Attachment of the main chain to the linkage unit in biosynthesis of teichoic acids

A continuum of anionic charge: structures and functions of D-alanyl-teichoic acids in gram-positive bacteria

Teichoic acids (TAs) are major wall and membrane components of most gram-positive bacteria. With few exceptions, they are polymers of glycerol-phosphate or ribitol-phosphate to which are attached glycosyl and D-alanyl ester residues. Wall TA is attached to peptidoglycan via a linkage unit, whereas lipoteichoic acid is attached to glycolipid intercalated in the membrane. Together with peptidoglycan, these polymers make up a polyanionic matrix that functions in (i) cation homeostasis; (ii) trafficking of ions, nutrients, proteins, and antibiotics; (iii) regulation of autolysins; and (iv) presentation of envelope proteins. The esterification of TAs with D-alanyl esters provides a means of modulating the net anionic charge, determining the cationic binding capacity, and displaying cations in the wall. This review addresses the structures and functions of D-alanyl-TAs, the D-alanylation system encoded by the dlt operon, and the roles of TAs in cell growth. The importance of dlt in the physiology of many organisms is illustrated by the variety of mutant phenotypes. In addition, advances in our understanding of D-alanyl ester function in virulence and host-mediated responses have been made possible through targeted mutagenesis of dlt. Studies of the mechanism of D-alanylation have identified two potential targets of antibacterial action and provided possible screening reactions for designing novel agents targeted to D-alanyl-TA synthesis.

Biosynthesis of linkage units for teichoic acids in gram-positive bacteria: distribution of related enzymes and their specificities for UDP-sugars and lipid-linked intermediates

The distribution and substrate specificities of enzymes involved in the formation of linkage units which contain N-acetylglucosamine (GlcNAc) and N-acetylmannosamine (ManNAc) or glucose and join teichoic acid chains to peptidoglycan were studied among membrane systems obtained from the following two groups of gram-positive bacteria: group A, including Bacillus subtilis, Bacillus licheniformis, Bacillus pumilus, Staphylococcus aureus, and Lactobacillus plantarum; group B, Bacillus coagulans. All the membrane preparations tested catalyzed the synthesis of N-acetylglucosaminyl pyrophosphorylpolyprenol (GlcNAc-PP-polyprenol). The enzymes transferring glycosyl residues to GlcNAc-PP-polyprenol were specific to either UDP-ManNAc (group A strains) or UDP-glucose (group B strains). In the synthesis of the disaccharide-bound lipids, GlcNAc-PP-dolichol could substitute for GlcNAc-PP-undecaprenol. ManNAc-GlcNAc-PP-undecaprenol, ManNAc-GlcNAc-PP-dolichol, Glc-GlcNAc-PP-undecaprenol, Glc-GlcNAc-PP-dolichol, and GlcNAc-GlcNAc-PP-undecaprenol were more or less efficiently converted to glycerol phosphate-containing lipid intermediates and polymers in the membrane systems of B. subtilis W23 and B. coagulans AHU 1366. However, GlcNAc-GlcNAc-PP-dolichol could not serve as an intermediate in either of these membrane systems. Further studies on the exchangeability of ManNAc-GlcNAc-PP-undecaprenol and Glc-GlcNAc-PP-undecaprenol revealed that in the membrane systems of S. aureus strains and other B. coagulans strains both disaccharide-inked lipids served almost equally as intermediates in the synthesis of polymers. In the membrane systems of other B. subtilis strains as well as B. licheniformis and B. pumilus strains, however, the replacement of ManNAc-GlcNAc-PP-undecaprenol by Glc-GlcNAc-PP-undecaprenol led to a great accumulation of (glycerol phosphate)-Glc-GlcNAc-PP-undecaprenol accompanied by a decrease in the formation of polymers.

The functions of autolysins in the growth and division of Bacillus subtilis

Some bacteria, such as streptococci, exhibit growth from discrete and well-defined zones. In Streptococcus faecalis, growth zones can be observed in the electron microscope, and the position of the zone can be used as a marker for cell cycle events. Growth of the cell surface of Bacillus subtilis appears to be by a much different mechanism from that of streptococci. Cell elongation takes place by the insertion at many sites in the cell cylinder of peptidoglycan components. The insertion occurs on the inner face of the wall, and upon cross linking, the new wall material becomes stress bearing and older wall is pushed to the surface. When old wall reaches the surface, it becomes susceptible to excision by autolysins, resulting in wall turnover; cell elongation, due to the stretching of the cross-linked peptidoglycan, therefore, accompanies turnover and does not require a specialized growth zone.

Structure and functions of linkage unit intermediates in the biosynthesis of ribitol teichoic acids in Staphylococcus aureus H and Bacillus subtilis W23

The stepwise formation and characterization of linkage unit intermediates and their functions in ribitol teichoic acid biosynthesis were studied with membranes obtained from Staphylococcus aureus H and Bacillus subtilis W23. The formation of labeled polymer from CDP-[14C]ribitol and CDP-glycerol in each membrane system was markedly stimulated by the addition of N-acetylmannosaminyl(beta 1----4)N-acetylglucosamine (ManNAc-GlcNAc) linked to pyrophosphorylyisoprenol. Whereas incubation of S. aureus membranes with CDP-glycerol and ManNAc-[14C]GlcNAc-PP-prenol led to synthesis of (glycerol phosphate) 1-3-ManNAc-[14C]GlcNAc-PP-prenol, incubation of B. subtilis membranes with the same substrates yielded (glycerol phosphate)1-2-ManNAc-[14C]GlcNAc-PP-prenol. In S. aureus membranes, (glycerol phosphate)2-ManNAc-[14C]GlcNAc-PP-prenol as well as (glycerol phosphate)3-ManNAc-[14C]GlcNAc-PP-prenol served as an acceptor for ribitol phosphate units, but (glycerol phosphate)-ManNAc-[14C]GlcNAc-PP-prenol did not. In B. subtilis W23 membranes, (glycerol phosphate)-ManNAc-[14C]GlcNAc-PP-prenol served as a better acceptor for ribitol phosphate units than (glycerol phosphate)2-ManNAc-[14C]GlcNAc-PP-prenol. In this membrane system (ribitol phosphate)-(glycerol phosphate)-ManNAc-[14C]GlcNAc-PP-prenol was formed from ManNAc-[14C]GlcNAc-PP-prenol, CDP-glycerol and CDP-ribitol. The results indicate that (glycerol phosphate)1-3-ManNAc-GlcNAc-PP-prenol and (glycerol phosphate)1-2-ManNac-GlcNAc-PP-prenol are involved in the pathway for the synthesis of wall ribitol teichoic acids in S. aureus H and B. subtilis W23 respectively.

Biosynthesis of wall teichoic acids in Staphylococcus aureus H, Micrococcus varians and Bacillus subtilis W23. Involvement of lipid intermediates containing the disaccharide N-acetylmannosaminyl N-acetylglucosamine

The precursors for linkage unit (LU) synthesis in Staphylococcus aureus H were UDP-GlcNAc, UDP-N-acetylmannosamine (ManNAc) and CDP-glycerol and synthesis was stimulated by ATP. Moraprenol-PP-GlcNAc-ManNAc-(glycerol phosphate)1-3 was formed from chemically synthesised moraprenol-PP-GlcNAc, UDP-ManNAc and CDP-glycerol in the presence of Triton X-100. LU intermediates formed under both conditions served as acceptors for ribitol phosphate residues, from CDP-ribitol, which comprise the main chain. The initial transfer of GlcNAc-1-phosphate from UDP-GlcNAc was very sensitive to tunicamycin whereas the subsequent transfer of ManNAc from UDP-ManNAc was not. Poly(GlcNAc-1-phosphate) and LU synthesis in Micrococcus varians, with endogenous lipid acceptor, UDP-GlcNAc and CDP-glycerol, was stimulated by UDP-ManNAc. Synthesis of LU on exogenous moraprenol-PP-GlcNAc, with Triton X-100, was dependent on UDP-ManNAc and CDP-glycerol and the intermediates formed served as substrates for polymer synthesis. Membranes from Bacillus subtilis W23 had much lower levels of LU synthesis, but UDP-ManNAc was again required for optimal synthesis in the presence of UDP-GlcNAc and CDP-glycerol. Conditions for LU synthesis on exogenous moraprenol-PP-GlcNAc were not found in this organism. LU synthesis on endogenous acceptor in the absence of UDP-ManNAc was explained by contamination of membranes with UDP-GlcNAc 2-epimerase. Under appropriate conditions, low levels of this enzyme were sufficient to convert UDP-GlcNAc into a mixture of UDP-Glc-NAc and UDP-ManNAc and account for LU synthesis. The results indicate the formation of prenol-PP-GlcNAc-ManNAc-(glycerol phosphate)1-3 which is involved in the synthesis of wall teichoic acids in S. aureus H, M. varians and B. subtilis W23 and their attachment to peptidoglycan.

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).

Lipopolymers, isoprenoids, and the assembly of the gram-positive cell wall

Several lines of evidence suggest that Gram-positive bacterial cell surface polymers are synthesized by stepwise addition of polymer subunits to an amphipathic acceptor. In the case of membrane-bound lipopolymers such as mannan and lipoteichoic acid, the finished product may be covalently linked to a lipid anchor. In the case of polymers that are transferred into preexisting cell wall, such as teichoic acid and peptidoglycan, two alternative fates might be possible: (1) transfer into wall with concomitant or later cleavage of the lipid anchor, with recycling of the lipid anchor or secretion of the lipid anchor into the growth medium, and (2) transfer into wall without cleavage of the lipid anchor, resulting in maintenance of the covalent relationship between lipid anchor and polymer chain. In the latter case, a close relationship should be established between the cell wall and the plasma membrane. A number of Gram-positive bacteria have been shown to be resistant to plasmolysis. Therefore, a model for the assembly of the Gram-positive cell wall is proposed which takes into account a role for lipopolymeric intermediates and which views the establishment of resistance to plasmolysis as the natural consequence of such a mechanism.

Elongation of teichuronic acid chains by a wall-membrane preparation from Micrococcus luteus

A wall-plus-membrane preparation from Micrococcus luteus catalyzes the incorporation of [14C]glucose from UDP-[14C]glucose, into two fractions of teichuronic acid, which is the cell wall polysaccharide consisting of alternating residues of glucose and N-acetylmannosaminuronic acid (ManNAcUA). Membrane-associated teichuronic acid was extracted from the wall-membrane fraction of reaction mixtures by sodium dodecyl sulfate. The synthesis of membrane-associated teichuronic acid required UDP-glucose, UDP-ManNAcUA, and UDP-N-acetylglucosamine and was inhibited by tunicamycin. Glucose incorporated into wall-bound teichuronic acid remained in wall fragments after extraction with sodium dodecyl sulfate, and its incorporation required UDP-glucose and UDP-ManNAcUA (but not UDP-N-acetylglucosamine) and was insensitive to tunicamycin. Radioactive material incorporated into wall-bound teichuronic acid could be released by treatment with mild acid or by digestion with lysozyme, indicating that the wall-bound teichuronic acid was covalently linked to peptidoglycan. There were about 600 pmol of wall-bound teichuronic acid acceptor sites for glucose per mg of protein as measured in incorporation reaction mixtures lacking UDP-ManNAcUA. In the presence of both UDP-glucose and UDP-ManNAcUA, elongation of teichuronic acid acceptor sites occurred, with the addition of six to eight disaccharide units to each acceptor site.

Teichoic and teichuronic acids: biosynthesis, assembly, and location

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