Using fusions with luxAB from Vibrio harveyi MAV to quantify induction and catabolite repression of the xyl operon in Staphylococcus carnosus TM300

Heterologous expression and characterization of a novel branching enzyme from the thermoalkaliphilic anaerobic bacterium Anaerobranca gottschalkii

The gene encoding the branching enzyme (BE) from the thermoalkaliphilic, anaerobic bacterium Anaerobranca gottschalkii was fused with a twin arginine translocation protein secretory-pathway-dependent signal sequence from Escherichia coli and expressed in Staphylococcus carnosus. The secreted BE was purified using hydrophobic interaction and gel filtration chromatography. The monomeric enzyme (72 kDa) shows maximal activity at 50 degrees C and pH 7.0. With amylose the BE displays high transglycosylation and extremely low hydrolytic activity. The conversion of amylose and linear dextrins was analysed by applying high-performance anion exchange chromatography and quantitative size-exclusion chromatography. Amylose (10(4)-4 x 10(7) g/mol) was converted to a major extent to products displaying molecular masses of 10(4)-4 x 10(5) g/mol, indicating that the enzyme could be applicable for the production of starch or dextrins with narrow molecular mass distributions. The majority of the transferred oligosaccharides, determined after enzymatic hydrolysis of the newly synthesized alpha-1,6 linkages, ranged between 10(3) and 10(4) g/mol, which corresponds to a degree of polymerisation (DP) of 6-60. The minimal donor chain length is DP 16. Furthermore, the obtained results support the hypotheses of a random endocleavage mechanism of BE and the occurrence of interchain branching.

Cooperative and non-cooperative DNA binding modes of catabolite control protein CcpA from Bacillus megaterium result from sensing two different signals

Carbon catabolite repression (CCR) of several operons in Bacillus subtilis and Bacillus megaterium is mediated by the cis-acting cre sequence and trans-acting catabolite control protein (CcpA). We describe purification of CcpA from B. megaterium and its interaction with regulatory sequences from the xyl operon. Specific interaction of CcpA with cre as scored by DNase I footprints at concentrations similar to the in vivo situation requires the presence of effectors. We have found two molecular effectors for CcpA activity, which lead to different recognition modes of DNA. The heat-stable phosphotransfer protein HPr from the PTS sugar uptake system triggers non-cooperative binding of CcpA to cre when phosphorylated at Ser46 (HPr-Ser46-P). Glucose 6-phosphate (Glc-6-P) triggers cooperative binding of CcpA to cre and two auxiliary cre* sites, one of which overlaps the -35 box of the xyl promoter. Binding to cre* depends on the presence of the functional cre sequence. A mutation in cre abolishes carbon catabolite repression in vivo and binding of CcpA to cre and cre* in vitro, indicating looping of the intervening DNA. The two triggers are not simultaneously active. The acidity of the buffer determines which of them activates CcpA when both are present in vitro. Glc-6-P is preferred at pH values below 5.4, and HPr-Ser46-P is preferred at neutral pH. The Ccpa dimers present at neutral pH form tetramers and higher oligomers at pH 4.6, explaining cooperativity of binding to DNA. CcpA is the first member of the LacI/GalR family of regulators, for which oligomerization without the leucine zipper at the C terminus is demonstrated.

Molecular and industrial aspects of glucose isomerase

Glucose isomerase (GI) (D-xylose ketol-isomerase; EC. 5.3.1.5) catalyzes the reversible isomerization of D-glucose and D-xylose to D-fructose and D-xylulose, respectively. The enzyme has the largest market in the food industry because of its application in the production of high-fructose corn syrup (HFCS). HFCS, an equilibrium mixture of glucose and fructose, is 1.3 times sweeter than sucrose and serves as a sweetener for use by diabetics. Interconversion of xylose to xylulose by GI serves a nutritional requirement in saprophytic bacteria and has a potential application in the bioconversion of hemicellulose to ethanol. The enzyme is widely distributed in prokaryotes. Intensive research efforts are directed toward improving its suitability for industrial application. Development of microbial strains capable of utilizing xylan-containing raw materials for growth or screening for constitutive mutants of GI is expected to lead to discontinuation of the use of xylose as an inducer for the production of the enzyme. Elimination of Co2+ from the fermentation medium is desirable for avoiding health problems arising from human consumption of HFCS. Immobilization of GI provides an efficient means for its easy recovery and reuse and lowers the cost of its use. X-ray crystallographic and genetic engineering studies support a hydride shift mechanism for the action of GI. Cloning of GI in homologous as well as heterologous hosts has been carried out, with the prime aim of overproducing the enzyme and deciphering the genetic organization of individual genes (xylA, xylB, and xylR) in the xyl operon of different microorganisms. The organization of xylA and xylB seems to be highly conserved in all bacteria. The two genes are transcribed from the same strand in Escherichia coli and Bacillus and Lactobacillus species, whereas they are transcribed divergently on different strands in Streptomyces species. A comparison of the xylA sequences from several bacterial sources revealed the presence of two signature sequences, VXW(GP)GREG(YSTAE)E and (LIVM)EPKPX(EQ)P. The use of an inexpensive inducer in the fermentation medium devoid of Co2+ and redesigning of a tailor-made GI with increased thermostability, higher affinity for glucose, and lower pH optimum will contribute significantly to the development of an economically feasible commercial process for enzymatic isomerization of glucose to fructose. Manipulation of the GI gene by site-directed mutagenesis holds promise that a GI suitable for biotechnological applications will be produced in the foreseeable future.

Specificity of DNA binding activity of the Bacillus subtilis catabolite control protein CcpA

CcpA was purified from Escherichia coli BL21 (lambda DE3)/pLysS carrying plasmid pTSC5, which was constructed by inserting the ccpA gene into the polycloning site of pGEM4. The purified protein migrated in sodium dodecyl sulfate-polyacrylamide gel electrophoresis with an apparent mass of 38 kDa but was eluted from a calibrated Bio-Gel P-100 column with an apparent mass of 75 kDa. Western blot (immunoblot) analysis revealed the presence of CcpA in E. coli BL21 (lambda DE3)/pLysS/pTSC5, which carries ccpA, and in wild-type Bacillus subtilis 168 but not in E. coli BL21 (lambda DE3)/pLysS/pGEM4 or in B. subtilis WLN-29, in which ccpA is inactivated by transposon Tn917 insertion. Purified CcpA bound to DNA containing amyO and retarded its mobility in electrophoretic mobility shift analysis. Complete retardation of the DNA required 75 ng of CcpA per assay. In DNase protection analysis, CcpA bound to DNA containing amyO and protected a region spanning amyO when either DNA strand was labeled. Mutant forms of amyO not effective in catabolite repression were not retarded by CcpA.

Catabolite repression of the Bacillus subtilis xyl operon involves a cis element functional in the context of an unrelated sequence, and glucose exerts additional xylR-dependent repression

Catabolite repression (CR) of xylose utilization by Bacillus subtilis involves a 14-bp cis-acting element (CRE) located in the translated region of the gene encoding xylose isomerase (xylA). Mutations of CRE making it more similar to a previously proposed consensus element lead to increased CR exerted by glucose, fructose, and glycerol. Fusion of CRE to an unrelated, constitutive promoter confers CR to beta-galactosidase expression directed by that promoter. This result demonstrates that CRE can function independently of sequence context and suggests that it is indeed a generally active cis element for CR. In contrast to the other carbon sources studied here, glucose leads to an additional repression of xylA expression, which is independent of CRE and is not found when CRE is fused to the unrelated promoter. This repression requires a functional xylR encoding Xyl repressor and is dependent on the concentrations of glucose and the inducer xylose in the culture broth. Potential mechanisms for this glucose-specific repression are discussed.