Nucleotide sequence and analysis of the Vibrio alginolyticus sucrase gene (scrB)

The Acquisition of the scr Gene Cluster Encoding Sucrose Metabolization Enzymes Enables Strains of Vibrio parahaemolyticus and Vibrio vulnificus to Utilize Sucrose as Carbon Source

Most strains of Vibrio parahaemolyticus are unable to utilize sucrose as carbon source, though few exceptions exist. We investigated a sucrose-positive V. parahaemolyticus strain by whole-genome sequencing (WGS) and confirmed the presences of a genomic island containing sucrose utilization genes. A 4.7 kb DNA cluster consisting of three genes: scrA encoding a sucrose uptake protein, scrK encoding a fructokinase, and scrB coding for a sucrose-6-phosphate hydrolase, was PCR amplified and inserted into the Vibrio/Escherichia coli shuttle vector pVv3. Two recombinant plasmids, only differing in the orientation of the insert with respect to the pVv3-lacZα-fragment, conferred the E. coli K12 transformants the ability to utilize sucrose. The introduction of the two plasmids into sucrose-negative V. parahaemolyticus and V. vulnificus strains also results in a change of the sucrose utilization phenotype from negative to positive. By performing a multiplex PCR targeting scrA, scrK, and scrB, 43 scr-positive V. parahaemolyticus isolates from our collection of retail strains were detected and confirmed to be able to use sucrose as carbon source. Strains unable to utilize the disaccharide were negative by PCR for the scr genes. For in-depth characterization, 17 sucrose-positive V. parahaemolyticus were subjected to WGS. A genomic island with a nucleotide identity of >95% containing scrA, scrB, scrK and three additional coding sequences (CDS) were identified in all strains. The additional genes were predicted as a gene coding for a transcriptional regulator (scrR), a porin encoding gene and a CDS of unknown function. Sequence comparison indicated that the genomic island was located in the same region of the chromosome II in all analyzed V. parahaemolyticus strains. Structural comparison of the genomes with sequences of the sucrose utilizing species V. alginolyticus revealed the same genomic island, which indicates a possible distribution of this genetic structure by horizontal gene transfer. The comparison of all genome sequences based on SNP differences reveals that the presence of sucrose utilizing genes is found in genetically diverse V. parahaemolyticus strains and is not restricted to a subset of closely related strains.

Diverse Horizontally-Acquired Gene Clusters Confer Sucrose Utilization to Different Lineages of the Marine Pathogen Photobacterium damselae subsp. damselae

The ability to metabolize sucrose is a variable trait within the family Vibrionaceae. The marine bacterium Photobacterium damselae subsp. damselae (Pdd), pathogenic for marine animals and humans, is generally described as negative for sucrose utilization (Scr⁻). Previous studies have reported sucrose-utilizing isolates (Scr⁺), but the genetic basis of this variable phenotype remains uncharacterized. Here, we carried out the genome sequencing of five Scr⁺ and two Scr⁻Pdd isolates and conducted a comparative genomics analysis with sixteen additional Pdd genomes sequenced in previous studies. We identified two different versions of a four-gene cluster (scr cluster) exclusive of Scr⁺ isolates encoding a PTS system sucrose-specific IIBC component (scrA), a fructokinase (scrK), a sucrose-6-phosphate hydrolase (scrB), and a sucrose operon repressor (scrR). A scrA deletion mutant did not ferment sucrose and was impaired for growth with sucrose as carbon source. Comparative genomics analyses suggested that scr clusters were acquired by horizontal transfer by different lineages of Pdd and were inserted into a recombination hot-spot in the Pdd genome. The incongruence of phylogenies based on housekeeping genes and on scr genes revealed that phylogenetically diverse gene clusters for sucrose utilization have undergone extensive horizontal transfer among species of Vibrio and Photobacterium.

Construction of homologous and heterologous synthetic sucrose utilizing modules and their application for carotenoid production in recombinant Escherichia coli

Sucrose is one of the most promising carbon sources for industrial fermentation. We expressed synthetic modules expressing genes of the PEP-PTS and non-PTS pathways in Escherichia coli K12 for comparison. We selected PEP-PTS pathway genes of Lactobacillus plantarum and Staphylococcus xylosus and non-PTS pathway genes of sucrose-utilizing (Scr(+)) E. coli EC3132. Switchable Scr(+) modules expressing E. coli EC3132 non-PTS genes conferred better sucrose-utilizing ability on Scr(-)E. coli K12 than E. coli EC3132. Scr(+) modules expressing S. xylosus PEP-PTS genes conferred a sucrose-utilizing ability on E. coli K12. Among L. plantarum PEP-PTS genes, SacA(LP) and SacK(LP) were functional in E. coli K12. CscA(EC)-CscB(EC)-CscK(EC) (non-PEP-PTS module) or ScrA(SX)-SacA(LP)-SacK(LP) (PEP-PTS module) was introduced to a diapolycopene-producing E. coli strain. In both Scr(+)E. coli K12, the sucrose-utilizing ability of the modules was not affected by diapolycopene formation, indicating that the modular Scr(+) systems could be employed for developing sustainable bioprocesses using sucrose.

Cloning, sequencing, and expression of cscA invertase from Escherichia coli B-62

We have isolated a 2.5-kb DNA fragment from plasmid pST5R7 encoding a sucrose utilization system from Escherichia coli B-62 which confers a sucrose-fermenting phenotype to transformed E. coli K-12 strains. DNA-sequence determination revealed one full-length open reading frame 98% identical to cscA, the sucrose-hydrolase (invertase) gene of the csc regulon from E. coli EC3132. Functional characterization indicates that high-level expression and limited periplasmic release of invertase is responsible for the sucrose-fermenting capacity of transformed E. coli K-12 strains carrying cscA.Key words: sucrose utilization, sucrose hydrolase, invertase, recombinant protein production.

Studies on identifying the catalytic role of Glu-204 in the active site of yeast invertase

In a previous study on yeast invertase (Reddy, A., and Maley, F. (1990) J. Biol. Chem. 265, 10817-10820), we identified Asp-23 through the procedures of affinity labeling and site-directed mutagenesis as a catalytic nucleophile. In the present study we undertook to determine other residues involved in the catalytic process. Earlier studies suggested histidine as a potential proton donor in the hydrolysis of sucrose, but by mutagenizing each of the enzyme's four histidines this amino acid was eliminated from consideration. Another candidate appeared to be cysteine, since iodine at about a 2-fold molar excess inactivated invertase by modifying both of the enzyme's cysteine residues. Dithiothreitol treatment restored the sulfhydryl groups and enzyme activity. Replacement of each of the cysteines with alanines revealed that C108A invertase retained full activity whereas C205A was reduced about 4-fold in its kcat. A comparison of the amino acid sequences of fructosylhydrolases revealed a conserved region coincident with Glu-204/Cys-205. Mutagenizing Glu-204 to Ala resulted in a 3, 000-fold reduction in the kcat of invertase indicating that Glu-204 plays a major role in catalysis. Based on these findings, a mechanism is proposed for the hydrolysis of sucrose which involves Asp-23 as a nucleophile and Glu-204 as an acid/base catalyst.

Molecular analysis of the scrA and scrB genes from Klebsiella pneumoniae and plasmid pUR400, which encode the sucrose transport protein Enzyme II Scr of the phosphotransferase system and a sucrose-6-phosphate invertase

The Klebsiella pneumoniae genes scrA and scrB are indispensable for sucrose (Scr) utilisation. Gene scrA codes for an Enzyme IIScr (IIScr) transport protein of the phosphoenolpyruvate-dependent carbohydrate: phosphotransferase system (PTS), while scrB encodes a sucrose 6-phosphate specific invertase. A 3.7 kbscr AB DNA fragment has been cloned from K. pneumoniae and expressed in Escherichia coli. Its nucleotide sequence was determined and the coding regions for scrA (1371 bp) and scrB (1401 bp) were identified by genetic complementation, enzyme activity test and radiolabelling of the gene products. In addition, the nucleotide sequence of the scrB gene from conjugative plasmid pUR400 isolated from Salmonella typhimurium was also determined and errors in the previously published sequence of the scrA gene of pUR400 were corrected. Extensive similarity was found between the sequences of ScrA and other Enzymes II, as well as between the two invertases and other sucrose hydrolysing enzymes. Based on the analysis of seven IIScr proteins, a hypothetical model of the secondary structure of IIScr is proposed.

Characterization of levJ, a sucrase/fructanase-encoding gene from Actinomyces naeslundii T14V, and comparison of its product with other sucrose-cleaving enzymes

A library of Actinomyces naeslundii T14V DNA was constructed in plasmid pUC18 and from this several sucrose-positive clones were isolated. Evidence was obtained that all these clones contained the same gene. One clone, which carried a plasmid that was named pPNG102, was chosen for further study. It was found that the enzyme specified by this plasmid hydrolyzed sucrose, raffinose, inulin and levan, but not dextran, and did not synthesize fructan or glucan from sucrose. The sequence of the insert in pPNG102 was determined and was found to contain a large ORF that specifies a polypeptide of 99,319 Da with similarity to other sucrases. This gene was named levJ. The deduced amino acid (aa) sequence contained both a potential signal sequence and potential C-terminal cell envelope attachment domain. Alignments revealed an internal 331-aa domain not present in other levanases and sucrases. A neighbour-joining tree showed that sucrases of eukaryotic origin form a cluster with eubacterial sucrase/fructanases, and this cluster does not include other eubacterial sucrases. It is postulated that certain eukaryotic sucrase-encoding genes are of eubacterial origin.

Nucleotide and derived amino acid sequences of an extracellular sucrase gene (invB) of Zymomonas mobilis ZM1 (ATCC10988)

DNA sequence analysis of a previously cloned 4.5 kb DNA fragment showed that the extracellular sucrase gene (invB) of Zymomonas mobilis was located in the 155 bp downstream of levansucrase gene (levU). The invB gene had an open reading frame of 1242 bp and the deduced amino acid sequence was 413 residues with a molecular weight of 46,107. The translated sequence of Z. mobilis invB was in good agreement with the 10 N-terminal amino acid residues determined by amino acid sequencing. The amino acid sequence of sucrase showed low similarity with that of other sucrases, but higher similarity with that of levansucrases.

Cloning, sequencing, and disruption of a levanase gene of Bacillus polymyxa CF43

The Bacillus polymyxa CF43 lelA gene, expressing both sucrose and fructan hydrolase activities, was isolated from a genomic library of B. polymyxa screened in Bacillus subtilis. The gene was detected as expressing sucrose hydrolase activity; B. subtilis transformants did not secrete the lelA gene product (LelA) into the extracellular medium. A 1.7-kb DNA fragment sufficient for lelA expression in Escherichia coli was sequenced. It contains a 548-codon open reading frame. The deduced amino acid sequence shows 54% identity with mature B. subtilis levanase and is similar to other fructanases and sucrases (beta-D-fructosyltransferases). Multiple-sequence alignment of 14 of these proteins revealed several previously unreported features. LelA appears to be a 512-amino-acid polypeptide containing no canonical signal peptide. The hydrolytic activities of LelA on sucrose, levan, and inulin were compared with those of B. subtilis levanase and sucrase, confirming that LelA is indeed a fructanase. The lelA gene in the chromosome of B. polymyxa was disrupted with a chloramphenicol resistance gene (cat) by "inter-gramic" conjugation: the lelA::cat insertion on a mobilizable plasmid was transferred from an E. coli transformant to B. polymyxa CF43, and B. polymyxa transconjugants containing the lelA::cat construct replacing the wild-type lelA gene in their chromosomes were selected directly. The growth of the mutant strain on levan, inulin, and sucrose was not affected.

Purification and characterisation of an Aspergillus niger invertase and its DNA sequence

A secreted invertase was purified 23-fold by ultrafiltration, ion-exchange, and gel filtration chromatography from the culture supernatant of 18 h sucrose-grown cultures of Aspergillus niger. The purified enzyme hydrolysed sucrose and raffinose but there was no detectable hydrolysis of inulin, melezitose or PNPG. Invertase activity was optimal at pH 5.5 and 50 degrees C. The molecular mass of reduced invertase was 115 kDa, as determined by SDS gel electrophoresis. The native molecular weight of between 225 kDa and 250 kDa, estimated by electrophoresis under non-denaturing conditions, suggests that the protein is a dimer of identical subunits. The suc1 gene encoding this protein was completely-sequenced. The translated sequence yields a protein of 566 amino acids with a calculated molecular mass of 61 kDa, suggesting that carbohydrates represent about 50% of the mass of the protein.

Sequence analysis of scrA and scrB from Streptococcus sobrinus 6715

The complete nucleotide sequences of Streptococcus sobrinus 6715 scrA and scrB, which encode sucrose-specific enzyme II of the phosphoenolpyruvate-dependent phosphotransferase system and sucrose-6-phosphate hydrolase, respectively, have been determined. These two genes were transcribed divergently, and the initiation codons of the two open reading frames were 192 bp apart. The transcriptional initiation sites were determined by primer extension analysis, and the putative promoter regions of these two genes overlapped partially. The gene encoding enzyme IIScr, scrA, contained 1,896 nucleotides, and the molecular mass of the predicted protein was 66,529 Da. The hydropathy plot of the predicted amino acid sequence indicated that enzyme IIScr was a relatively hydrophobic protein. The gene encoding sucrose-6-phosphate hydrolase, scrB, contained 1,437 nucleotides. The molecular mass of the predicted protein was 54,501 Da, and the encoded enzyme was hydrophilic. The predicted amino acid sequences of the two open reading frames exhibited approximately 45 and 70% identity with those encoded by scrA and scrB, respectively, from Streptococcus mutans GS5. Homology also was observed between the N-terminal region of the S. sobrinus 6715 enzyme IIScr and other enzyme IIs specific for the glucopyranoside molecule, all of which generate glucopyranoside-6-phosphate during translocation and phosphorylation of the respective substrates. The sequence of the C-terminal domain of the S. sobrinus 6715 enzyme IIScr shared significant homology with enzyme IIIGlc from Escherichia coli and Salmonella typhimurium and with the C-terminal domain of enzyme IIBgl from E. coli, indicating that the two functional domains, enzyme IIScr and enzyme IIIScr, were covalently linked as a single polypeptide in S. sobrinus 6715. The deduced amino acid sequence of the gene product of S. sobrinus scrB shared strong homology with sucrase from Bacillus subtilis, Klebsiella pneumoniae, and Vibrio alginolyticus, suggesting conservation based on the physiological roles of these proteins.

Molecular characterization of a fructanase produced by Bacteroides fragilis BF-1

The Bacteroides fragilis BF-1 fructanase-encoding gene (fruA) was cloned and expressed in Escherichia coli from the recombinant plasmid pBS100. The fruA gene consisted of 1,866 bp encoding a protein of 622 amino acids with a calculated M(r) of 70,286. The apparent M(r) of the fructanase, determined by in vitro cell-free transcription-translation and sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis, was approximately 71,500. An alignment of the amino acid sequences of the B. fragilis BF-1 fructanase and the Bacillus subtilis levanase revealed that 45.5% of the amino acids were identical. The fruA gene was expressed in E. coli from its own promoter; however, no E. coli promoter-like sequence was evident upstream from the gene. A major E. coli transcription start point and a single B. fragilis BF-1 transcription start point were located. Expression of the fruA gene was constitutive in E. coli(pBS100) and B. fragilis BF-1. The ratio of sucrase activity to inulinase activity (S/I ratio) was constant for enzyme preparations from E. coli (pBS100), indicating that both activities were associated with the fructanase. For B. fragilis BF-1, the S/I ratio varied considerably depending on the carbon source used for growth, suggesting that a separate sucrase is produced in addition to the fructanase in B. fragilis BF-1. Localization experiments and TnphoA mutagenesis indicated that the fructanase was exported to the periplasm. Sequence analysis of the N-terminal region of the fructanase revealed a putative 30-amino-acid signal peptide. The enzymatic properties of the purified fructanase were investigated. The enzyme was able to hydrolyze sucrose, raffinose, inulin, and levan but not melezitose, indicating that it was a beta-D-fructofuranosidase which was able to hydrolyze beta(2-->6)-linked fructans.

Characterization of a sucrase gene from Staphylococcus xylosus

The Staphylococcus xylosus gene scrB, encoding a sucrase, has been isolated from a genomic library of S. xylosus constructed in Escherichia coli. The gene was detected by its ability to confer utilization of the glucose and fructose residues of raffinose in an E. coli strain that is not able to metabolize galactose. It was found to reside within a 1.8-kb DNA fragment, the nucleotide sequence of which was determined. One large open reading frame, which is preceded by a ribosome binding site, is encoded on the fragment. Its deduced amino acid sequence yields a protein with a molecular mass of 57.377 kDa which shows significant homology with bacterial sucrose-6-phosphate hydrolases and sucrases. Overexpression of scrB in E. coli by the bacteriophage T7 polymerase promoter system resulted in the production of a protein with an apparent molecular mass of 58 kDa. Disruption of the scrB gene in the S. xylosus genome rendered S. xylosus unable to utilize sucrose. Thus, the ScrB sucrase is essential for sucrose metabolism in S. xylosus.

Sucrose fermentation by Fusobacterium mortiferum ATCC 25557: transport, catabolism, and products

Studies of sucrose utilization by Fusobacterium mortiferum ATCC 25557 have provided the first definitive evidence for phosphoenolpyruvate-dependent sugar:phosphotransferase activity in the family Bacteroidaceae. The phosphoenolpyruvate-dependent sucrose:phosphotransferase system and the two enzymes required for the dissimilation of sucrose 6-phosphate are induced specifically by growth of F. mortiferum on the disaccharide. Monomeric sucrose 6-phosphate hydrolase (M(r), 52,000) and a dimeric ATP-dependent fructokinase (subunit M(r), 32,000) have been purified to electrophoretic homogeneity. The physicochemical and catalytic properties of these enzymes have been examined, and the N-terminal amino acid sequences for both proteins are reported. The characteristics of sucrose 6-phosphate hydrolase and fructokinase from F. mortiferum are compared with the same enzymes from both gram-positive and gram-negative species. Butyric, acetic, and D-lactic acids are the end products of sucrose fermentation by F. mortiferum. A pathway is proposed for the translocation, phosphorylation, and metabolism of sucrose by this anaerobic pathogen.

Two generalized transducing phages in Vibrio parahaemolyticus and Vibrio alginolyticus

Two bacteriophages named phi VP253 and phi VP143 isolated after ultraviolet induction from lysogenic strains of Vibrio parahaemolyticus have been shown to be generalized transducing phages. So far, seven different auxotrophic markers of a V. parahaemolyticus strain could be transduced at the frequencies ranging from 2.2 x 10(-7) to 7.5 x 10(-5) per infected cell at the m.o.i. of approximately 1.0. The phage phi VP143, but not phi VP253, lysed 20 of the 28 strains of V. alginolyticus and the occurrence of generalized transduction by this phage in this Vibrio species has been confirmed. Molecular size of the genomes of both phages were estimated to be approximately 48 kb as judged from electrophoretic mobilities of the DNAs digested with HindIII endonuclease. The results and similarity of the two phages in morphology and other properties suggest very close relatedness of the phages.

Characterization of the major promoter for the plasmid-encoded sucrose genes scrY, scrA, and scrB

Sucrose genes from a Salmonella thompson plasmid were cloned in Escherichia coli K-12. A physical map and a genetic map of the genes were constructed, revealing strong homology with the scr regulon from the Salmonella typhimurium plasmid pUR400. Two promoters were examined after being subcloned into transcriptional fusion vectors. Primer extension analysis and site-directed mutagenesis were used to identify the precise location of the promoter of scrY, scrA, and scrB. Transcription from this promoter was regulated over a 1,000-fold range by the combined effects of ScrR-mediated repression and catabolite repression. A putative cyclic AMP receptor protein binding site centered 72.5 bp upstream of the start point of transcription of scrY appeared to be essential for full activity of the scrY promoter. Transcription from the putative scrK promoter was far less sensitive to repression by ScrR. In ScrR+ cells, readthrough transcription from the putative scrK promoter into scrY accounted for less than 10% of scrY expression.

Cloning, nucleotide sequence, and enzymatic characterization of an alpha-amylase from the ruminal bacterium Butyrivibrio fibrisolvens H17c

A Butyrivibrio fibrisolvens amylase gene was cloned and expressed by using its own promoter on the recombinant plasmid pBAMY100 in Escherichia coli. The amylase gene consisted of an open reading frame of 2,931 bp encoding a protein of 976 amino acids with a calculated Mr of 106,964. In E. coli(pBAMY100), more than 86% of the active amylase was located in the periplasm, and TnphoA fusion experiments showed that the enzyme had a functional signal peptide. The B. fibrisolvens amylase is a calcium metalloenzyme, and three conserved putative calcium-binding residues were identified. The amylase showed high sequence homology with other alpha-amylases in the three highly conserved regions which constitute the active centers. These and other conserved regions were located in the N-terminal half, and no similarity with any other amylase was detected in the remainder of the protein. Deletion of approximately 40% of the C-terminal portion of the amylase did not result in loss of amylolytic activity. The B. fibrisolvens amylase was identified as an endo-alpha-amylase by hydrolysis of the Phadebas amylase substrate, hydrolysis of gamma-cyclodextrin to maltotriose, maltose, and glucose and the characteristic shape of the blue value and reducing sugar curves. Maltotriose was the major initial hydrolysis product from starch, although extended incubation resulted in its hydrolysis to maltose and glucose.

Nucleotide sequence and analysis of the Vibrio alginolyticus scr repressor-encoding gene (scrR)

The nucleotide sequence of the Vibrio alginolyticus scr repressor-encoding gene (scrR) was determined. The deduced amino acid sequence of the scr repressor was homologous with the gal, lac and cyt repressors of Escherichia coli and contained a helix-turn-helix DNA binding domain. Although the scrR gene encoded a protein which was required for the regulation of the V. alginolyticus sucrose utilization system, a particular deletion in the scrR gene could not be complemented in trans. The lack of complementation was discussed in terms of the possible involvement of a cis regulatory element or interference by the truncated scr repressor.

Metronidazole activation and isolation of Clostridium acetobutylicum electron transport genes

An Escherichia coli F19 recA, nitrate reductase-deficient mutant was constructed by transposon mutagenesis and shown to be resistant to metronidazole. This mutant was a most suitable host for the isolation of Clostridium acetobutylicum genes on recombinant plasmids, which activated metronidazole and rendered the E. coli F19 strain sensitive to metronidazole. Twenty-five E. coli F19 clones containing different recombinant plasmids were isolated and classified into five groups on the basis of their sensitivity to metronidazole. The clones were tested for nitrate reductase, pyruvate-ferredoxin oxidoreductase, and hydrogenase activities. DNA hybridization and restriction endonuclease mapping revealed that four of the C. acetobutylicum insert DNA fragments on recombinant plasmids were linked in an 11.1-kb chromosomal fragment. DNA sequencing and amino acid homology studies indicated that this DNA fragment contained a flavodoxin gene which encoded a protein of 160 amino acids that activated metronidazole and made the E. coli F19 mutant very sensitive to metronidazole. The flavodoxin and hydrogenase genes which are involved in electron transfer systems were linked on the 11.1-kb DNA fragment from C. acetobutylicum.

Cloning and sequencing of the sacA gene: characterization of a sucrase from Zymomonas mobilis

The Zymomonas mobilis gene (sacA) encoding a protein with sucrase activity has been cloned in Escherichia coli and its nucleotide sequence has been determined. Potential ribosome-binding site and promoter sequences were identified in the region upstream of the gene which were homologous to E. coli and Z. mobilis consensus sequences. Extracts from E. coli cells, containing the sacA gene, displayed a sucrose-hydrolyzing activity. However, no transfructosylation activity (exchange reaction or levan formation) could be detected. This sucrase activity was different from that observed with the purified extracellular protein B46 from Z. mobilis. These two proteins showed different electrophoretic mobilities and molecular masses and shared no immunological similarity. Thus, the product of sacA (a polypeptide of 58.4-kDa molecular mass) is a new sucrase from Z. mobilis. The amino acid sequence, deduced from the nucleotide sequence of sacA, showed strong homologies with the sucrases from Bacillus subtilis, Salmonella typhimurium, and Vibrio alginolyticus.

Nucleotide sequence and analysis of the Vibrio alginolyticus sucrose uptake-encoding region

The nucleotide sequence of the Vibrio alginolyticus sucrose uptake-encoding region was determined, and contained two genes, scrA and scrK. The scrA gene encodes an enzyme IISucrose (EIIScr) protein of the phosphoenolpyruvate dependent phosphotransferase system and the scrK gene encodes a fructokinase. The deduced amino acid (aa) sequence for the V. alginolyticus EIIScr protein was homologous with the EIIScr proteins from Streptococcus mutans, Salmonella typhimurium (pUR400 system) and Bacillus subtilis. The deduced aa sequence for the V. alginolyticus fructokinase was homologous with the Escherichia coli enzymes, 6-phosphofructokinase (isoenzyme 2) and ribokinase. Transposon phoA mutagenesis experiments indicated that the EIIScr protein was a membrane-bound protein with a region that extended into the periplasm.

Sequencing and expression of a cellodextrinase (ced1) gene from Butyrivibrio fibrisolvens H17c cloned in Escherichia coli

The nucleotide sequence of a 2.314 kb DNA segment containing a gene (ced1) expressing cellodextrinase activity from Butyrivibrio fibrisolvens H17c was determined. The B. fibrisolvens H17c gene was expressed from a weak internal promoter in Escherichia coli and a putative consensus promoter sequence was identified upstream of a ribosome binding site and a GTG start codon. The complete amino acid sequence (547 residues) was deduced and homology was demonstrated with the Clostridium thermocellum endoglucanase D (EGD), Pseudomonas fluorescens var. cellulosa endoglucanase (EG), and a cellulase from the avocado fruit (Persea americana). The ced1 gene product Ced1 showed cellodextrinase activity and rapidly hydrolysed short-chain cellodextrins to yield either cellobiose or cellobiose and glucose as end products. The Ced1 enzyme released cellobiose from p-nitrophenyl-beta-D-cellobioside and the enzyme was not inhibited by methylcellulose, an inhibitor of endoglucanase activity. Although the major activity of the Ced1 enzyme was that of a cellodextrinase it also showed limited activity against endoglucanase specific substrates [carboxymethylcellulose (CMC), lichenan, laminarin and xylan]. Analysis by SDS-polyacrylamide gel electrophoresis with incorporated CMC showed a major activity band with an apparent Mr of approximately 61,000. The calculated Mr of the ced1 gene product was 61,023.

Nucleotide sequence of the celA gene encoding a cellodextrinase of Ruminococcus flavefaciens FD-1

The nucleotide sequence of a 3.6 kb DNA fragment containing a cellodextrinase gene (celA) from Ruminococcus flavefaciens FD-1 was determined. The gene was expressed from its own regulatory region in Escherichia coli and a putative consensus promoter sequence was identified upstream of a ribosome binding site and a TTG start codon. The complete amino acid sequence of the CelA enzyme (352 residues) was deduced and showed no significant homology to cellulases from other organisms. Two lysozyme-type active sites were found in the amino-terminal third of the enzyme. In E. coli the cloned Cel A protein was translocated into the periplasm. The lack of a typical signal sequence, and the results of transposon phoA mutagenesis experiments indicated that CelA is secreted by a mechanism other than a leader peptide.

Expression and regulation of a Bacteroides fragilis sucrose utilization system cloned in Escherichia coli

A Bacteroides fragilis strain isolated from human feces was the source of chromosomal DNA in the construction of plasmid pBS100. The cloned 6-kilobase insert of plasmid pBS100 conferred a sucrose positivity phenotype on transformed cells of Escherichia coli JA221. E. coli JA221(pBS100) cells were able to utilize sucrose as the sole source of carbon because of the presence of sucrase enzyme and sucrose uptake activities. Sucrase activity was inducible in B. fragilis but constitutive in E. coli JA221(pBS100) cells. In sucrose-minimal medium, both B. fragilis and E. coli JA221(pBS100) produced intracellular and extracellular sucrase activities throughout the growth cycle. Osmotic shock experiments performed on E. coli JA221(pBS100) indicated that up to 55% of the sucrase activity was localized in the periplasmic space, 30% was in the cytoplasm, and the remaining 15% was in the cell-free extracellular supernatant fluid. B. fragilis and E. coli JA221(pBS100) actively transported sucrose. Sucrose uptake was induced by sucrose in B. fragilis, whereas the uptake activity in E. coli JA221(pBS100) was constitutive. E. coli JA221(pBS100) appeared to transport sucrose by a phosphotransferase-independent system. B. fragilis transported sucrose only under strictly anaerobic conditions. No uptake activity was detected under aerobic conditions with or without addition of catalase.