Intracellular proteolytic activity during sporulation ofBacillus megaterium

Intracellular proteolytic activity increased during incubation of the sporogenic strain ofBacillus 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 respeot 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 proteinsin vitro at a rate up to 150 μg mg_-1 h_-1 and native proteins at a rate up to 70 μg mg^-1 h^-1. Degradation of proteinsin vivo proceeded from the beginning of transfer to the sporulation medium at a constant rate of 40 μg mg^-1 h^-1 and the inactivation of beta-galactosidase at a rate of 70 μg 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 ofBacillus 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 ofBacillus megaterium KM synthesized the exocellular protease in the sporulation medium, but its protein turnover was found to decrease substantially after 3 – 4 h. The intraeellular proteolytic system of the sporogenic strain J and the asporogenic strain KM were also inhibited by EDTA and PMSF.     

Turnover of mucopeptide during the life cycle ofBacillus megaterium

Sporogenic and asporogenicBacillus megaterium strains, as well asBacillus cereus degraded the murein component of the cell wall labelled with¹⁴C-diaminopimelic acid to TCA-soluble compounds during growth. The rate of murein turnover was about 15% during one generation in all three cases. The addition of chloramphenicol instantaneously markedly decreased the degradation rate, whereas in the presence of penicillin the degradation proceeded at the beginning at a rate comparable with that in the control and decreased only after a certain time interval. The cell wall degradation was considerably or completely stopped during the stationary phase of growth. In sporogenic strains ofBacillus megaterium andBacillus cereus the release of mature spores was associated with a new wave of the wall degradation, during which the wall of the sporangial cell was completely digested to TCA-soluble fragments. Free spores contained practically no mucopeptide component (cortex or spore wall) originating from the wall of the vegetative cell. A possible existence of a stable fraction of the cell wall not subject to turnover was investigated by measuring the³H/¹⁴C ratio in cells labelled simultaneously with³H (or¹⁴C)-diaminopimelic acid and¹⁴C (or³H)-leucine. The ratio changed during five generations, remaining constant later. This indicates that a certain portion of murein could be stable. The murein degradation during growth was not associated with secretion or release of a significant quantity of autolytic enzymes into the medium. The wall was apparently attacked from the inside. On the other hand, the release of the spore was accompanied by an increasing autolytic activity in the medium. This latter activity reached values corresponding to 3–8 μg lysozyme/ml. The results published here were presented at the 2nd Harden Conference “Cell walls and cell membranes”, Wye, Kent, England in September 1970.     

Turnover of murein in cellular and filamentous populations ofBacillus megaterium

Bacillus megaterium grows in the form of filaments at temperatures above 45°C. The rate of turnover of the cell wall begins to decrease gradually under these conditions. At the same time sensitivity of the filamentous forms to lysozyme decreases. Filaments outgrown at 48°C retain the decreased rate of turnover of the cell wall for a certain time after transfer to 30°C, in spite of the fact that septa are formed and filaments are converted to cells. However, a population incubated longer than 2 h at 48°C often ceases to grow and the growth is not restored even after transfer to 30°C. Three clones of the asporogenic strainBacillus megaterium KM differing somewhat in their ability to form filaments at 35°C differ mutually also in the rate of turnover of the cell wall. However, the decreased rate of the turnover cannot be unambiguously correlated with the increased tendency to form filaments.     

Protein degradation during sporulation ofBacillus megaterium: Effect of actinomycin D

The first acceleration of protein degradation in cells ofBacillus megaterium was found at the stage 0–I of sporulation, the second one at the stage II–III, where the sporulation process became irreversible. These accelerations were reduced by actinomycin D inhibiting RNA and protein syntheses by more than 95%. In the presence of the antibiotic, only 8% of prelabeled proteins were degraded. Actinomycin D did not lower either the concentration of ATP or the proteolytic activity in the homogenate prepared from sporulating cells. This indicates that the inhibition of protein catabolism by actinomycin D was not owing to the absence of ATP or proteolytic enzymes. Actinomycin probably inhibited an unknown step preceding the proteolytic attack of the protein molecules during sporulation, because it had no significant effect on proteolysis during vegetative growth.     

Effect of actinomycin D on viability,sporulation and nucleotide pool ofBacillus megaterium

A transient 7-fold rise of ppGpp concentration, 2-3-fold increase of pppGpp concentration and 50 % drop of the concentration of GTP inBacillus megaterium cells immediately after their transfer to the sporulation medium were observed. Actinomycin D, in concentrations inhibiting RNA synthesis by 95%, blocked the rise of the (p)ppGpp pool and caused an instant several-fold increase of the GTP level. When the cells were exposed to actinomycin D in the sporulation medium for a 1-h period (time 0–1 h, 1–2 h or 2.20–3.20-h), they were able to form colonies on nutrient agar after being kept, in addition for 1–2 h in the sporulation medium free of the antibiotic. The ability of sporulation was, however, markedly limited. The share of cells that could sporulate increased when the irreversible sporulation phase was reached.     

Characterization of degradation products of the cell wall released during growth and sporulation ofBacillus megaterium

Asporogenic and sporogenic strains ofBacillus megaterium KM release during growth heterogeneous fragments of the cell wall into the medium the non-dialyzable fraction representing 50–90% by the total. During lysis of sporangia the non-dialyzable fraction represents only 30% of the soluble fraction of autolyzed walls. Gel filtration on Sephadex permits to separate the non-dialyzable fragments of the cell wall released during growth into two fractions contaning simultaneously peptidoglycan and phosphorus. The two fractions contain peptidoglycan components in the same ratio as in the cell wall. Only one peptidoglycan macromolecular fraction, smaller than the fractions released during growth, was detected by gel filtration in the material released during lysis of sporangia.     

Growth ofBacillus megaterium in phosphate-limited medium

Batch cultures of Bacillus megaterium grown in phosphate-limited media were compared with control cultures grown in phosphate-sufficient media. The effects of phosphate limitation on growth were determined by viable cells counts. Intracellular levels of protein, RNA, poly-3-hydroxybutyrate, carbohydrate and oxygen uptake were significantly affected by phosphate limitation. Electron micrographs of sectioned cells revealed differences in the structure; in particular the thick, rigid cell wall was absent from cells grown in phosphate-limited media, and such cells were larger, pleomorphic, and after 2 d were insensitive to lysozyme.     

Dual effect of amino acids on the development of intracellular proteolytic activity in the irreversible sporulation phase ofBacillus megaterium

Amino acids added to a population ofBacillus megaterium immediately after its transfer to a sporulation medium stimulated growth, delayed sporulation by 1 h, and delayed the development of intracellular cytoplasmic serine proteinase (ISP) activity. However, the ISP activity in late sporulation stages exceeded twice that of the control population. Amino acids supplemented at T₃, i.e., at the time when engulfed forespores were developing, caused a decrease of specific ISP activity. The course of the phenylmethane sulfonyl fluoride (PMSF)-resistant activity in the cytoplasm was not affected by amino acids. Intracellular degradation of proteins prelabeled at the end of the growth phase was decreased by amino acids during the reversible sporulation phase but was only slightly affected later.     

Characteristics of intracellular proteolytic activities ofBacillus megaterium

Intracellular proteolytic activities ofB. megaterium KM occur soluble in the cytoplasm and periplasm and insoluble in the membrane. Two proteolytic enzymes were found in the cytoplasmic fraction by gel filtration on Sephadex G 150 and by polyacrylamide gel electrophoresis. The first enzyme called C_I was stable, had a relative molecular mass ofM _r=105000 (M=105 kg/mol) and was inhibited by EDTA and PMSF, whereas the second, designated C_II, was labile and had a relative molecular mass ofM _r=46000 (M=46 kg/mol). Because of its lability it could not be characterized in detail. In the “periplasm” only a single proteolytic enzyme P (M _r=28000;M=28 kg/mol) inhibited by EDTA could be demonstrated. The extracellular enzyme exhibited similar properties. The membrane proteolytic activity was sensitive to PMSF and EDTA. The membrane enzymes have not yet been solubilized. In cells of the mutant KM 12 that does not produce the extracellular proteinase, only one type of proteinase, in all its properties identical with the cytoplasmic proteinase C_I, could be demonstrated.     

Lethal germination of spores ofBacillus megaterium 9885

Washed spore suspensions germinated promptly without prior heat shock in a basal germination solution containingl-leucine.Germination was inhibited by dipicolinic acid. The inhibition was reversed by eitherl-leucine or phosphate.Phosphate accelerated the rate and increased the extent of germination, which was accompanied by an uncommonly large fall in the optical density of the suspension, but phosphate also caused a massive lysis after germination. This was accompanied by a sudden shedding of the spore coats. The suspensions consisted of shrivelled, cellular walls and membranes attached to the empty spore coats.Lysis of the germinated cells was prevented by fairly high concentrations of Ca or Mg.During germination, exogenous Ca we used Ca⁴⁵ was absorbed by the cells. Both cells and sonically disrupted cellular particles firmly retained the calcium, and evidence suggested that much of the Ca was bound in the cytoplasmic membranes.The cations contained in plain agar enabled spores which germinated on tryptone soya agar plates to develop into colonies; in the corresponding broth medium these spores lysed upon germination.Hypertonic sucrose delayed but did not prevent lysis.     

Synthesis of exocellular proteins during the exponential and stationary phase of growth ofBacillus megaterium

Synthesis of exocellular metalloprotease and cellular and exocellular proteins in the sporogenic strainBacillus megaterium J-27 and asporogenic strain KM 1 was investigated. Both organisms excrete the enzyme into the medium during growth and during the stationary phase. In the asporogenic strain the excretion decreases at the end of the exponential phase. In the sporogenic strain it continues during the transition to the stationary phase at the original rate and proteolytic activity in the medium increases two to three times during 2 h after the end of the exponential phase. Both organisms synthesize relatively more exocellular proteins during the exponential phase than during the stationary phase. The proportion of exooellular protein synthesized during the exponential phase does not exceed 3 % of total proteins, during the stationary phase this proportion usually decreases to less than 1 %.     

Synthesis and deposition of spore coat proteins during sporulation of Bacillus megaterium

Rabbit (anti-spore coat protein) IgG was prepared by immunization with coat proteins extracted with sodium dodecyl sulfate and dithiothreitol from isolated spore coats of Bacillus megaterium ATCC 12872. Coat proteins were detected from 3 hr after the end of exponential growth (t3) in the mother cell cytoplasmic fraction by sandwich enzyme immunoassay using this antibody. The proteins in the forespore coat protein fraction increased from t3 and reached a plateau at t10. Immunoblot analysis for the coat proteins in sporulating cells revealed the sequential synthesis of various proteins in the mother cell cytoplasmic fraction and simultaneous deposition of the same proteins as in the forespore coat fraction. These results suggest that turnover of precursor proteins of the spore coat is very rapid if precursor proteins are produced and they are proteolytically processed to produce mature proteins. Specific antibody to the 48,000-dalton protein, which is a major protein, did not cross-react with any other major (36,000, 22,000, 19,500, and 17,500-dalton) proteins. Specific antibody to the 22,000-dalton protein did not cross-react with the 48,000, 36,000, 19,500, 17,500, and 16,000-dalton proteins, but did cross-react with the 44,000, 25,000, and 12,000-dalton proteins.     

Protease activity in cells ofBacillus megaterium during derepression

A proteolytic activity hydrolyzing denatured proteins of Bacillus megaterium labelled with 35S or 14C amino acids was detected in cells of the asporogenic strain of Bacillus megaterium. The substrate is hydrolyzed by the enzyme or enzymes at optimum pH around 7, their activity being almost completely inhibited by EDTA and o-phenanthroline. PMSF, the inhibitor of serine proteases, is slightly inhibitory. Gel filtration on a Sephadex column separated the protease activity to two or three fractions. The protease activity in cells with the repressed synthesis of protease corresponds to 5-20 mug of substrate degraded per hour by 1 mg of protein at 37 degrees C. It increases five to ten-fold during the derepression. When the intracellular protease activity increases the extracellular enzyme begins to be excreted into the medium. The intracellular protease activity rapidly decreases after the addition of chloramphenicol or of a mixture of amino acids to the derepressed culture. Half or even more of the protease activity is released from the cells during their conversion to protoplasts by means of lysozyme. This "periplasmic" activity remains mostly in the supernatant also after mesosomes have been centrifuged down from the periplasm. A portion of the activity bound in protoplasts sediments together with membrane fraction after their lysis.     

Observation of nuclear fusion in living cells ofBacillus megaterium

Summary Fusion of nuclear bodies was observed by phase contrast microscopy in living cells ofBacillus megaterium grown on nutrient agar with added sodium monofluoroacetate. The nuclear fusion is perhaps carried out in the stage of the resting nucleus. The resting nuclear bodies in adjoining cells are fused in about twenty minutes. A part of this work was published in the Japanese Journal of Genetics34, 88, 1959 (in Japanese).     

Functional half-life of the exocellular protease mRNA ofBacillus megaterium

Functional half-life of the exocellular protease mRNA was determined in exponentially growing and stationary cells of the asporogenic strain ofBacillus megaterium, KM and in the sporogenic strain ofB. megaterium 27 during sporulation. No reserve of the protease mRNA could be detected in the cells and the half-lives were determined to be 6–7 min in the exponential and stationary cells ofB. megaterium KM and 7.5 – 8.5 min inB. megaterium 27. The mean half-life of mRNA for cell proteins was determined to be 3.5–4.5 min. Thus, as compared with the mean half-life of mRNA for cell proteins that of mRNA for the exocellular protease is slightly longer.     

The growth of cells ofBacillus megaterium in a hypertonic phosphate buffer

The morphological changes in cells grown in a phosphate medium were described. The synthesis of certain macromolecules under these conditions was characterized quantitatively and simultaneous structural changes in the cells demonstrated. It was shown that structural alterations in the cell wall resulting in striking changes of the cell shape were not caused by an altered rate of synthesis of the mucopolymers.