Enzymes synthesizing and hydrolyzing murein in Escherichia coli. Topographical distribution over the cell envelope

Envelopes from regions of the cell which in vivo show very little, if any, murein synthesis were isolated using the minicell-producing strain P678-54. Envelopes from minicells, representing in fact cell ends, were able to synthesize murein and to carry out transpeptidation in vitro; also all four murein hydrolase activities tested, carboxypeptidase, endopeptidase, amidase and transglycosylase, were found to be present. The specific activities of the murein synthesizing and degrading enzymes in envelopes derived from cell poles and from actively growing cells were similar. The topological distribution of murein-synthesizing enzymes and of murein hydrolases over the cell envelope is discussed.

Regulation of murein biosynthesis and septum formation in filamentous cells of Escherichia coli PAT 84

Both the beta-lactam antibiotic, cephalexin, and the deoxyribonucleic acid synthesis inhibitor, nalidixic acid, are known to inhibit cell division in Escherichia coli and induce the formation of filaments. The biosynthesis of murein was investigated in these filaments and compared with the murein synthesized by the normally dividing rods of E. coli PAT 84. Differences were found in the extent of peptide side-chain cross-linkage. Filamentous cells had higher extents of cross-linkages in their newly synthesized murein. Quantitative analyses of the D-alanine carboxypeptidase and transpeptidase reactions in the different cells revealed that the carboxypeptidase activity of the filamentous cells was partially inhibited. These results were similar to those previously found with filaments that were obtained after growth of the thermosensitive division mutant at its restrictive temperature. We conclude that the formation of new cell ends (septa) depends on the proper balance between the activities of the D-alanine carboxypeptidase that regulates the availability of precursor doners and the transpeptidase, which catalyzes cross-linking and attachment of newly synthesized murein.

Quantitative determination of N-acetylglucosamine residues at the non-reducing ends of peptidoglycan chains by enzymic attachment of [14C]-D-galactose

The ability of human milk galactosyltransferase to attach D-galactose residues quantitatively to the C-4 of N-acetylglucosamine moieties at the ends of oligosaccharides has been utilized for the specific labeling and quantitative determination of the chain length of the glycan moiety of the bacterial cell wall. The average polysaccharide chain length of the soluble, uncrosslinked peptidoglycan secreted by Micrococcus luteus cells on incubation with penicillin G was studied with this technique and found to be approximately 70 hexosamines long. Furthermore, the peptidoglycan chain length of Escherichia coli sacculi of different cell shapes and dimensions was determined both in rod-shaped cells and in filaments induced by temperature shift of a division mutant or by addition of cephalexin or nalidixic acid. The average chain length found in most of these sacculi was between 70 and 100 hexosamines long. Small spherical 'mini' cells had chain lengths similar to those of the isogenic rod-like cells.

Codon-dependent rearrangement of the three-dimensional structure of phenylalanine tRNA, exposing the T-psi-C-G sequence for binding to the 50S ribosomal subunit

Codon-anticodon interaction induces an allosteric rearrangement of the three-dimensional structure of Phe-tRNAPhe that exposes the T-psi-C-G sequence for binding to the C-G-A-A sequence of the 5S rRNA within the 50S ribosomal subunit. The conformational change in the tRNAPhe structure was followed by the binding of C-G-[3H]A-[3H]A to the T-psi-C-G sequence, as measured by equilibrium dialysis at 10 mM Mg2+. C-G-A-A (14 pmol) was bound to tRNAPhe in the complete system containing elongation factor Tu-GTP-Phe-tRNA-(uridylyl-3',5')7-uridine-30S ribosomes (100 pmol). At a Mg2+ concentration lower than 5 mM the rearrangement was dependent on elongation factor-Tu, whereas GTP could be replaced by guanylyl imidodiphosphonate. In the absence of elongation factor-Tu-GTP a sigmoidal C-G-A-A binding curve with respect to Mg2+ concentration was obtained, showing half-saturation at 6 mM Mg2+. To achieve the change in the tRNAPhe structure in the absence of 30S ribosomes, a twofold higher concentration of (uridylyl-3',5')7-uridine had to be used. A sigmoidal curve was obtained again when the Mg2+ dependence of the C-G-A-A binding was followed, with 12 pmol of C-G-A-A being bound to 200 pmol of Phe-tRNA. Since T-psi-C-G exposure should influence the binding of Phe-tRNA to 70S ribosomes, Phe-tRNA binding to 70S ribosomes was examined. In the "nonenzymatic" binding (i.e., no elongation factor-Tu-GTP) of Phe-tRNA a sigmoidal Mg2+ dependence was found, whereas the "enzymatic" binding (elongation factor-Tu-GTP present) showed a hyperbolic curve. With 30S ribosomes as controls, only hyperbolic binding curves were found. The Mg2+ dependence of AA-tRNA binding thus reflects the rearrangement of the tRNA structure.

Peptidoglycan biosynthesis in a thermosensitive division mutant of Escherichia coli

Peptidoglycan biosynthesis during a bacterial division cycle was investigated in the thermosensitive division mutant Escherichia coli PAT 84 Synchronous cell division of this organism was initiated by a shift down from restrictive to permissive growth temperature. Cells harvested at different times after a shift down of temperature served as representatives of the various stages during cell division. These cells were made permeable to peptidoglycan nucleotide-sugar precursors by pretreatment with ether and were found capable of catalyzing the polymerization of externally added precursors as well as the covalent attachment of the newly synthesized peptidoglycan strands to those preexisting in the cell wall. Differences were observed in the rats of peptidoglycan synthesis and in the extent of peptide side-chain corss-linkage at the various stages of division. Nonseptate filaments, formed at the restrictive temperature, incorporated significantly more peptidoglycan which was more cross-linked than in normally dividing cells grown at the permissive temperature. Quantitative analyses of the carboxypeptidase and transpeptidase reactions in cells at different stages of division were performed and the inhibitory effect of a number of beta-lactam antibiotics was investigated. Of special significance was the finding that low doses of penicillin or growth at restrictive temperature, which did not affect transpeptidation, partially inhibited the carboxypeptidase activity. This inhibition was paralleled by an increase in incorporation of newly synthesized peptidoglycan into the preexisting cell wall. We therefore propose that carboxypeptidase activity regulates the availability of peptidoglycan precursor(s) for attachment to the preexisting peptidoglycan by transpepidation.

Novel type of murein transglycosylase in Escherichia coli

The purification and properties of a novel type of murein transglycosylase from Escherichia coli are described. The purified enzyme appears as a single band on sodium dodecyl sulfate-polyacrylamide gels and has an apparent molecular weight of approximately 65,000 as estimated by gel filtration and gel electrophoresis. It degrades pure murein sacculi from E. coli almost completely into low-molecular-weight products. The two prominent muropeptide fragments in the digest are the disaccharide-tripeptide N-acetylglucosamine-N-acetylmuramic acid-L-alanine-D-iso-glutamic acid-meso-diaminopimelic acid and the corresponding disaccharide-tetrapeptide N-acetylglucosamine-N-acetylmuramic acid-L-alanine-D-iso-glutamic acid-meso-diaminopimelic acid-D-alanine. The unique feature of these compounds is that the disaccharide has no reducing end group and that the muramic acid residue possesses an internal 1 leads to 6 anhydro linkage. The new lytic enzyme is designated as a murein: murein transglycosylase. Its possible role in the rearrangement of murein during cell growth and division is discussed.

Sphere-rod morphogenesis of Escherichia coli

The morphogenetic capacity of E. coli was studied by converting the rod-shaped cells into spheres and then determining whether these spheres could revert to rods. The morphogenesis of cells was followed by immobilizing them in a viscous Methocel-containing medium. Two different types of spheres were prepared: cells which retained a mechanically intact sacculus, and osmotically sensitive sphaeroplasts lacking a sacculus. The sphaeroplasts were not able to revert to rods although they were able to synthesize a new sacculus. In contrast, spheres which had retained an intact sacculus were able to reshape themselves into rods. The were also able to form new ends at (or near) the sites of the ends on the original rods.

Morphogenetic aspects of murein structure and biosynthesis

The shape of Escherichia coli is fixed by the form of the sacculus. This sacculus is a macromolecule made up from the polymer murein. In an investigation of the possible factors determining the shape of the sacculus, we attempted to resolve between two fundamental alternatives. (i) Is the shape of the sacculus automatically fixed by its chemical composition? or (ii) does a special morphogenetic system exist which determines the shape of the sacculus? An analysis of sacculi from cells grown in poor and rich media and harvested at different stages of growth was made. Significant variations in the composition of murein were found, whereas the general shape of the cells remained unchanged. This finding stands opposed to the assumption of a strict correlation between chemistry and shape of the sacculus. The second alternative was investigated by attempting to change artificially the shape of the sacculus by modifying the form of the hypothetical morphogenetic system. Rod-shaped cells were converted into spherical spheroplasts which were subsequently allowed to reform a new spherical sacculus. In chemical composition this spherical sacculus was found to be indistinguishable from the rod-shaped sacculus. This finding is taken as evidence for the existence of a distinct morphogenetic apparatus in the cell wall whose form is reflected by the shape of the sacculus.

Zum Wirkungsmechanismus von Penicillin

A search for possible degradation products of murein, originating from the action of penicillin on growing cells of E. coli B has been made. 2.6-Diaminopimelic acid-containing oligopeptides were isolated from penicillin-induced spheroplasts; the chemical structure of one of these components is identical with that of the peptide side chain of the muropeptides of normal cells. Other peptides, possibly enzymatic breakdown products of the latter, were also identified. The spheroplasts further contained uridine diphosphate muramylpeptides, regarded as precursors of the murein.