spores to dry heat, but loss of both proteins was necessary to sensitize alpha-beta- spores to dry heat. The lack of ExoA and Nfo also sensitized alpha-beta-, but not wild-type, spores to desiccation. In contrast, loss of ExoA and Nfo did not sensitize growing cells or wild-type or alpha-beta- spores to hydrogen peroxide or t-butylhydroperoxide. Loss of ExoA and Nfo also did not increase the spontaneous mutation frequency of growing cells. exoA expression took place not only in growing cells, but also in the forespore compartment of the sporulating cell. These results, together with those from previous work, suggest that ExoA and Nfo are additional factors that protect B. subtilis spores from DNA damage accumulated during spore dormancy.

Compartmentalization of gene expression during Bacillus subtilis spore formation

Gene expression in members of the family Bacillaceae becomes compartmentalized after the distinctive, asymmetrically located sporulation division. It involves complete compartmentalization of the activities of sporulation-specific sigma factors, sigma(F) in the prespore and then sigma(E) in the mother cell, and then later, following engulfment, sigma(G) in the prespore and then sigma(K) in the mother cell. The coupling of the activation of sigma(F) to septation and sigma(G) to engulfment is clear; the mechanisms are not. The sigma factors provide the bare framework of compartment-specific gene expression. Within each sigma regulon are several temporal classes of genes, and for key regulators, timing is critical. There are also complex intercompartmental regulatory signals. The determinants for sigma(F) regulation are assembled before septation, but activation follows septation. Reversal of the anti-sigma(F) activity of SpoIIAB is critical. Only the origin-proximal 30% of a chromosome is present in the prespore when first formed; it takes approximately 15 min for the rest to be transferred. This transient genetic asymmetry is important for prespore-specific sigma(F) activation. Activation of sigma(E) requires sigma(F) activity and occurs by cleavage of a prosequence. It must occur rapidly to prevent the formation of a second septum. sigma(G) is formed only in the prespore. SpoIIAB can block sigma(G) activity, but SpoIIAB control does not explain why sigma(G) is activated only after engulfment. There is mother cell-specific excision of an insertion element in sigK and sigma(E)-directed transcription of sigK, which encodes pro-sigma(K). Activation requires removal of the prosequence following a sigma(G)-directed signal from the prespore.