Membrane synthesis in Bacillus subtilis. I. Isolation and properties of strains bearing mutations in glycerol metabolism

The role of cell-envelope synthesis for envelope growth and cytoplasmic density in Bacillus subtilis

All cells must increase their volumes in response to biomass growth to maintain intracellular mass density within physiologically permissive bounds. Here, we investigate the regulation of volume growth in the Gram-positive bacterium Bacillus subtilis. To increase volume, bacteria enzymatically expand their cell envelopes and insert new envelope material. First, we demonstrate that cell-volume growth is determined indirectly, by expanding their envelopes in proportion to mass growth, similarly to the Gram-negative Escherichia coli, despite their fundamentally different envelope structures. Next, we studied, which pathways might be responsible for robust surface-to-mass coupling: We found that both peptidoglycan synthesis and membrane synthesis are required for proper surface-to-mass coupling. However, surprisingly, neither pathway is solely rate-limiting, contrary to wide-spread belief, since envelope growth continues at a reduced rate upon complete inhibition of either process. To arrest cell-envelope growth completely, the simultaneous inhibition of both envelope-synthesis processes is required. Thus, we suggest that multiple envelope-synthesis pathways collectively confer an important aspect of volume regulation, the coordination between surface growth, and biomass growth.

Phosphatidylglycerol homeostasis in glycerol-phosphate auxotrophs of Staphylococcus aureus

Background

The balanced synthesis of membrane phospholipids, fatty acids and cell wall constituents is a vital facet of bacterial physiology, but there is little known about the biochemical control points that coordinate these activities in Gram-positive bacteria. In Escherichia coli, the glycerol-phosphate acyltransferase (PlsB) plays a key role in coordinating fatty acid and phospholipid synthesis, but pathogens like Staphylococcus aureus have a different acyltransferase (PlsY), and the headgroup of the major membrane phospholipid, phosphatidylglycerol (PtdGro), is used as a precursor for lipoteichoic acid synthesis.

Results

The PlsY acyltransferase in S. aureus was switched off by depriving strain PDJ28 (ΔgpsA) of the required glycerol supplement. Removal of glycerol from the growth medium led to the rapid cessation of phospholipid synthesis. However, the continued utilization of the headgroup caused a reduction in PtdGro coupled with the accumulation of CDP-diacylglycerol and phosphatidic acid. PtdGro was further decreased by its stimulated conversion to cardiolipin. Although acyl-acyl carrier protein (ACP) and malonyl-CoA accumulated, fatty acid synthesis continued at a reduced level leading to the intracellular accumulation of unusually long-chain free fatty acids.

Conclusions

The cessation of new phospholipid synthesis led to an imbalance in membrane compositional homeostasis. PtdGro biosynthesis was not coupled to headgroup turnover leading to the accumulation of pathway intermediates. The synthesis of cardiolipin significantly increased revealing a stress response to liberate glycerol-phosphate for PtdGro synthesis. Acyl-ACP accumulation correlated with a decrease in fatty acid synthesis; however, the coupling was not tight leading to the accumulation of intracellular fatty acids.

Spo0A links de novo fatty acid synthesis to sporulation and biofilm development in Bacillus subtilis

During sporulation in Bacillus subtilis, the committed-cell undergoes substantial membrane rearrangements to generate two cells of different sizes and fates: the mother cell and the forespore. Here, we demonstrate that the master transcription factor Spo0A reactivates lipid synthesis during development. Maximal Spo0A-dependent lipid synthesis occurs during the key stages of asymmetric division and forespore engulfment. Spo0A reactivates the accDA operon that encodes the carboxylase component of the acetyl-CoA carboxylase enzyme, which catalyses the first and rate-limiting step in de novo lipid biosynthesis, malonyl-CoA formation. The disruption of the Spo0A-binding box in the promoter region of accDA impairs its transcriptional reactivation and blocks lipid synthesis. The Spo0A-insensitive accDA(0A) cells were proficient in planktonic growth but defective in sporulation (σ(E) activation) and biofilm development (cell cluster formation and water repellency). Exogenous fatty acid supplementation to accDA(0A) cells overcomes their inability to synthesize lipids during development and restores sporulation and biofilm proficiencies. The transient exclusion of the lipid synthesis regulon from the forespore and the known compartmentalization of Spo0A and ACP in the mother cell suggest that de novo lipid synthesis is confined to the mother cell. The significance of the Spo0A-controlled de novo lipid synthesis during B. subtilis development is discussed.

Induction of Staphylococcus aureus Lactose Permease in the Absence of Glycerolipid Synthesis

Glycerol auxotrophs of S. aureus were isolated and shown to cease phospholipid synthesis immediately when deprived of glycerol. Second-step mutants with temperature-sensitive inducibility of the lac system were also isolated. When cells were induced by temperature shift to produce the products of the lac system in the absence of glycerol, the permease activity, relative to 6-phospho-beta-galactosidase activity, was between 30 and 50% that of glycerol-supplemented cultures. However, the phosphotransferase activity for beta-galactosides in isolated membranes was found to be normal when compared to the level of beta-galactosidase. This indicated that the permeation system was induced and integrated into the membrane, but did not function efficiently for transport. Readdition of glycerol in the presence of chloramphenicol resulted in a slow increase in efficiency of the transport activity. Glycerol deprivation after induction led to a small loss of permease efficiency.

Synthesis of sn-glycerol 3-phosphate, a key precursor of membrane lipids, in Bacillus subtilis

The Bacillus subtilis gpsA gene was cloned by complementation of an Escherichia coli gpsA strain auxotrophic for sn-glycerol 3-phosphate. The gene was sequenced and found to encode an NAD(P)H-dependent dihydroxyacetone phosphate reductase with a deduced molecular mass of 39.5 kDa. The deduced amino acid sequence showed strong conservation with that of the E. coli homolog and to other procaryotic and eucaryotic dihydroxyacetone phosphate reductases. The physical location of gpsA on the B. subtilis chromosome was at about 200 degrees. Disruption of the chromosomal gpsA gene yielded B. subtilis strains auxotrophic for glycerol, indicating that the gpsA gene product is responsible for synthesis of the sn-glycerol 3-phosphate required for phospholipid synthesis. We also found that transformation of the classical B. subtilis glycerol auxotrophs with a gpsA-containing genomic fragment yielded transformants that grew in the absence of glycerol. In agreement with prior work, our attempts to determine the reductase activity in B. subtilis extracts were unsuccessful. However, expression of the B. subtilis gpsA gene in E. coli gave reductase activity that was only slightly inhibited by sn-glycerol 3-phosphate. Since the E. coli GpsA dihydroxyacetone phosphate reductase is very sensitive to allosteric inhibition by sn-glycerol 3-phosphate, these results indicate that the B. subtilis gpsA-encoded reductase differs from that of E. coli. It seems that B. subtilis regulates sn-glycerol 3-phosphate synthesis at the level of gene expression rather than through the E. coli mechanism of strong allosteric inhibition of an enzyme produced in excess.

Regulation of the synthesis of unsaturated fatty acids by growth temperature in Bacillus subtilis

Bacillus subtilis synthesizes, almost exclusively, saturated fatty acids, when grown at 37 degrees C. When cultures were transferred from 37 degrees C to 20 degrees C, a chloramphenicol- and rifampicin-sensitive synthesis of a C-16 unsaturated fatty acid was observed. Synthesis of this compound reached a plateau after 5 h at 20 degrees C, reaching levels of 20% of the total fatty acid content. [14C]-labelled fatty acids attached as thioesters to acyl-carriers compounds, such as coenzyme A (CoA) or acyl-carrier protein (ACP) synthesized de novo by glycerol-requiring auxotrophs deprived of glycerol to arrest phospholipid synthesis, could not be desaturated at 20 degrees C. Desaturation of these fatty acids was readily observed when glycerol was restored to the cultures allowing resumption of transfer of acyl-moieties from acyl-thioesters to phospholipid. It was also observed that depletion of the pools of CoA and ACP by starvation of pantothenate auxotrophs had no effect on the observed synthesis of unsaturated fatty acid at 20 degrees C. The overall results indicate that synthesis of unsaturated fatty acids in B. subtilis is a cold-inducible process and that phospholipids are obligate intermediates in this fatty acid desaturation pathway.

Synthesis of peptidoglycan and teichoic acid in Bacillus subtilis: role of the electrochemical proton gradient

The effects of several ionophores and uncouplers on glycerol and N-acetylglucosamine incorporation by Bacillus subtilis 61360, a glycerol auxotroph, were tested at different pH values. In particular, the effect of valinomycin on the synthesis of teichoic acid and peptidoglycan was examined in more detail in both growing cells and in vitro biosynthetic systems. Valinomycin inhibited synthesis of wall teichoic acid and peptidoglycan in whole cells but not in the comparable in vitro systems. It did not inhibit formation of free lipid or lipoteichoic acid. The results were consistent with a role for the electrochemical proton gradient in maintaining full activity of cell wall synthetic enzymes in intact cells. Such an energy source would be required for a model in which rotation or reorientation of synthetic enzyme complexes is envisaged for the translocation of wall precursor molecules across the cytoplasmic membrane (Harrington and Baddiley, J. Bacteriol. 155:776-792, 1983).

Effect of cerulenin on macromolecule synthesis in chemoheterotrophically and photoheterotrophically grown Rhodopseudomonas sphaeroides

The antibiotic cerulenin causes the immediate cessation of phospholipid biosynthesis in both chemoheterotrophic and photoheterotrophic cultures of Rhodopseudomonas sphaeroides. Macromolecule biosynthesis in photoheterotrophic cells was unaffected by cerulenin for the first 2 h after antibiotic addition and then continued at a reduced rate for an additional 8 h. In contrast, macromolecule biosynthesis in chemoheterotrophic cells was severely affected by cerulenin within the first 2 h of treatment. Pulse-labeling of protein after cerulenin addition revealed that all subcellular fractions were equally affected by the action of cerulenin with chemoheterotrophic cell fractions more profoundly affected than those derived from photoheterotrophic cells. Protein insertion into the intracytoplasmic membrane of photoheterotrophic cells continued for up to 6 h after the onset of cerulenin treatment. Residual macromolecule synthesis was correlated with the presence of the photosynthetic membrane system under all conditions of growth.

Characterization of glycerol nonutilizing and protoperithecial mutants of Neurospora

Mutants defective in polyol metabolism and/or in protoperithecial development were selected in Neurospora tetrasperma, a species in which protoperithecial development occurs at nonpermissively high temperature if certain polyols are used in lieu of sucrose as carbon source. Mutants selected for nonutilization of one of the four polyols tested, glycerol, mannitol, sorbitol, or xylitol, were usually found to be nonutilizers of the other three polyols as well. Mutants blocked at various stages of protoperithecial development complemented pairwise to produce more advanced developmental stages, usually mature protoperithecia and, when of opposite mating type, mature perithecia. About one-third of the mutants manifested both polyol auxotrophy and defective protoperithecial development upon initial isolation, but protoperithecial defectiveness in such mutants usually showed erratic segregation in crosses and/or instability to repeated vegetative transfer, whereas polyol auxotrophy usually did not and was, therefore, studied further. Two glycerol nonutilizing strains were introgressed into N. crassa to facilitate genetic analysis. One, glp-4, lacked both inducible and constitutive glycerol kinase and mapped to linkage group VI, between ad-1 and rib-1; the other, glp-5, lacked glyceraldehyde kinase and mapped to linkage group I, proximal to ad-9. Another mutant, gly-u(234), has been reported by other investigators to lack inducible glycerol kinase but to map to linkage group I, distal to ad-9.

Morphology and anionic polymer content in the cell wall of a glycerol-requiring mutant of Bacillus subtilis

A glycerol-requiring mutant of Bacillus subtilis formed irregular spheres and showed disturbed septum formation, when subjected to growth limitation by the supply of glycerol. Under phosphate limitation the cells were also round and developed asymmetric septa. In magnesium-limited cultures the cells contained a thickened wall, as compared with that of the parent strain grown under the same conditions. Chemical analysis revealed the presence of teichoic acid as the major anionic polymer in the wall of the glycerol-, as well as the magnesium-limited cells of the glycerol-requiring B. subtilis mutant. Under phosphate limitation teichuronic acid was the only anionic polymer present in the wall. Thus, in this respect, there were no apparent differences between mutant organisms and the parent strain when grown under magnesium and phosphate limitation, respectively and the observed morphological deviations could not be correlated with an altered anionic polymer content of the wall.

Enzyme secretion in Escherichia coli: synthesis of alkaline phosphatase and acid hexose phosphatase in the absence of phospholipid synthesis

De novo synthesis of two periplasmic enzymes in Escherichia coli, alkaline phosphatase and acid hexose phosphatase, have been studied in the presence and absence of new phospholipid synthesis. Alkaline phosphatase synthesis was initiated by a temperature shift in a strain carrying a phoA amber mutation and a temperature-sensitive suppressor mutation; acid hexose phosphatase was studied after relief of catabolite repression. Glycerol auxotrophs (gpsA) were used to control phospholipid synthesis. Synthesis of both enzymes proceeded at a normal rate for 0.5 to 1.0 generation of growth, although it was then curtailed. It is concluded that secretion of these enzymes is not obligatorily coupled to new net phospholipid synthesis.

Isolation and characterization of a glycerol auxotroph of Rhodopseudomonas capsulata: effect of lipid synthesis on the synthesis of photosynthetic pigments

A glycerol auxotroph was isolated from Rhodopseudomonas capsulata for use as a system for studying membrane synthesis and function. When the mutant was deprived of glycerol, net phospholipid synthesis ceased immediately and a small amount of free fatty acids accumulated. A turnover of lipid occurred in both deprived and supplemented cultures. Deoxyribonucleic acid and protein synthesis continued for one doubling of cell massand then slowed down in deprived cells. Net ribonucleic acid synthesis slowed down more dramatically. Oxidative phosphorylation activity of membrane preparations from aerobically and semi-anaerobically grown cells appeared unaffected by glycerol deprivation, indicating that simultaneous lipid synthesis is not a requirement for new oxidative phosphorylating activity. In the absence of net phospholipid synthesis, bacteriochlorophyll and carotenoid syntheses were reduced to 30% of the activity of supplemented cultures. Delta-Aminolevulinic acid synthase, the first enzyme on the bacteriochlorophyll pathway that is subject to regulatory control, increased in activity in deprived cultures. Lascelles and Szilagyi (1965) showed an association between phospholipid synthesis and pigment production. They found an increased lipid content associated with pigmented cells. The present results indicate that not only is there an association between lipid and pigment synthesis, but also there is actually a dependence of bacteriochlorophyll synthesis on phospholipid synthesis.

Mesosomes: membranous bacterial organelles

Images

Inhibition of de novo fatty acid synthesis by the antibiotic cerulenin in Bacillus subtilis: effects on citrate-Mg2+ transport and synthesis of macromolecules

Inhibition of de novo fatty acid biosynthesis by the antibiotic cerulenin in Bacillus subtilis stopped de novo synthesis of neutral lipids and phospholipids. The bacteria ceased growing but remained completely viable. Addition of 12-methyltetradecanoic acid and palmitic acid to the culture medium of cerulenin-treated cells restored growth of the bacteria, albeit at a reduced rate. Although the de novo synthesis of all lipid components of the membrane was blocked, citrate-Mg(2+) transport activity remained inducible, and induced cells did not lose this transport activity when treated with cerulenin. Shortly after the addition of cerulenin, the rate of ribonucleic acid synthesis dropped rapidly and was followed by a slower decrease in the rate of protein synthesis. The rate of deoxyribonucleic acid synthesis remained almost unaffected. The rapid decrease of ribonucleic acid synthesis in cerulenin-treated cells might be due to the inhibition of de novo fatty acid biosynthesis or it might be due to a secondary effect of cerulenin in B. subtilis cells.

Synthesis of chloroplast membrane polypeptides during synchronous growth of Chlamydomonas reinhardtii

The synthesis of the major chloroplast membrane polypeptides has been studied during synchronous growth of Chlamydomonas reinhardtii. Under these conditions, chlorophyll is synthesized during the latter part of the light period and cell division takes place during the dark period. The profile of the chloroplast membrane polypeptides of C. reinhardtii has been well characterized and shown to contain two major classes by size (Hoober, J. 1970. J. Biol. Chem.245:4327). Polypeptides of group I have a mol wt range of 50,000-55,000 daltons. The second region consists of at least three polypeptide groups, IIa, IIb, and IIc, having mol wt of 40,000, 31,000, and 27,000 daltons, respectively. The synthesis of these polypeptides has been measured using a double-labeling technique and a computer-aided statistical analysis. The rate of labeling of group I polypeptides is highest during the early light period and decreases after 6 h of growth. Group IIa is labeled from the beginning of the light period, but little synthesis of IIb occurs before 3 h, and significant amounts of label are not found in IIc before 5 h of growth. After approximately 8 h of light, groups IIb and IIc are synthesized at rates significantly greater than those of the other membrane polypeptides. The synthesis of the major polypeptide groups ceases in the dark. We conclude that the biosynthesis of the chloroplast membranes is a sequential or stepwise process.

Mutants of Escherichia coli defective in membrane phospholipid synthesis: macromolecular synthesis in an sn-glycerol 3-phosphate acyltransferase Km mutant

sn-Glycerol 3-phosphate (G3P) auxotrophs of Escherichia coli have been selected from a strain which cannot aerobically catabolize G3P. The auxotrophy resulted from loss of the biosynthetic G3P dehydrogenase (EC 1.1.1.8) or from a defective membranous G3P acyltransferase. The apparent K(m) of the acyltransferase for G3P was 11- to 14-fold higher (from about 90 mum to 1,000 to 1,250 mum) in membrane preparations from the mutants than those of the parent. All extracts prepared from revertants of the G3P dehydrogenase mutants showed G3P dehydrogenase activity, but most contained less than 10% of the wild-type level. Membrane preparations from revertants of the acyltransferase mutants had apparent K(m)'s for G3P similar to that of the parent. Strains have been derived in which the G3P requirement can be satisfied with glycerol in the presence of glucose, presumably because the glycerol kinase was desensitized to inhibition by fructose 1,6-diphosphate. Investigations on the growth and macromolecular synthesis in a G3P acyltransferase K(m) mutant revealed that upon glycerol deprivation, net phospholipid synthesis stopped immediately; growth continued for about one doubling; net ribonucleic acid (RNA), deoxyribonucleic acid (DNA), and protein nearly doubled paralleling the growth curve; the rate of phospholipid synthesis assessed by labeling cells with (32)P-phosphate, (14)C-acetate, or (3)H-serine was reduced greater than 90%; the rates of RNA and DNA synthesis increased as the cells grew and then decreased as the cells stopped growing; the rate of protein synthesis showed no increase and declined more slowly than the rates of RNA and DNA synthesis when the cells stopped growing. The cells retained and gained in the capacity to synthesize phospholipids upon glycerol deprivation. These data indicate that net phospholipid synthesis is not required for continued macromolecular synthesis for about one doubling, and that the rates of these processes are not coupled during this time period.

Bacteriophage T4-mediated release of envelope components from Escherichia coli

When Escherichia coli B, labeled by prior growth in (14)C-glucose, are infected with T4 phage there is a rapid release of (14)C-nondialyzable material into the medium. About half of this material is derived from the cell envelope as evidenced by its content of phospholipid and lipopolysaccharide and its buoyant density upon isopycnic ultracentrifugation of 1.19 g/cm(3). It is similar in its gross chemical and physical properties to envelope material released at a lower rate from growing uninfected cells or from cells whose protein synthesis is inhibited by chloramphenicol (22). The rate of release of this envelope material at a multiplicity of infection (MOI) of 10 is greatest in the first minute after infection, and release is completed by 4 min. The rate of its release, as a function of MOI at 2 min after infection, is greatest at low MOI (e.g., MOI 2 and 4); in addition, the release does not continue above MOI 30. The main conclusion derived from the data is that phage, as part of the process of adsorption and injection of DNA, cause an increased release of envelope substance from the cells. With the assumption that all of the envelope material released is derived from the outer envelope, it is estimated that uninfected cells release 20 to 30% of their outer envelope per hour, whereas infected cells release 30% in 2 min at MOI 30. Further, because release does not continue at high MOI, this phenomenon is not considered to be a direct cause of lysis from without. Data are also presented on the amounts of other non-dialyzable (14)C-components released and on the differences in the kinetics of release from chloramphenicol-treated cells compared to phage-infected cells. To avoid the possibility that the release is due to phage lysozyme which is an adventitious "contaminant" of wild-type phage, a phage mutant (T4BeG59s) devoid of this enzyme was used in these experiments.

Immunological properties of teichoic acids

Images

Inhibition of lipid synthesis in Escherichia coli cells by the antibiotic cerulenin

The antibiotic cerulenin markedly inhibits the growth of Escherichia coli. The effects of the antibiotic on cellular syntheses were studied by measuring the incorporation of labeled precursors into lipids and macromolecules. During the first 40 min after the addition of cerulenin to a culture of growing cells, lipid synthesis was inhibited more than 90% and ribonucleic acid and deoxyribonucleic acid synthesis about 25%, whereas protein synthesis was not affected. At later periods after cerulenin addition (1 to 2 h), the inhibition of cell growth and of lipid and protein synthesis was complete. Upon removal of cerulenin from the culture, growth was restored and lipid synthesis resumed more rapidly than did the synthesis of protein. Addition of both palmitate and oleate, but not of either fatty acid alone, reversed the inhibition of growth by cerulenin. These findings support the conclusion that the antibiotic effects of cerulenin are due to a specific inhibition of fatty acid synthesis.

Abnormal septation and inhibition of sporulation by accumulation of L- -glycerophosphate in Bacillus subtilis mutants

Accumulation of l-alpha-glycerophosphate, in cells of Bacillus subtilis mutants lacking the nicotinamide adenine dinucleotide-independent glycerophosphate dehydrogenase activity, suppresses both growth and sporulation. After growth has stopped, the cells slowly develop one and later more asymmetric septa that are thicker than normal prespore septa and apparently contain too much cell wall material to allow further membrane development into forespores or spores. l-Malate prevents accumulation of glycerophosphate and restores sporulation of the mutant. Glucose or gluconate cannot resotre sporulation, because they still effect glycerophosphate accumulation via de novo synthesis. If that accumulation is blocked in a double mutant, which is unable to make glycerophosphate from or to metabolize it into Embden-Meyerhof compounds, then nonsuppressing amounts of glucose or gluconate can restore sporulation.

Regulation of macromolecular biosynthesis in a mutant of Escherichia coli defective in membrane phospholipid biosynthesis

Nucleic acid and protein synthesis were studied in temperature-sensitive mutants defective in phospholipid synthesis. The defect is due to a single mutation in glycerol 3-phosphate acyltransferase (EC 2.3.1.15). The results show that at the restrictive temperature not only does phospholipid synthesis cease, but DNA, RNA, and protein synthesis also cease. Active transport continues, however, indicating that the cells do not become leaky or lose their energy supply. These results suggest that phospholipid synthesis is coupled to DNA, RNA, and protein synthesis.

Consequences of glycerol deprivation on the synthesis of membrane components in a glycerol auxotroph of Staphylococcus aureus

In a glycerol auxotroph of Staphylococcus aureus, the deprivation of glycerol affected the formation of certain membrane components. (i) There was synthesis of fatty acids at the predeprivation rate even though the fatty acids synthesized accumulated as free fatty acids rather than as esterified fatty acids; (ii) there was a complete cessation of phospholipid and vitamin K isoprenologue biosynthesis; (iii) there was conservation of the glycerol esters of the complex phospholipids and glucolipids; (iv) there was an immediate decrease in the rate of synthesis of monoglucoslydiglyceride (30%) and diglucosyldiglyceride (60%); (v) there was a 50% decrease in the rate of synthesis of the polar and nonpolar carotenoids; (vi) there was synthesis of protoheme, heme a, and nonspecific membrane protein at the predeprivation rate; and (vii) there was an abrupt cessation in the formation of new, functional glycine transport activity.

An acceptor-dependent polyglycerolphosphate polymerase

A method for the extraction of polyglycerolphosphate polymerase from Bacillus subtilis membranes is described. Further purification by ion-exchange chromatography yields an enzyme fraction totally dependent on the addition of a heat-stable factor for activity. This factor was isolated from cell membranes and acts as an acceptor of glycerolphosphate units. The acceptor contains glycerolphosphate, glucosamine, and fatty acids, but its precise chemical structure has not yet been elucidated.

An effect of hydroxyurea on Staphylococcus epidermidis: temporary loss of viability

Although hydroxyurea (HU) is recognized as a specific inhibitor of deoxyribonucleic acid (DNA) synthesis, it did not have this effect in a strain of Staphylococcus epidermidis. In this case, HU caused a loss of colony-forming ability but did not prevent cell division in liquid medium. DNA synthesis apparently was affected only secondarily. About 10% of the population recovered colony-forming ability during the latter part of the growth cycle or when growth was slowed by chloramphenicol. Recovery also occurred when HU was removed from the medium. HU prevented cellular autolysis.

Control of fatty acid synthesis in bacteria

When glycerol-requiring auxotrophs of Bacillus subtilis are deprived of glycerol, the synthesis of fatty acids continues at an apparent rate of 20 to 50% that of supplemented cultures. The newly synthesized fatty acids are not incorporated into phospholipid and accumulate as free fatty acids. These molecules undergo a much more rapid turnover than phospholipid fatty acids, and the rate of turnover is sufficient to indicate that the rate of fatty acid synthesis in glycerol-deprived cultures is similar to that in supplemented ones. The average chain length of the free fatty acids is greater than that of the phospholipid fatty acids. Cells deprived of required amino acids also show a diminution in the apparent rate of fatty acid synthesis; however, in this case, the fatty acids accumulate in phospholipid, and no increase of the free fatty acid fraction is observed. It is argued on the basis of these findings that the control of lipid synthesis does not operate at the level of transacylation but must act on one or more of the reactions of the fatty acid synthetase.

Osmotic-sensitive mutant of Salmonella typhimurium

A strain (DA82) having peculiar osmotic properties was isolated in Salmonella typhimurium. The mutant shows increased elasticity of its cell wall and makes spherical instead of elongated cells, regardless of the osmolality of the medium. The strain withstands dilution in distilled water without disruption or death and grows normally in 0.1 molal NaCl broth (240 milliosmol), but it dies exponentially in low-osmolality broth (40 milliosmol). Addition of salts or sucrose instantly stops death and allows growth and cell division to proceed. Death is not due to lysis because this appears at later times and at a much lower rate. Osmotic inactivation is temperature-dependent: higher death rates occur at higher incubation temperatures. Inhibition of protein synthesis by chloramphenicol (20 mug/ml) prevents osmotic death. At 37 C and at lower temperatures, the phenomenon of osmotic death is transient. After a variable interval, growth of the osmotic-sensitive strain resumes. It is assumed that the strain's osmotic behavior is due to membrane defectiveness. The membrane disfunction and the wall defect shown by the strain may be consequences of a single genetic alteration or the results of independent mutations.

The fluid mosaic model of the structure of cell membranes

A fluid mosaic model is presented for the gross organization and structure of the proteins and lipids of biological membranes. The model is consistent with the restrictions imposed by thermodynamics. In this model, the proteins that are integral to the membrane are a heterogeneous set of globular molecules, each arranged in an amphipathic structure, that is, with the ionic and highly polar groups protruding from the membrane into the aqueous phase, and the nonpolar groups largely buried in the hydrophobic interior of the membrane. These globular molecules are partially embedded in a matrix of phospholipid. The bulk of the phospholipid is organized as a discontinuous, fluid bilayer, although a small fraction of the lipid may interact specifically with the membrane proteins. The fluid mosaic structure is therefore formally analogous to a two-dimensional oriented solution of integral proteins (or lipoproteins) in the viscous phospholipid bilayer solvent. Recent experiments with a wide variety of techniqes and several different membrane systems are described, all of which abet consistent with, and add much detail to, the fluid mosaic model. It therefore seems appropriate to suggest possible mechanisms for various membrane functions and membrane-mediated phenomena in the light of the model. As examples, experimentally testable mechanisms are suggested for cell surface changes in malignant transformation, and for cooperative effects exhibited in the interactions of membranes with some specific ligands. Note added in proof: Since this article was written, we have obtained electron microscopic evidence (69) that the concanavalin A binding sites on the membranes of SV40 virus-transformed mouse fibroblasts (3T3 cells) are more clustered than the sites on the membranes of normal cells, as predicted by the hypothesis represented in Fig. 7B. T-here has also appeared a study by Taylor et al. (70) showing the remarkable effects produced on lymphocytes by the addition of antibodies directed to their surface immunoglobulin molecules. The antibodies induce a redistribution and pinocytosis of these surface immunoglobulins, so that within about 30 minutes at 37 degrees C the surface immunoglobulins are completely swept out of the membrane. These effects do not occur, however, if the bivalent antibodies are replaced by their univalent Fab fragments or if the antibody experiments are carried out at 0 degrees C instead of 37 degrees C. These and related results strongly indicate that the bivalent antibodies produce an aggregation of the surface immunoglobulin molecules in the plane of the membrane, which can occur only if the immunoglobulin molecules are free to diffuse in the membrane. This aggregation then appears to trigger off the pinocytosis of the membrane components by some unknown mechanism. Such membrane transformations may be of crucial importance in the induction of an antibody response to an antigen, as well as iv other processes of cell differentiation.

Effect of glycerol deprivation on the phospholipid metabolism of a glycerol auxotroph of Staphylococcus aureus

A study of the effects of glycerol deprivation on the content and metabolism of the phospholipids of a glycerol auxotroph of Staphylococcus aureus showed that (i) there was an increase in the proportions of lysylphosphatidylglycerol (LPB) and a concomitant decrease in the proportion of phosphatidylglycerol. The total phospholipid content per sample and the proportion of cardiolipin did not change, but the phosphatidic acid increased transiently and then fell to pretreatment levels. (ii) The loss of (32)P from the lipids during the chase in a pulse-chase experiment was essentially the same in phosphatidylglycerol, cardiolipin, and phosphatidic acid during glycerol deprivation or growth in the presence of glycerol. LPG lost half the radioactivity in slightly more than two doubling times when grown with glycerol. In the absence of glycerol, (32)P accumulated in LPG for about 20 min and then stopped, after which time there was no apparent turnover. (iii) During glycerol deprivation, the initial (32)P incorporation decreased sixfold compared to that of the control with glycerol. The initial incorporation into LPG decreased only 2.5-fold, whereas that of PG decreased 45-fold. (iv) During glycerol deprivation, the free fatty acid content increased from 1.2 to 12.5% of the total extractable fatty acids and then slowly decreased. The increase was largely iso- and anti-iso-branched 21-carbon-atom fatty acids. In glycerol-supplemented cultures, the major fatty acids were branched 14- to 18-carbon fatty acids. The decrease in longer chain free fatty acids after 60 min represented their esterification into lipids. (v) During glycerol deprivation ribonucleic acid synthesis and cell growth continued for 40 min and protein synthesis continued for 90 min. Then synthesis and growth stopped. (vi) After the addition of glycerol to glycerol-deprived cells, (32)P and (14)C-glycerol were incorporated into the phospholipids without lag; ribonucleic acid, protein synthesis, and cell growth began after a 5- to 10-min lag at the pretreatment rate. The initial rate of lipid synthesis after the addition of glycerol was three times greater than the growth rate. This rapid rate continued for about 25 min until the lipid content and proportions of LPG and phosphatidylglycerol were restored.

Temperature-sensitive mutant of Bacillus subtilis that accumulates membrane-associated protein inclusions

At 47 C, a temperature-sensitive mutant of Bacillus subtilis 168 accumulates membrane-associated protein inclusions and exhibits a pleiotropic phenotype indicative of a defect in lipid synthesis. The mutant bacteria cease growing at 47 C, and the turbidity of the culture gradually declines. The lack of growth is not due to the death or lysis of the cells, since viability does not decrease for about 1 hr and the "lysis" can be delayed for several hours by increasing the osmotic pressure of the medium. Synthesis of deoxyribonucleic acid and ribonucleic acid stops at 47 C although a residual synthesis of protein occurs. When the temperature is raised, the mutant fails to increase the proportion of 17:0 branched-chain fatty acids and to decrease the proportion of 18:0 and 18:1 fatty acids. The membrane-associated inclusions can be seen by phase-contrast or electron microscopy and remain attached to protoplast membranes during isolation. The inclusions are mostly protein and are digested with Pronase.

Mode of cell wall growth of Bacillus megaterium

The growth of the cell wall of Bacillus megaterium KM was examined by labeling the cell wall uniformly with (3)H-alpha, epsilon-diaminopimelic acid and examining by radioautography the distribution of labeled cell wall material in daughter cells. The results indicate that the label is distributed uniformly to all daughter cells and rule out a single growth point, either central or apical, for these cells. The results strongly suggest that cell wall growth occurs by uniform deposition of new wall over the whole cell surface.

On the process of cellular division in Escherichia coli. VI. Use of a methocel-autoradiographic method for the study of cellular division in Escherichia coli

A technique allowing chain formation by Escherichia coli for the analysis of the segregation of certain cellular components by autoradiographic methods has been developed. Deoxyribonucleic acid labeled with (3)H-thymidine does not appear to segregate preferentially with one or another of the bacteria. Membrane lipids labeled with either (3)H-oleate or (3)H-glycerol are evenly distributed among all progeny cells. A similar pattern of distribution of cell wall material is observed when (3)H-diaminopimelate is used as the marker.

Phospholipid metabolism in the absence of net phospholipid synthesis in a glycerol-requiring mutant of Bacillus subtilis

A glycerol-requiring auxotroph of Bacillus subtilis showed no net synthesis of phospholipid when deprived of glycerol. Although there was no net synthesis of phospholipid, we found that: (i) fatty acids and (32)P were slowly incorporated into phospholipid; (ii) in pulse-chase experiments, both (32)P and (14)C in the glycerol portion of the phospholipids were lost from phosphatidlyglycerol (PG) and lysylphosphatidylglycerol and accumulated in cardiolipin (CL); (iii) the proportions of the phospholipids in the membrane changed with a loss of PG and an accumulation of CL. The addition of glycerol to the glycerol-deprived cells resulted in a rapid incorporation of glycerol and restoration to the predeprivation metabolism and PG to CL ratio.

Induction of citrate transport in Bacillus subtilis during the absence of phospholipid synthesis

Citrate transport can be induced in a glycerol-requiring mutant of Bacillus subtilis even after deprival of glycerol. Under these conditions de novo phospholipid synthesis is stopped although ribonucleic acid, deoxyribonucleic acid, and protein syntheses continue. It is concluded that the inducibility of bacterial transport systems must not necessarily require de novo phospholipid synthesis.