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Query: UMLS:C0023241 (Legionella)
 6,990 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Lipid A, a major component of the outer membranes of Escherichia coli and other Gram-negative bacteria, is usually constructed around a beta-1',6-linked glucosamine disaccharide backbone. However, in organisms like Acidithiobacillus ferrooxidans, Leptospira interrogans, Mesorhizobium loti, and Legionella pneumophila, one or both glucosamine residues are replaced with the sugar 2,3-diamino-2,3-dideoxy-d-glucopyranose. We now report the identification of two proteins, designated GnnA and GnnB, involved in the formation of the 2,3-diamino-2,3-dideoxy-d-glucopyranose moiety. The genes encoding these proteins were recognized because of their location between lpxA and lpxB in A. ferrooxidans. Based upon their sequences, the 313-residue GnnA protein was proposed to catalyze the NAD(+)-dependent oxidation of the glucosamine 3-OH of UDP-GlcNAc, and the 369-residue GnnB protein was proposed to catalyze the subsequent transamination to form UDP 2-acetamido-3-amino-2,3-dideoxy-alpha-d-glucopyranose (UDP-GlcNAc3N). Both gnnA and gnnB were cloned and expressed in E. coli using pET23c+. In the presence of l-glutamate and NAD(+), both proteins were required for the conversion of [alpha-(32)P]UDP-GlcNAc to a novel, less negatively charged sugar nucleotide shown to be [alpha-(32)P]UDP-GlcNAc3N. The latter contained a free amine, as judged by modification with acetic anhydride. Using recombinant GnnA and GnnB, approximately 0.4 mg of the presumptive UDP-GlcNAc3N was synthesized. The product was purified and subjected to NMR analysis to confirm the replacement of the GlcNAc 3-OH group with an equatorial NH(2). As shown in the accompanying papers, UDP-GlcNAc3N is selectively acylated by LpxAs of A. ferrooxidans, L. interrogans, and M. loti. UDP-GlcNAc3N may be useful as a substrate analog for diverse enzymes that utilize UDP-GlcNAc.
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PMID:Oxidation and transamination of the 3"-position of UDP-N-acetylglucosamine by enzymes from Acidithiobacillus ferrooxidans. Role in the formation of lipid a molecules with four amide-linked acyl chains. 1504 94

Legionella pneumophila is an opportunistic pathogen that in the environment colonizes biofilms and replicates within amoebae. The bacteria employ the intracellular multiplication/defective organelle trafficking (Icm/Dot) type IV secretion system to grow intracellularly in a specific vacuole. Using Acanthamoeba castellanii as a host cell, we have previously identified lcsC (Legionella cytotoxic suppressor), a paralogue of the lipid A disaccharide synthase lpxB, as a cytotoxic factor of L. pneumophila. A bioinformatic analysis of the genome revealed that L. pneumophila is unique in harbouring two paralogues of lpxB and two and three paralogues of the lipid A biosynthesis acyltransferases lpxA and lpxD, respectively. LcsC (lpxB1) forms a transcriptional unit with gnnA, encoding a putative UDP-GlcNAc oxidase in the biosynthetic pathway leading to 3-aminoglucosamine analogues of lipid A. LpxB2 clusters with lpxD2, lpxA2 and lpxL paralogues, encoding secondary acyltransferases. LcsC/lpxB1 and lpxB2 were found to partially complement the growth defect of an Escherichia coli lpxB conditional mutant strain, indicating that both corresponding enzymes possess lipid A disaccharide synthase activity. The two L. pneumophila lpxB paralogues are not functionally equivalent, since expression of lcsC/lpxB1 but not lpxB2 in an L. pneumophila icmG mutant is cytotoxic for A. castellanii, and LPS purified from the two strains triggers CD14-dependent tumour necrosis factor (TNF)alpha production by macrophages with a different potency. The lpxB and lpxA paralogues are expressed under various growth conditions, including broth, biofilms and in A. castellanii. While the flagellar gene flaA is mainly expressed in late stationary phase, the lpxB and lpxA paralogues are preferentially expressed in the exponential and early stationary phases. Upon exposure to hypotonic stress and nutrient deprivation, lpxA1, and to a lesser extent lcsC/lpxB1, is upregulated. The differential regulation of lpxB or lpxA paralogues in response to changing environmental conditions might allow L. pneumophila to adapt its lipid A structure.
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PMID:Expression of Legionella pneumophila paralogous lipid A biosynthesis genes under different growth conditions. 1797 91

Legionaminic acid is a nine-carbon alpha-keto acid that is similar in structure to other members of the sialic acid family that includes neuraminic acid and pseudaminic acid. It is found as a component of the lipopolysaccharide in several bacterial species and is perhaps best known for its presence in the O-antigen of the causative agent of Legionnaires' disease, Legionella pneumophila. In this work, the enzymes responsible for the biosynthesis and activation of N, N'-diacetyllegionaminic acid are identified for the first time. A cluster of three L. pneumophila genes bearing homology to known sialic acid biosynthetic genes ( neuA,B,C) were cloned and overexpressed in Escherichia coli. The NeuC homologue was found to be a hydrolyzing UDP- N, N'-diacetylbacillosamine 2-epimerase that converts UDP- N, N'-diacetylbacillosamine into 2,4-diacetamido-2,4,6-trideoxymannose and UDP. Stereochemical and isotopic labeling studies showed that the enzyme utilizes a mechanism involving an initial anti elimination of UDP to form a glycal intermediate and a subsequent syn addition of water to generate product. This is similar to the hydrolyzing UDP- N-acetylglucosamine 2-epimerase (NeuC) of sialic acid biosynthesis, but the L. pneumophila enzyme would not accept UDP-GlcNAc as an alternate substrate. The NeuB homologue was found to be a N, N'-diacetyllegionaminic acid synthase that condenses 2,4-diacetamido-2,4,6-trideoxymannose with phosphoenolpyruvate (PEP), although the in vitro activity of the recombinant enzyme (isolated as a MalE fusion protein) was very low. The synthase activity was dependent on the presence of a divalent metal ion, and the reaction proceeded via a C-O bond cleavage process, similar to the reactions catalyzed by the sialic acid and pseudaminic acid synthases. Finally, the NeuA homologue was shown to possess the CMP- N, N'-diacetyllegionaminic acid synthetase activity that generates the activated form of legionaminic acid used in lipopolysaccharide biosynthesis. Together, the three enzymes constitute a pathway that converts a UDP-linked bacillosamine derivative into a CMP-linked legionaminic acid derivative.
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PMID:Biosynthesis of CMP-N,N'-diacetyllegionaminic acid from UDP-N,N'-diacetylbacillosamine in Legionella pneumophila. 1827 54