Biosynthetic elongation of isolated teichuronic acid polymers via glucosyl- and N-acetylmannosaminuronosyltransferases from solubilized cytoplasmic membrane fragments of Micrococcus luteus

Membrane Translocation and Assembly of Sugar Polymer Precursors

Bacterial polysaccharides play an essential role in cell viability, virulence, and evasion of host defenses. Although the polysaccharides themselves are highly diverse, the pathways by which bacteria synthesize these essential polymers are conserved in both Gram-negative and Gram-positive organisms. By utilizing a lipid linker, a series of glycosyltransferases and integral membrane proteins act in concert to synthesize capsular polysaccharide, teichoic acid, and teichuronic acid. The pathways used to produce these molecules are the Wzx/Wzy-dependent, the ABC-transporter-dependent, and the synthase-dependent pathways. This chapter will cover the initiation, synthesis of the various polysaccharides on the cytoplasmic face of the membrane using nucleotide sugar precursors, and export of the nascent chain from the cytoplasm to the extracellular milieu. As microbial glycobiology is an emerging field in Gram-positive bacteria research, parallels will be drawn to the more widely studied polysaccharide biosynthesis systems in Gram-negative species in order to provide greater understanding of these biologically significant molecules.

The Cell Wall Teichuronic Acid Synthetase (TUAS) Is an Enzyme Complex Located in the Cytoplasmic Membrane of Micrococcus luteus

The cell wall teichuronic acid (TUA) of Micrococcus luteus is a long-chain polysaccharide composed of disaccharide repeating units [-4-β-D-ManNAcAp-(1→6)α-D-Glcp−1-]_n, which is covalently anchored to the peptidoglycan on the inner cell wall and extended to the outer surface of the cell envelope. An enzyme complex responsible for the TUA chain biosynthesis was purified and characterized. The 440 kDa enzyme complex, named teichuronic acid synthetase (TUAS), is an octomer composed of two kinds of glycosyltransferases, Glucosyltransferase, and ManNAcA-transferase, which is capable of catalyzing the transfer of disaccharide glycosyl residues containing both glucose and the N-acetylmannosaminuronic acid residues. TUAS displays hydrophobic properties and is found primarily associated with the cytoplasmic membrane. The purified TUAS contains carotinoids and lipids. TUAS activity is diminished by phospholipase digestion. We propose that TUAS serves as a multitasking polysaccharide assembling station on the bacterial membrane.

Genome sequence of the Fleming strain of Micrococcus luteus, a simple free-living actinobacterium

Micrococcus luteus (NCTC2665, "Fleming strain") has one of the smallest genomes of free-living actinobacteria sequenced to date, comprising a single circular chromosome of 2,501,097 bp (G+C content, 73%) predicted to encode 2,403 proteins. The genome shows extensive synteny with that of the closely related organism, Kocuria rhizophila, from which it was taxonomically separated relatively recently. Despite its small size, the genome harbors 73 insertion sequence (IS) elements, almost all of which are closely related to elements found in other actinobacteria. An IS element is inserted into the rrs gene of one of only two rrn operons found in M. luteus. The genome encodes only four sigma factors and 14 response regulators, a finding indicative of adaptation to a rather strict ecological niche (mammalian skin). The high sensitivity of M. luteus to beta-lactam antibiotics may result from the presence of a reduced set of penicillin-binding proteins and the absence of a wblC gene, which plays an important role in the antibiotic resistance in other actinobacteria. Consistent with the restricted range of compounds it can use as a sole source of carbon for energy and growth, M. luteus has a minimal complement of genes concerned with carbohydrate transport and metabolism and its inability to utilize glucose as a sole carbon source may be due to the apparent absence of a gene encoding glucokinase. Uniquely among characterized bacteria, M. luteus appears to be able to metabolize glycogen only via trehalose and to make trehalose only via glycogen. It has very few genes associated with secondary metabolism. In contrast to most other actinobacteria, M. luteus encodes only one resuscitation-promoting factor (Rpf) required for emergence from dormancy, and its complement of other dormancy-related proteins is also much reduced. M. luteus is capable of long-chain alkene biosynthesis, which is of interest for advanced biofuel production; a three-gene cluster essential for this metabolism has been identified in the genome.

Biosynthesis of teichuronic acid in the bacterial cell wall. Purification and characterization of the glucosyltransferase of Micrococcus luteus

This report describes what is, to our knowledge, the first purification to near homogeneity of an enzyme involved in the biosynthesis of the teichuronic acid of Micrococcus luteus cell walls. The glucosyltransferase of M. luteus, which participates in the biosynthesis of teichuronic acid, was solubilized from cytoplasmic membrane fragments by extraction with buffer solutions containing the detergents Thesit (dodecyl alcohol polyoxyethylene ether; 1 mg/ml) and 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate (0.5 mg/ml). The detergent-solubilized enzyme was purified 150-fold, with a recovery of 13% by adsorbent column chromatography, ion-exchange chromatography, gel filtration, and preparative nondenaturing gradient polyacrylamide gel electrophoresis. On the basis of its mobility on native gradient gel, the glucosyltransferase was estimated to have a molecular mass of 440 kDa. The purified native enzyme was a multisubunit protein consisting of subunits of two sizes; their molecular masses were determined to be 52.5 and 54 kDa, respectively, by observation of the mobility of the protein bands in sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The isoelectric point of the enzyme was approximately 5.

Polymer length of teichuronic acid released from cell walls of Micrococcus luteus

Teichuronic acid released from its phosphodiester linkage to peptidoglycan in the cell walls of Micrococcus luteus by mild acid treatment is resolved into a ladderlike series of bands by electrophoresis on polyacrylamide gels in the presence of borate. Each band of the ladder differs from its nearest neighbor by one disaccharide repeat unit, ----4)-2-acetamido-2-deoxy-beta-D-mannopyranuronosyl-(1----6)- alpha-D-glucopyranosyl-(1-. Acid-fragmented teichuronic acid, after conversion to the phenylamine derivative, was fractionated by preparative-scale molecular sieve column chromatography, which produced a series of elution peaks. Fast-atom-bombardment mass spectrometry of the smallest member of the series determined its molecular weight and established its identity as the phenylamine derivative of one disaccharide repeat unit of teichuronic acid. Homologous fractions of the same series were used to index the ladder of bands obtained by polyacrylamide gel electrophoresis from samples containing a more extensive distribution of polymer lengths. Nearly native teichuronic acid consists of polymers with a broad range of molecular sizes ranging from 20 to 55 disaccharide units. The most abundant species are those which have 25 to 40 repeat units. Prolonged treatment of teichuronic acid with the acid conditions used to release it from peptidoglycan causes gradual fragmentation of the teichuronic acid.