Blocking chromosome translocation during sporulation of Bacillus subtilis can result in prespore-specific activation of sigmaG that is independent of sigmaE and of engulfment

Coupling of σG activation to completion of engulfment during sporulation of Bacillus subtilis survives large perturbations to DNA translocation and replication

Spore formation in Bacillus subtilis is characterized by activation of RNA polymerase sigma factors, including the late-expressed σ(G). During spore formation an asymmetric division occurs, yielding the smaller prespore and the larger mother cell. At division, only 30% of the chromosome is in the prespore, and the rest is then translocated into the prespore. Following completion of engulfment of the prespore by the mother cell, σ(G) is activated in the prespore. Here we tested the link between engulfment and σ(G) activation by perturbing DNA translocation and replication, which are completed before engulfment. One approach was to have large DNA insertions in the chromosome; the second was to have an impaired DNA translocase; the third was to use a strain in which the site of termination of chromosome replication was relocated. Insertion of 2.3 Mb of Synechocystis DNA into the B. subtilis genome had the largest effect, delaying engulfment by at least 90 min. Chromosome translocation was also delayed and was completed shortly before the completion of engulfment. Despite the delay, σ(G) became active only after the completion of engulfment. All results are consistent with a strong link between completion of engulfment and σ(G) activation. They support a link between completion of chromosome translocation and completion of engulfment.

The genomic basis for the evolution of a novel form of cellular reproduction in the bacterium Epulopiscium

Background

Epulopiscium sp. type B, a large intestinal bacterial symbiont of the surgeonfish Naso tonganus, does not reproduce by binary fission. Instead, it forms multiple intracellular offspring using a process with morphological features similar to the survival strategy of endospore formation in other Firmicutes. We hypothesize that intracellular offspring formation in Epulopiscium evolved from endospore formation and these two developmental programs share molecular mechanisms that are responsible for the observed morphological similarities.

Results

To test this, we sequenced the genome of Epulopiscium sp. type B to draft quality. Comparative analysis with the complete genome of its close, endospore-forming relative, Cellulosilyticum lentocellum, identified homologs of well-known sporulation genes characterized in Bacillus subtilis. Of the 147 highly conserved B. subtilis sporulation genes used in this analysis, we found 57 homologs in the Epulopiscium genome and 87 homologs in the C. lentocellum genome.

Conclusions

Genes coding for components of the central regulatory network which govern the expression of forespore and mother-cell-specific sporulation genes and the machinery used for engulfment appear best conserved. Low conservation of genes expressed late in endospore formation, particularly those that confer resistance properties and encode germinant receptors, suggest that Epulopiscium has lost the ability to form a mature spore. Our findings provide a framework for understanding the evolution of a novel form of cellular reproduction.

Regulation of growth of the mother cell and chromosome replication during sporulation of Bacillus subtilis

During spore formation, Bacillus subtilis divides asymmetrically, resulting in two cells with different fates. Immediately after division, the transcription factor σ(F) becomes active in the smaller prespore, followed by activation of σ(E) in the larger mother cell. We recently showed that a delay in σ(E) activation resulted in the novel phenotype of two spores (twins) forming within the same mother cell. Mother cells bearing twins are substantially longer than mother cells with single spores. Here we explore the regulation of the growth and DNA replication of the mother cell. We find that length correlates with chromosome number in the mother cell. We show that replication and growth could occur after asymmetric division in mother cells with no active σ(E). In contrast, when σ(E) was active, replication and growth ceased. In growing mother cells, with no active σ(E), Spo0A-directed transcription levels remained low. In the presence of active σ(E), Spo0A-directed gene expression was enhanced in the mother cells. Artificial Spo0A activation blocked mother cell growth in the absence of σ(E). Spo0A activation blocked growth even in the absence of SirA, the Spo0A-directed inhibitor of the initiation of replication. Together, the results indicate that the burst of Spo0A-directed expression along with the activation of σ(E) provides mechanisms to block the DNA replication and growth of the mother cell.

Inactivation of σE and σG in Clostridium acetobutylicum illuminates their roles in clostridial-cell-form biogenesis, granulose synthesis, solventogenesis, and spore morphogenesis

Central to all clostridia is the orchestration of endospore formation (i.e., sporulation) and, specifically, the roles of differentiation-associated sigma factors. Moreover, there is considerable applied interest in understanding the roles of these sigma factors in other stationary-phase phenomena, such as solvent production (i.e., solventogenesis). Here we separately inactivated by gene disruption the major sporulation-specific sigma factors, σ(E) and σ(G), and performed an initial analysis to elucidate their roles in sporulation-related morphogenesis and solventogenesis in Clostridium acetobutylicum. The terminal differentiation phenotype for the sigE inactivation mutant stalled in sporulation prior to asymmetric septum formation, appeared vegetative-like often with an accumulation of DNA at both poles, frequently exhibited two longitudinal internal membranes, and did not synthesize granulose. The sigE inactivation mutant did produce the characteristic solvents (i.e., butanol and acetone), but the extent of solventogenesis was dependent on the physiological state of the inoculum. The sigG inactivation mutant stalled in sporulation during endospore maturation, exhibiting engulfment and partial cortex and spore coat formation. Lastly, the sigG inactivation mutant did produce granulose and exhibited wild-type-like solventogenesis.

A small protein required for the switch from {sigma}F to {sigma}G during sporulation in Bacillus subtilis

A cascade of alternative sigma factors governs the program of developmental gene expression during sporulation in Bacillus subtilis. Little is known, however, about how the early-acting sigma factors are inactivated and replaced by the later-acting factors. Here we identify a small protein, Fin (formerly known as YabK), that is required for efficient switching from σ(F)- to σ(G)-directed gene expression in the forespore compartment of the developing sporangium. The fin gene, which is conserved among Bacillus species and species of related genera, is transcribed in the forespore under the control of both σ(F) and σ(G). Cells mutant for fin are unable to fully deactivate σ(F) and, conversely, are unable to fully activate σ(G). Consistent with their deficiency in σ(G)-directed gene expression, fin cells are arrested in large numbers following the engulfment stage of sporulation, ultimately forming 50-fold fewer heat-resistant spores than the wild type. Based in part on the similarity of Fin to the anti-σ(G) factor CsfB (also called Gin), we speculate that Fin is an anti-σ(F) factor which, by disabling σ(F), promotes the switch to late developmental gene expression in the forespore.

Loss of compartmentalization of σ(E) activity need not prevent formation of spores by Bacillus subtilis

Compartmentalization of the activities of RNA polymerase sigma factors is a hallmark of formation of spores by Bacillus subtilis. It is initiated soon after the asymmetrically located sporulation division takes place with the activation of σ(F) in the smaller cell, the prespore. σ(F) then directs a signal via the membrane protease SpoIIGA to activate σ(E) in the larger mother cell by processing of pro-σ(E). Here, we show that σ(E) can be activated in the prespore with little effect on sporulation efficiency, implying that complete compartmentalization of σ(E) activity is not essential for spore formation. σ(E) activity in the prespore can be obtained by inducing transcription in the prespore of spoIIGA or of sigE*, which encodes a constitutively active form of σ(E), but not of spoIIGB, which encodes pro-σ(E). We infer that σ(E) compartmentalization is partially attributed to a competition between the compartments for the activation signaling protein SpoIIR. Normally, SpoIIGA is predominantly located in the mother cell and as a consequence confines σ(E) activation to it. In addition, we find that CsfB, previously shown to inhibit σ(G), is independently inhibiting σ(E) activity in the prespore. CsfB thus appears to serve a gatekeeper function in blocking the action of two sigma factors in the prespore: it prevents σ(G) from becoming active before completion of engulfment and helps prevent σ(E) from becoming active at all.

A feeding tube model for activation of a cell-specific transcription factor during sporulation in Bacillus subtilis

Spore formation by Bacillus subtilis takes place in a sporangium consisting of two chambers, the forespore and the mother cell, which are linked by pathways of intercellular communication. One pathway, which couples the activation of the forespore transcription factor sigma(G) to the action of sigma(E) in the mother cell, has remained mysterious. Traditional models hold that sigma(E) initiates a signal transduction pathway that specifically activates sigma(G) in the forespore. Recent experiments indicating that the mother cell and forespore are joined by a channel have led to the suggestion that a specific regulator of sigma(G) is transported from the mother cell into the forespore. As we report here, however, the requirement for the channel is not limited to sigma(G). Rather, it is also required for the persistent activity of the early-acting forespore transcription factor sigma(F) as well as that of a heterologous RNA polymerase (that of phage T7). We infer that macromolecular synthesis in the forespore becomes dependent on the channel at intermediate stages of development. We propose that the channel is a gap junction-like feeding tube through which the mother cell nurtures the developing spore by providing small molecules needed for biosynthetic activity, including sigma(G)-directed gene activation.

Processing of a membrane protein required for cell-to-cell signaling during endospore formation in Bacillus subtilis

Activation of the late prespore-specific RNA polymerase sigma factor sigma(G) during Bacillus subtilis sporulation coincides with completion of the engulfment process, when the prespore becomes a protoplast fully surrounded by the mother cell cytoplasm and separated from it by a double membrane system. Activation of sigma(G) also requires expression of spoIIIJ, coding for a membrane protein translocase of the YidC/Oxa1p/Alb3 family, and of the mother cell-specific spoIIIA operon. Here we present genetic and biochemical evidence indicating that SpoIIIAE, the product of one of the spoIIIA cistrons, and SpoIIIJ interact in the membrane, thereby linking the function of the spoIIIJ and spoIIIA loci in the activation of sigma(G). We also show that SpoIIIAE has a functional Sec-type signal peptide, which is cleaved during sporulation. Furthermore, mutations that reduce or eliminate processing of the SpoIIIAE signal peptide arrest sporulation following engulfment completion and prevent activation of sigma(G). SpoIIIJ-type proteins can function in cooperation with or independently of the Sec system. In one model, SpoIIIJ interacts with SpoIIIAE in the context of the Sec translocon to promote its correct localization and/or topology in the membrane, so that it can signal the activation of sigma(G) following engulfment completion.

Expression of the sigmaF-directed csfB locus prevents premature appearance of sigmaG activity during sporulation of Bacillus subtilis

During sporulation, sigma(G) becomes active in the prespore upon the completion of engulfment. We show that the inactivation of the sigma(F)-directed csfB locus resulted in premature activation of sigma(G). CsfB exerted control distinct from but overlapping with that exerted by LonA to prevent inappropriate sigma(G) activation. The artificial induction of csfB severely compromised spore formation.

Perturbations to engulfment trigger a degradative response that prevents cell-cell signalling during sporulation in Bacillus subtilis

During sporulation in Bacillus subtilis, the mother cell membranes migrate around the forespore in a phagocytic-like process called engulfment. Developmental gene expression requires the successful completion of this key morphological event. Here we show that perturbations to engulfment block the accumulation of proteins secreted into the space between the mother cell and forespore membranes. Our data support a model in which engulfment defects cause the proteolytic clearance of these secreted proteins. Importantly, we show that this degradative response is reversible; once proper engulfment is restored, secreted proteins again accumulate. In particular, we have found that the forespore signalling protein SpoIVB fails to accumulate when engulfment is impaired and, as a result, late mother cell gene expression under the control of sigma(K) is blocked. If engulfment is restored, SpoIVB accumulates and cell-cell signalling resumes. Thus, this degradative pathway functions like a developmental checkpoint ensuring that mother cell gene expression does not commence unless morphogenesis proceeds normally.

Separation of chromosome termini during sporulation of Bacillus subtilis depends on SpoIIIE

Bacillus subtilis undergoes a highly distinctive division during spore formation. It yields two unequal cells, the mother cell and the prespore, and septum formation is completed before the origin-distal 70% of the chromosome has entered the smaller prespore. The mother cell subsequently engulfs the prespore. Two different probes were used to study the behavior of the terminus (ter) region of the chromosome during spore formation. Only one ter region was observed at the time of sporulation division. A second ter region, indicative of chromosome separation, was not distinguishable until engulfment was nearing completion, when one was in the mother cell and the other in the prespore. Separation of the two ter regions depended on the DNA translocase SpoIIIE. It is concluded that SpoIIIE is required during spore formation for chromosome separation as well as for translocation; SpoIIIE is not required for separation during vegetative growth.