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B J Stevens

Publications and source records attributed to B J Stevens.

At least 19 recordsLinked to original sources
PubMed1992

Mechanism of action of cyclic beta-1,2-glucan synthetase from Agrobacterium tumefaciens: competition between cyclization and elongation reactions.

We have examined some aspects of the mechanism of cyclic beta-1,2-glucan synthetase from Agrobacterium tumefaciens (235-kDa protein, gene product of the chvB region). The enzyme produces cyclic beta-1,2-glucans containing 17 to 23 glucose residues from UDP-glucose. In the presence of added cyclic beta-1,2-glucans (> 0.5 mg/ml) (containing 17 to 23 glucose residues), the enzyme instead synthesizes larger cyclic beta-1,2-glucans containing 24 to 30 glucose residues. This is achieved by de novo synthesis and not by disproportion reactions with the added product. This is interpreted as inhibition of the specific cyclization reaction for the synthesis of cyclic beta-1,2-glucans containing 17 to 23 glucose residues but with no concomitant effect on the elongation (polymerization) reaction. Temperature and detergents both affect the distribution of sizes of cyclic beta-1,2-glucans, but glucans containing 24 to 30 glucose residues are not produced. We suggest that the size distribution of cyclic beta-1,2-glucan products depends on competing elongation and cyclization reactions.

Agrobacterium tumefaciens↗
PubMed1990

Use of oligonucleotide probes to identify members of two-component regulatory systems in Xanthomonas campestris pathovar campestris.

Two-component regulatory systems comprising a sensor and a regulator protein, both with highly conserved amino acid domains, and commonly genetically linked, have been described in a range of bacterial species and are involved in sensing environmental stimuli. We used two oligonucleotide probes matching the postulated coding regions for domains of sensor and regulator proteins respectively in Xanthomonas campestris pathovar campestris (Xcc) to identify possible two-component regulatory systems in Xcc. Two different fragments of Xcc DNA with homology to both of these probes were cloned. The DNA sequence of part of one of these fragments encompassed a potential open reading frame (ORF), the predicted amino acid sequence of which had extensive homology with regulator proteins of two-component regulatory systems. Analysis of the predicted amino acid sequence for the 3' end of an adjacent ORF revealed a very high level of homology with the C-terminal end of sensor proteins. Strains of Xcc with Tn5-induced mutations in the regulator gene were affected in extracellular polysaccharide production, and also in resistance to salt and chloramphenicol. No effects of mutation in the second clone were observed.

Amino Acid Sequence↗
PubMed1988

Changes in composition and structure of wheat bran resulting from the action of human faecal bacteria in vitro.

Cell-wall material of wheat bran was incubated with human faecal bacteria for 24-72 h and the resulting structural changes were studied by methylation analysis. Of the carbohydrate content, approximately 39% was degraded after 24 h, increasing to only 44% after 72 h. Arabinoxylans and mixed-linkage beta-D-glucans from the aleurone layer were degraded preferentially. After treatment of the bran with alkali, the extent of degradation was increased three-fold as a result of saponification of ester cross-links which facilitated increased degradation of the polymers from both the aleurone and outer, lignified, layers. There was evidence that ester linkages between the glucuronosyl residues, attached to O-2 of the (1----4)-linked xylosyl residues, and phenolic groups of lignin were also saponified. The treatment with alkali also rendered the cellulose more susceptible to bacterial attack. The alkali-soluble acidic arabinoxylan fractions of the bran were degraded readily by bacterial action, but the xyloglucans cross-linked to arabinoxylans by phenolics were relatively resistant.

Arabinose↗
PubMed1985

Immunolocalization of the 100 K nucleolar protein in CHO cells.

The localization of a major nucleolar protein with a molecular weight of 100,000 has been followed in Chinese hamster ovary cells using specific antibodies to this protein and immunocytochemical techniques. By immunofluorescence and immunocytochemistry at ultrastructural level in situ, the 100 K protein was detected abundantly in the nucleolus of interphase cells. In exponentially growing cells, the dense fibrillar component was shown to contain more 100 K protein than the granular RNP component but both the nucleolar components were positively immunostained. Fibrillar centers consistently showed weaker or no staining. The 100 K association with the preribosomal RNP components of the interphase nucleolus supports evidence for a role of this protein in pre-rRNA transcription and preribosomal processing.

Animals↗
PubMed1985

Immunolocalization of the 100 kDa nucleolar protein during the mitotic cycle in CHO cells.

The localization of a major nucleolar protein with a molecular weight of 100,000 has been followed during mitosis in Chinese hamster ovary CHO cells using specific antibodies to this protein and immunocytochemical techniques. The 100 kDa protein was visualized at discrete sites on metaphase chromosomes, corresponding to nucleolus organizer regions, and in large, immunostained nucleolar remnants that are discarded in the cytoplasm after nucleolar disintegration. After mitosis, the 100 kDa protein was shown to play an early role in nucleolar reformation. It was first detected in small deposits around the anaphase chromosomes. In telophase, the protein accumulated simultaneously in prenucleolar bodies and in the reforming nucleoli. The early presence of the 100 kDa protein in the telophase nucleus suggests that it is essential for the reestablishment of nucleolar function after mitosis. Thus this protein is present throughout the CHO cell cycle, an observation which supports the hypothesis that it plays a fundamental role in cell organization.

Animals↗
PubMed1979

Factors affecting the incidence and anti-salmonella activity of the anaerobic caecal flora of the young chick.

Thirty-two different types of anaerobic bacteria isolated from chickens have been tested for anti-salmonella activity in vitro. Under the conditions of the test only Bacteroides hypermegas and a Bifidobacterium sp. were shown to inhibit the salmonellas and this was attributed to the production of volatile fatty acids (VFA's) coupled with a low pH. When these organisms were tested in newly hatched chicks no inhibition of S. typhimurium occurred. Possible explanations for this observation are considered. The pH value and concentration of VFA's in the caecal material were determined in chicks from 0-84 days. In vitro tests with S. typhimurium indicated that, whilst the organism would be able to multiply at the pH and concentration of VFA's found during the first few days after hatching, the rapid increase in VFA concentration during the first 21 days would make this increasingly difficult. The significance of the developing caecal flora in relation to VFA production and pH is discussed. Because certain feed additives are known to influence the carriage of salmonellas, the sensitivity of various caecal anaerobes to these compounds was determined in vitro, generally at 1, 10 and 100 microgram/ml. The additives tested included flavomycin, furazolidone, nitrovin, tetracyline, tylosin, sulphaquinoxaline, virginiamycin and zinc bacitracin. All the organisms tested were inhibited by 100 microgram/ml furazolidone; none were inhibited by 500 microgram/ml sulphaquinoxaline. Changes occurring in the VFA concentration, pH value and microflora of the caeca of chicks fed for 49 days or longer on a normal starter diet or the same diet containing 10 or 100 mg/kg nitrovin have been compared. When the chicks were fed on the diet containing 100 mg/kg nitrovin, the Gram-negative non-sporing anaerobes were eliminated as a significant part of the caecal flora. However, the VFA concentration combined with a low pH in chicks from 2 weeks onwards was still sufficient to inhibit salmonella multiplication. Other possibly interrelated factors which might lead to an increased salmonella carrier rate in the nitrovin-treated chickens are discussed.

Anaerobiosis↗