Effects of mutational loss of specific intracellular proteases on the sporulation of Bacillus subtilis

Construction and properties of an intracellular serine protease mutant of Bacillus subtilis

An intracellular serine protease (ISP-1) mutant of Bacillus subtilis was created by introducing a frameshift into the coding region of the cloned gene. Intracellular protease activity in the mutant was very low, yet sporulation in both nutrient broth and minimal medium was normal. The rate of bulk protein turnover in the mutant was slightly slower than that in the wild-type strain. These results suggest that the gene for ISP-1 is not essential and that ISP-1 is not the major enzyme involved in protein turnover during sporulation.

Cloning and sequencing of the major intracellular serine protease gene of Bacillus subtilis

A Bacillus subtilis 2.7-kilobase DNA fragment containing an intracellular protease gene was cloned into Escherichia coli. The transformants produced an intracellular protease of approximately 35,000 Mr whose activity was inhibited by both phenylmethylsulfonyl fluoride and EDTA. Introduction of the fragment on a multicopy vector, pUB110, into B. subtilis caused a marked increase in the level of the intracellular protease. The nucleotide sequence of the cloned fragment showed the presence of an open reading frame for a possible proenzyme of the major intracellular serine protease (ISP-I) of B. subtilis with an NH2-terminal 17- or 20-amino-acid extension. The total amino acid sequence of the protease deduced from the nucleotide sequence showed considerable homology with that of an extracellular serine protease, subtilisin. The transcriptional initiation site of the ISP-I gene was identified by nuclease S1 mapping. No typical conserved sequence for promoters was found upstream of the open reading frame. An ISP-I-negative mutant of B. subtilis was constructed by integration of artificially deleted gene into the chromosome. The mutant sporulated normally in a nutritionally rich medium but showed decreased sporulation in a synthetic medium. The chloramphenicol resistance determinant of a plasmid integrated at the ISP-I locus was mapped by PBS1 transduction and was found to be closely linked to metC (99.5%).

Activation of intracellular serine proteinase in Bacillus subtilis cells during sporulation

Cells of Bacillus subtilis 168 (trpC2) growing and sporulating in a single chemically defined medium carried out intracellular protein degradation and increased their levels of intracellular serine protease-1 in a manner very similar to what had previously been reported for cells sporulating in nutrient broth. The results were interpreted to mean that these processes are intrinsic to sporulation rather than medium dependent. To determine the cause of these increases in specific activity of proteinases, we purified the protease, prepared rabbit immunoglobulins directed against it, and monitored changes in protease antigen levels by performing rocket immunoelectrophoresis. In cells sporulating in nutrient broth, the protease antigen levels increased about 7-fold, whereas the specific activity increased about 150-fold, for an activation of about 20-fold. In cells sporulating in the single chemically defined sporulation medium, the protease antigen increased about 10-fold, whereas the specific activity increased at least 400-fold, for an activation of about 40-fold. These results were interpreted to mean that a posttranslational event activated the protease in vivo; a previously described endogenous proteinase inhibitor was confirmed to be present in the strain used. Chloramphenicol added to the cultures inhibited both the increases in antigen levels and in the specific activity of the proteinase.

Stability and synthesis of the penicillin-binding proteins during sporulation

The penicillin-binding proteins (PBPs) of Bacillus subtilis were examined after incubation of vegetative and sporulating cultures with chloramphenicol, an inhibitor of protein synthesis. The results indicate that the sporulation-specific increases in vegetative PBPs 2B and 3 and the appearance of two new PBPs, 4* and 5*, depend on concurrent protein synthesis, which is most likely to be de novo synthesis of the PBPs rather than synthesis of an activator or processing enzyme. It was also learned that in vivo the PBPs differ in their individual stabilities, which helps to explain some of the quantitative changes that occur in the PBP profile during sporulation. All the membrane-bound PBPs, except possibly PBP 1, were found to be stable in the presence of crude extracts of sporulating cells that contained proteolytic activity.

Degradation of ornithine transcarbamylase in sporulating Bacillus subtilis cells

When Bacillus subtilis cells grew and sporulated on glucose-nutrient broth, ornithine transcarbamylase (OTCase) was synthesized in the early stationary phase and then inactivated. The loss of OTCase activity was much slower in a mutant that was deficient in a major intracellular serine protease (ISP). Immunochemical analysis showed that synthesis of OTCase decreased to a low, but detectable, level during its inactivation and that loss of activity was paralleled by loss of cross-reactive protein. Because the antibodies were capable of detecting denatured and fragmented forms of OTCase, we conclude that inactivation involved or was rapidly followed by degradation in vivo. Native OTCase was not degraded in crude extracts or when purified ISP and OTCase were incubated together under a variety of conditions. Synthesis of OTCase was not shut off normally in the ISP-deficient mutant. When the effects of continued synthesis were minimized, OTCase was degraded only slightly slower in the mutant than in its parent. Thus, the mutant had unanticipated pleiotropic characteristics, and it was unlikely that ISP played a major role in the degradation of OTCase in vivo.

A temporally regulated promoter from Bacillus subtilis is transcribed only by an RNA polymerase with a 37,000 dalton sigma factor

A 1,250 base pair Bacillus subtilis chromosomal HindIII restriction fragment (S fragment) has been cloned into the B. subtilis expression-probe plasmid pGR71. The S fragment induces the expression of the pGR71 chloramphenicol resistance gene shortly after the initiation of sporulation. The transcriptional promoter responsible for the expression of this temporally regulated genetic element has been identified and mapped in vitro. This promoter is recognized exclusively by the minor B. subtilis RNA polymerase that contains the 37,000 dalton sigma factor.

Bacillopeptidase F: two forms of a glycoprotein serine protease from Bacillus subtilis 168

Bacillopeptidase F is a serine endopeptidase excreted by Bacillus subtilis 168 after the end of exponential growth. As a step toward discovering a physiological function for this protease, an enzymological and immunological study was undertaken. When bacillopeptidase F was purified at pH 10, a number of enzymically active, rapidly moving electrophoretic forms were observed, as had been previously reported. Rabbit antiserum was prepared against one form. When the enzyme was purified at pH 6.0 in the presence of the covalent inhibitor phenylmethylsulfonyl fluoride, using the rabbit antiserum to detect the bacillopeptidase F protein, no fast-moving electrophoretic forms were observed. Instead, only two forms of the enzyme were isolated. One form had a molecular weight of 33,000, and the other had a molecular weight of 50,000, as determined by equilibrium sedimentation methods. Both forms appeared to be glycoproteins, both contained compounds, released on acid hydrolysis, which cochromatographed with phosphoserine and galactosamine, and the two gave identical immunoprecipitin lines in Ouchterlony double-diffusion tests. The smaller form had a pI of 4.4, whereas the larger had a pI of 5.4. The data suggest that bacillopeptidase F is distinct from all other proteases of B. subtilis.

Characterization of Bacillus subtilis mutants with a temperature-sensitive intracellular protease

A colony screening procedure was devised to detect Bacillus subtilis mutants containing temperature-sensitive trypsin-like intracellular protease activity. The enzyme was characterized as a non-sulfhydryl serine protease on the basis of inhibitor studies. It was also inhibited by D- or L-histidine but not by any other amino acid tested. The long-term survival at 45 degrees C of these mutants in a minimal salts medium was decreased, with rapid lysis occurring within 24 h. A D-histidine function in long-term survival and inhibition accounted for the presence of additional protease mutants among survivors of histidine auxotrophs selected for their ability to utilize D-histidine. In addition to being lysed when incubated at 45 degrees C under nongrowth conditions, all of the protease mutants had a decreased rate of protein turnover and produced spores deficient in a major low-molecular-weight spore coat polypeptide. The morphology of the undercoat layers was altered, but there was no effect on spore heat resistance or on germination. The missing spore coat polypeptide appeared to be processed from a larger precursor by cleavage to produce N-terminal histidine. A defect in this protease could account for the lack of processing and thus the absence of this polypeptide in spore coats.

A decrease in S-adenosylmethionine synthetase activity increases the probability of spontaneous sporulation

Starting with a relaxed (relA) strain, mutants with reduced activity of adenosine triphosphate:L-methionine S-adenosyl transferase (EC 2.5.1.6; SAM synthetase) were isolated in Bacillus subtilis. One such mutant (gene symbol metE1) had only 3% of the normal SAM synthetase activity but grew almost as well as the parent strain. Another mutant was isolated (gene symbol spdC1) as being able to sporulate continually at a high frequency; it had one-half the normal SAM synthetase activity at 33 degrees C. Both mutants continually and spontaneously entered spore development at a higher frequency than the parent strain in a medium containing excess glucose, ammonium ions, and phosphate. Sporulation was prevented by a high concentration of SAM (1 mM or more) or by the combination of adenosine and methionine (0.5 mM or more each), both of which are precursors of SAM. In contrast to this continual increase in the spore titer, addition of decoyinine, an inhibitor of GMP synthetase, rapidly initiated massive sporulation. Various amino acid analogs also induced sporulation in the relA strain, the methionine analogs ethionine and selenomethionine being most effective.

Germination properties of a spore coat-defective mutant of Bacillus subtilis

The presence of the gerE36 mutation in strains of Bacillus subtilis 168 resulted in poor germination of their spores in a range of germinants, as measured by the fall in absorbance of spore suspensions. Although resistant to heat and organic solvents, spores were sensitive to lysozyme; electron microscopy revealed that their coat structure was incomplete. These spores responded to germinants by losing heat resistance and changing from phase bright to phase gray. The release of dipicolinic acid and the fall in absorbance of spore suspensions reached only 75 and 50% of wild-type levels, respectively, but followed the same time course as the loss of heat resistance. Although the germination response was incomplete, the concentration of L-alanine required to elicit it was the same for the mutant as for the wild type. The properties of mutant spores suggest that an intact spore coat is not required for the initial interaction between germinant and spore, but that the coat layers may contain molecules important in later stages of germination. In transduction with phage SPP1, the gerE36 mutation mapped between citF and ilvB and was 90% cotransduced with citF2. The gerE mutation identifies the location of a gene important for the progress of late stages of spore formation.

Enzyme changes during Bacillus subtilis sporulation caused by deprivation of guanine nucleotides

When sporulation is initiated by nutrient limitation, e.g., at the end of growth, certain biochemical processes occur in sequence. To determine which of these processes occur, even when the cells sporulate in the presence of a rapidly metabolizable carbon source, we induced sporulation of Bacillus subtilis by deprivation of guanine nucleotides, in a synthetic medium containing excess glucose, ammonium ions, and phosphate. The deprivation was produced either by decoyinine addition to a standard strain or by guanosin limitation of a guanine auxotroph. At 1 h after the onset of this deprivation, an extensive turnover of proteins began whose appearance was chloramphenicol sensitive. At least one enzyme (aspartate transcarbamylase) lost 70% of its activity within 15 min, indicating its rapid destruction. Whereas the magnitude of the above two changes was similar to that observed during sporulation at the end of growth in nutrient sporulation medium, protease (intracellular and extracellular) increased to less than one-tenth of the specific activity in nutrient sporulation medium, and alkaline phosphatase increased to less than one-half. However, glucose dehydrogenase, an enzyme made only in forespores, increased to the same specific activity under both conditions, presumably because the forespore compartment is protected from media (e.g., glucose) influences by the double membrane (two bilayers with opposite polarity).

Synthesis and inactivation of carbamyl phosphate synthetase isozymes of Bacillus subtilis during growth and sporulation

Pyrimidine-repressible carbamyl phosphate synthetase P was synthesized in parallel with aspartate transcarbamylase during growth of Bacillus subtilis on glucose-nutrient broth. Both enzymes were inactivated at the end of exponential growth, but at different rates and by different mechanisms. Unlike the inactivation of aspartate transcarbamylase, the inactivation of carbamyl phosphate synthetase P was not interrupted by deprivation for oxygen or in a tricarboxylic acid cycle mutant. The arginine-repressible isozyme carbamyl phosphate synthetase A was synthesized in parallel with ornithine transcarbamylase during the stationary phase under these growth conditions. Again, both enzymes were subsequently inactivated, but at different rates and by apparently different mechanisms. The inactivation of carbamyl phosphate synthetase A was not affected in a protease-deficient mutatn the inactivation of ornithine transcarbamylase was greatly slowed.

Characterization of a thermosensitive sporulation mutant of Bacillus subtilis affected in the structural gene of an intracellular protease

A thermosensitive sporulation mutant (ts-15) of Bacillus subtilis has been isolated. This mutant when grown at the restrictive temperature (42 degrees C) is unable to sporulate, shows no intracellular protease activity and no protein turnover. These three traits were recovered in two revertants (ts-15R1 and ts-15R2) and were also transmitted together by transformation into the wild type. Immunological studies have shown that when ts-15 is grown at 42 degrees C it synthesizes a 'cryptic' protein with apparently the same antigenic properties as the wild type or as ts-15 mutant grown at the permissive temperature (30 degrees C). The intracellular proteases from the wild type and from ts-15 grown at 30 degrees C and 42 degrees C were completely purified and their properties were studied with respect to their molecular weights, substrate specificity, inhibition pattern, heat inactivation and antigenicity. The molecular weight of the enzyme from the wild type or ts-15 grown at 30 degrees C was 64000--65000 in the absence of sodium dodecylsulfate and 31000--32000 in the presence of sodium dodecylsulfate. It was assumed therefore that the active enzyme is formed from two similar subunits. However, the intracellular protease from ts-15 grown at 42 degrees C showed the same molecular weight of 32000--34000 in the presence or in the absence of sodium dodecylsulfate. On the basis of this experiment and others described in the paper we concluded that the mutation in ts-15 is most likely a point mutation in a structural gene of an intracellular protease and results in an inability to assemble the two subunits into an active form.

On the appearance of Bacillus subtilis intracellular serine protease in the cell membrane and culture medium. Comparison of the enzyme and other Bacillus subtilis serine proteases

While about 80% of the cell-bound intracellular serine protease of Bacillus subtilis A-50 have been recovered in the soluble fraction upon disruption of cells, the rest of the enzyme was found to be associated with the membrane fraction. Soluble cytoplasmic intracellular serine protease, as well as membrane-bound serine protease liberated by non-ionic detergent treatment, have been isolated in a pure state and shown to be identical. The same protease might also be found extracellularly, due presumably to cell lysis or altered membrane permeability. Intracellular serine protease of Bacillus subtilis A-50 was clearly related to Bacillus subtilis serine proteases W1 and bacillopeptidase F described as extracellular enzymes.

Benzeneboronic acid selectively inhibits sporulation of Bacillis subtilis

m-Aminobenzeneboronic acid at levels of 0.2 mM in nutrient broth medium selectively inhibited sporulation without appreciably altering vegetative growth. Significant inhibitory effects were seen even when it was added as late as 6 h after the end of logarithmic growth. The pH changes associated with growth and sporulation of Bacillus subtilis in nutrient broth were not significantly altered by the inhibitor. When it was present in cultures of actively growing cells, its inhibitory effect could not be reversed by simple dilution. The compound caused extensive clumping, of cells, which appeared not to be related to the ability of boronates to esterify to diols.

Intracellular serine protease of Bacillus subtilis: sequence homology with extracellular subtilisins

Intracellular serine protease was isolated from stationary-grown Bacillus subtilis A-50 cells and purified to homogeneity. The molecular weight of the enzyme is 31,000 +/- 1,000, with an isoelectric point of 4.3. Its amino acid composition is characteristically enriched in glutamic acid content, differing from that of extra-cellular subtilisins. The enzyme is completely inhibited with phenylmethylsulfonyl fluoride and ethylenediaminetetraacetic acid. Intracellular protease possesses negligible activity towards bovine serum albumin and hemoglobin, but has 5- to 20-fold higher specific activity against p-nitroanilides of benzyloxycarbonyl tripeptides than subtilisin BPN'. Esterolytic activity of the enzyme is also higher than that of subtilisin BPN'. The enzyme is sequence homologous with secretory subtilisins throughout 50 determined NH2-terminal residues, indicating the presence of duplicated structural genes for serine proteases in the B. subtilis genome. The occurrence of two homologous genes in the cell might accelerate the evolution of serine protease not only by the loosening of selective constrainst, but also by creation of sequence variants by means of intragenic recombination. Three molecular forms of intracellular protease were found, two of them with NH2-terminal glutamic acid and one minor form, three residues longer, with asparagine as NH2 terminus. These data indicate the possible presence of an enzyme precursor proteolytically modified during cell growth.

Characterization and function of intracellular proteases in sporulating Bacillus cereus

Intracellular proteases from sporulating Bacillus cereus have been purified by ammonium sulfate fractionation, heat treatment and DEAE cellulose column chromatography. After the last purification step, two protease activities, with an activity ratio of about thirty to one are resolved. Both proteases are resistant to o-phenanthroline but sensitive to phenyl methyl sulfonyl fluoride. Their separation by polyacrylamide gel electrophoresis and DEAE cellulose column chromatography, their difference in heat sensitivity and a mutation affecting only the major intracellular protease (IP1) suggest that the two are products of distinct genes. An IP1 mutant previously shown to produce coat defective spores (4) also turnsover protein with a reduced rate during late sporulation stages. Correlated with the slower turnover rate in this mutant is the more rapid disappearance of IP1. A partial revertant of this mutant has a protein turnover rate intermediate between that of the original mutant and wild type. These correlations imply that IP1 has an important role in protein turnover during sporulation.

Intracellular proteolytic activity during sporulation of Bacillus megaterium

Intracellular proteolytic activity increased during incubation of the sporogenic strain of Bacillus megaterium KM in a sporulation medium together with excretion of an extracellular metalloprotease. The exocellular protease activity in a constant volume of the medium reached a 100-fold value with respect to the intracellular activity. Maximal values of the activity of both the extracellular and intracellular enzyme were reached after 3-5 h of incubation. After 7 h 20-50% cells formed refractile spores. The intracellular proteolytic system hydrolyzed denatured proteins in vitro at a rate up to 150 mug mg-1 h-1 and native proteins at a rate up to 70 mug mg-1 h-1. Degradation of proteins in vivo proceeded from the beginning of transfer to the sporulation medium at a constant rate of 40 mug mg-1 h-1 and the inactivation of beta-galactosidase at a rate of 70 mug mg-1 h-1. The intracellular proteolytic activity was inhibited to 65-88% by EDTA, to 23-76% by PMSF. Proteolysis of denatured proteins was inhibited both by EDTA and PMSF more pronouncedly than proteolysis of native proteins; 50-65% of the activity were localized in protoplasts. Another strain of Bacillus megaterium (J) characterized by a high (up to 90%) and synchronous sporulation activity was found to behave in a similar way, but the rate of protein turnover in this strain was almost twice as high. The asporogenic strain of Bacillus megaterium KM synthesized the exocellular protease in the sporulation medium, but its protein turnover was found to decrease substantially after 3-4 h. The intracellular proteolytic system of the sporogenic strain J and the asporogenic strain KM were also inhibited by EDTA and PMSF.

Genetic aspects of bacterial endospore formation

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Apparent inactivation of inosine 5'-monophosphate dehydrogenase in sporulating Bacillus subtilis Is an artifact of in vitro proteolysis

The intracellular levels of inosine 5'-monophosphate dehydrogenase were found to decline very slowly during stationary phase in Bacillus subtilis when adequate precautions were taken to prevent proteolysis after cell rupture.

Production and possible function of serine protease during sporulation of Bacillus subtilis

The production of extracellular protease during sporulation in Bacillus subtilis 168 was investigated. Two proteases are produced, an alkaline serine protease and a neutral metalloprotease. In vivo inhibition of the serine protease with phenylmethylsulfonylfluoride indicated that the metalloprotease was degraded by the serine protease during sporulation. The experiments with phenylmethylsulfonylfluoride also show that the serine protease is necessary for the sequential process of sporulation and that it is required continuously for the first 2 to 3 h of the 8-h process.

Criteria for categorizing early biochemical events occurring during sporulation of Bacillus subtilis

Two criteria are suggested for assessing the relevance of biochemical events occurring early in sporulation. The first is thymidine starvation, a condition known to inhibit sporulation. This also inhibits the production of metalloprotease, serine protease, and ribonuclease; alpha-amylase production, however, is unaffected. The second is the effect of a regulator mutation which increases the production of the proteases. In the mutant, ribonuclease is produced in correspondingly large quantities whereas alpha-amylase production is unaffected. We conclude that, whereas the serine protease is part of the main sequence of events leading to formation of the spore, the metalloprotease is a side effect, i.e., connected with the main sequence but not part of it. Ribonuclease could, on present evidence, be either in the main sequence or a side effect associated with it. Amylase, however, seems to be separately regulated and neither directly nor indirectly connected with the sporulation sequence.

Oxygen-dependent inactivation of glutamine phosphoribosylpyrophosphate amidotransferase in stationary-phase cultures of Bacillus subtilis

Glutamine phosphoribosylpyrophosphate amidotransferase (ATase) activity is rapidly inactivated in stationary-phase cells of Bacillus subtilis. The inactivation of APase requires both the cessation of rapid cell growth and the presence of oxygen. ATase is inactivated in two protease-deficient mutant strains at a rate similar to that seen in the wild type, and is stable in anaerobic cell-free extracts of the parent strain. These results suggest that the inactivation of ATase is not the result of general proteolysis. The inactivation of ATase in stationary-phase cultures can be inhibited by oxygen starvation. This oxygen requirement does not reflect a dependence on the generation of metabolic energy, but appears to be a direct requirement for molecular oxygen. ATase synthesis is repressed by the addition of adenosine, and is inactivated only after the cessation of exponential growth. Addition of chloramphenicol or rifampin to exponential- and stationary-phase cells does not inhibit ATase inactivation, suggesting that protein or ribonucleic acid synthesis is not required for inactivation. ATase is inactivated at the end of exponential growth in cells that have exhausted a required amino acid.