Acetic acid bacteria encode two levansucrase types of different ecological relationship.

Acetic acid bacteria (AAB) are associated with plants and insects. Determinants for the targeting and occupation of these widely different environments are unknown. However, most of these natural habitats share plant-derived sucrose, which can be metabolized by some AAB via polyfructose building levansucrases (LS) known to be involved in biofilm formation. Here, we propose two LS types (T) encoded by AAB as determinants for habitat selection, which emerged from vertical (T1) and horizontal (T2) lines of evolution and differ in their genetic organization, structural features and secretion mechanism, as well as their occurrence in proteobacteria. T1-LS are secreted by plant-pathogenic α- and γ-proteobacteria, while T2-LS genes are common in diazotrophic, plant-growth-promoting α-, ß- and γ-proteobacteria. This knowledge may be exploited for a better understanding of microbial ecology, plant health and biofilm formation by sucrase-secreting proteobacteria in eukaryotic hosts.

Dissection of hexosyl- and sialyltransferase domains in the bifunctional capsule polymerases from Neisseria meningitidis W and Y defines a new sialyltransferase family.

Crucial virulence determinants of disease causing Neisseria meningitidis species are their extracellular polysaccharide capsules. In the serogroups W and Y, these are heteropolymers of the repeating units (→6)-α-d-Gal-(1→4)-α-Neu5Ac-(2→)n in NmW and (→6)-α-d-Glc-(1→4)-α-Neu5Ac-(2→)n in NmY. The capsule polymerases, SiaDW and SiaDY, which synthesize these highly unusual polymers, are composed of two predicted GT-B fold domains separated by a large stretch of amino acids (aa 399-762). We recently showed that residues critical to the hexosyl- and sialyltransferase activity are found in the predicted N-terminal (aa 1-398) and C-terminal (aa 763-1037) GT-B fold domains, respectively. Here we use a mutational approach and synthetic fluorescent substrates to define the boundaries of the hexosyl- and sialyltransferase domains. Our results reveal that the active sialyltransferase domain extends well beyond the predicted C-terminal GT-B domain and defines a new glycosyltransferase family, GT97, in CAZy (Carbohydrate-Active enZYmes Database).

Induced accumulation of glucuronosyldiacylglycerol in tomato and soybean under phosphorus deprivation.

Glucuronosyldiacylglycerol (GlcADG) is a plant glycolipid that accumulates in Arabidopsis and rice in response to phosphorus (P) starvation. It has been suggested that GlcADG functions to mitigate the stress induced by P depletion. Biosynthesis of GlcADG requires sulfolipid (SQDG) synthase, which is coded for in plant genomes. This indicates the possibility that GlcADG may be a general constituent of membrane lipids in plants. In this study, we investigated the SQDG synthases found in the genomes of higher plants, ferns, mosses, algae and cyanobacteria. In addition, we analyzed GlcADG accumulation, and the expression of SQDG synthase homologs in tomato and soybean plants grown under P-limited conditions. LC-MS analysis of lipids from these plants confirmed that GlcADG accumulated during P deprivation, as previously observed in Arabidopsis and rice. We also observed upregulation of SQDG synthase transcripts in these plants during P deprivation. These data suggest that GlcADG is present not only in model plants, but also in various other plant species, and that this lipid molecule performs an important physiological function as a mitigator of P-deprivation stress in plants.

Mutational analysis of the role of calcium ions in the Lactobacillus reuteri strain 121 fructosyltransferase (levansucrase and inulosucrase) enzymes.

Bacterial fructosyltransferase enzymes belonging to glycoside hydrolase family 68 (GH68) are not known to require a metal cofactor. Here, we show that Ca2+ ions play an important structural role in the Lactobacillus reuteri 121 levansucrase (Lev) and inulosucrase (Inu) enzymes. Analysis of the Bacillus subtilis Lev 3D structure [Meng, G. and Futterer, K. (2003) Nat. Struct. Biol. 10, 935-941] has provided evidence for the presence of a bound metal ion, most likely Ca2+. Characterization of site-directed mutants in the putative Ca2+ ion-binding sites of Lb. reuteri Lev and Inu revealed that the Inu Asp520 and Lev Asp500 residues play an important role in Ca2+ binding. Sequence alignments of family GH68 proteins showed that this Ca2+ ion-binding site is (largely) present only in proteins of Gram-positive origin.

Sequence and structure-based prediction of fructosyltransferase activity for functional subclassification of fungal GH32 enzymes.

Sucrolytic enzymes catalyse sucrose hydrolysis or the synthesis of fructooligosaccharides (FOSs), a prebiotic in human and animal nutrition. FOS synthesis capacity differs between sucrolytic enzymes. Amino-acid-sequence-based classification of FOS synthesizing enzymes would greatly facilitate the in silico identification of novel catalysts, as large amounts of sequence data lie untapped. The development of a bioinformatics tool to rapidly distinguish between high-level FOSs synthesizing predominantly sucrose hydrolysing enzymes from fungal genomic data is presented. Sequence comparison of functionally characterized enzymes displaying low- and high-level FOS synthesis revealed conserved motifs unique to each group. New light is shed on the sequence context of active site residues in three previously identified conserved motifs. We characterized two enzymes predicted to possess low- and high-level FOS synthesis activities based on their conserved motif sequences. FOS data for the enzymes confirmed our successful prediction of their FOS synthesis capacity. Structural comparison of enzymes displaying low- and high-level FOS synthesis identified steric hindrance between nystose and a long loop region present only in low-level FOS synthesizers. This loop is proposed to limit the synthesis of FOS species with higher degrees of polymerization, a phenomenon observed among enzymes displaying low-level FOS synthesis. Conserved sequence motifs surrounding catalytic residues and a distant structural determinant were identifiers of FOS synthesis capacity and allow for functional annotation of sucrolytic enzymes directly from amino acid sequence. The tool presented may also be useful to study the structure-function relationships of ß-fructofuranosidases by identifying mutations present in a group of closely related enzymes displaying similar function.

Nonomuraea sp. ID06-A0189 inulin fructotransferase (DFA III-forming): gene cloning, characterization and conservation among other Nonomuraea species.

The inulin fructotransferase (DFA III-forming)(EC 4.2.2.18) gene in Nonomuraea sp. ID06-A0189 was amplified from genomic DNA, sequenced and expressed in Escherichia coli. The 1326-bp gene, designated as Nsp-ift, encodes a protein composed of a putative 37-amino-acid signal peptide and 404-amino-acid mature protein. A putative ribosomal binding sequence was identified 12 bases upstream from the start codon. However, a typical bacterial promoter could not be found by in silico analysis. The deduced amino-acid sequence of the enzyme was most similar to that of inulin fructotransferase (DFA I-forming) in Frankia sp. EAN1pec. Phylogenetic analysis of deduced amino-acid sequences indicated that Nonomuraea sp. ID06-A0189 and Frankia sp. EAN1pec inulin fructotransferases formed a distinct clade from those from Arthrobacter sp. H65-7, A. globiformis and Bacillus sp. snu-7 that showed 57, 56 and 56% identity to that of Nsp-ift, respectively. The Nsp-ift without a putative signal peptide was successfully expressed in E. coli and partially purified using His-tag affinity chromatography. The recombinant enzyme displayed optimum temperature between 65 and 70 °C, optimum pH between 5.5 and 6.0 and remained stable up to 70 °C. The properties were identical to those of the original enzyme. Of 10 Nonomuraea species tested by Southern hybridization, enzyme activity measurements and PCR, only Nonomuraea sp. ID06-A0189 has the Nsp-ift gene, suggesting that Nsp-ift is not highly conserved in this genus.

Purification and properties of a heat-stable inulin fructotransferase from Arthrobacter ureafaciens.

An inulin fructotransferase producing difructose dianhydride I (EC 2.4.1.200) was purified from Arthrobacter ureafaciens A51-1. It had maximum activity at pH 5.5 and 45 degrees C, and was stable up to 80 degrees C. This is the highest thermal stability for this enzyme reported to date. The molecular mass was estimated to be 38000 by SDS-PAGE, and 61000 by gel filtration. It was therefore estimated to be a dimer.

Linkage mapping and nucleotide polymorphisms of the 6-SFT gene of cool-season grasses.

Fructan plays an important role as an alternate carbohydrate and may contribute to drought and cold-stress tolerances in various plant species. The gene coding for sucrose:fructan 6-fructosyltransferase (6-SFT; EC 2.4.1.10), an enzyme that catalyzes the formation and extension of beta-2,6-linked fructans (levans), is important to fructan synthesis in many cool-season grasses, including cereal species. In this study, we compared a conserved sequence from the 6-SFT gene in barley with comparable sequences in 20 other cool-season grasses. We detected several DNA length polymorphisms, including variations in one simple-sequence repeat (SSR) in a 6-SFT intron of the barley cultivars Steptoe and Morex. Using the 'Steptoe' x 'Morex' doubled-haploid mapping population, the 6-SFT gene was genetically mapped to the distal region in the short arm of barley chromosome 1 (7H), where it is closely linked with trait locus Rpg1. Primers designed from other conserved regions of the barley 6-SFT gene successfully amplified 351- or 354-bp sequences of this gene from diverse cool season grass species. Sequence identities of the PCR products were greater than 80% among the 21 species. Phylogeny, as determined using these DNA sequences, is similar to that obtained from rDNA ITS sequences, and congruent with our current knowledge of genome relationships.

Identification of four families of peptidoglycan lytic transglycosylases.

The lytic transglycosylases are a class of autolysins which cleave the bacterial cell wall heteropolymer peptidoglycan (murein) to facilitate its biosynthesis and turnover. A search of the National Center for Biotechnology Information (NCBI) databases using the primary sequences of the six characterized lytic transglycosylases of Escherichia coli, a membrane-bound form of the enzyme from Pseudomonas aeruginosa, and the endolysins of lambda bacteriophage permitted the identification of a total of 127 known and hypothetical enzymes from a wide variety of bacteria and bacteriophage. These amino acid sequences have been arranged into four families based on alignments, and consensus motifs have been identified. Family 1 represents a superfamily comprising 86 sequences which are subdivided into five (1A--1E) subfamilies.

Evolution of penicillin resistance in Streptococcus pneumoniae; the role of Streptococcus mitis in the formation of a low affinity PBP2B in S. pneumoniae.

Penicillin-resistant strains of Streptococcus pneumoniae possess forms of penicillin-binding proteins (PBPs) that have a low affinity for penicillin compared to those from penicillin-sensitive strains. PBP genes from penicillin-resistant isolates are very variable and have a mosaic structure composed of blocks of nucleotides that are similar to those found in PBP genes from penicillin-sensitive isolates and blocks that differ by up to 21%. These chromosomally encoded mosaic genes have presumably arisen following transformation and homologous recombination with PBP genes from a number of closely related species. This study shows that PBP2B genes from many penicillin-resistant isolates of S. pneumoniae contain blocks of nucleotides originating from Streptococcus mitis. In several instances it would appear that this material alone is sufficient to produce a low affinity PBP2B. In other examples PBP2B genes possess blocks of nucleotides from S. mitis and at least one additional unidentified species. Mosaic structure was also found in the PBP2B genes of penicillin-sensitive isolates of S. mitis or S. pneumoniae. These mosaics did not confer penicillin resistance but nevertheless reveal something of the extent to which localized recombination occurs in these naturally transformable streptococci.

Role of class A penicillin-binding proteins in PBP5-mediated beta-lactam resistance in Enterococcus faecalis.

Peptidoglycan polymerization complexes contain multimodular penicillin-binding proteins (PBP) of classes A and B that associate a conserved C-terminal transpeptidase module to an N-terminal glycosyltransferase or morphogenesis module, respectively. In Enterococcus faecalis, class B PBP5 mediates intrinsic resistance to the cephalosporin class of beta-lactam antibiotics, such as ceftriaxone. To identify the glycosyltransferase partner(s) of PBP5, combinations of deletions were introduced in all three class A PBP genes of E. faecalis JH2-2 (ponA, pbpF, and pbpZ). Among mutants with single or double deletions, only JH2-2 DeltaponA DeltapbpF was susceptible to ceftriaxone. Ceftriaxone resistance was restored by heterologous expression of pbpF from Enterococcus faecium but not by mgt encoding the monofunctional glycosyltransferase of Staphylococcus aureus. Thus, PBP5 partners essential for peptidoglycan polymerization in the presence of beta-lactams formed a subset of the class A PBPs of E. faecalis, and heterospecific complementation was observed with an ortholog from E. faecium. Site-directed mutagenesis of pbpF confirmed that the catalytic serine residue of the transpeptidase module was not required for resistance. None of the three class A PBP genes was essential for viability, although deletion of the three genes led to an increase in the generation time and to a decrease in peptidoglycan cross-linking. As the E. faecalis chromosome does not contain any additional glycosyltransferase-related genes, these observations indicate that glycan chain polymerization in the triple mutant is performed by a novel type of glycosyltransferase. The latter enzyme was not inhibited by moenomycin, since deletion of the three class A PBP genes led to high-level resistance to this glycosyltransferase inhibitor.