Characterization of transcription initiation and termination signals of the proteinase genes of Lactococcus lactis Wg2 and enhancement of proteolysis in L. lactis

Sequence and analysis of pBM02, a novel RCR cryptic plasmid from Lactococcus lactis subsp cremoris P8-2-47

This paper reports the complete nucleotide sequence of the 3.85 kbp plasmid pBM02 from Lactococcus lactis subsp. cremoris P8-2-47. Analysis of the sequence predicted six ORFs larger than 25 amino acids. They all were transcribed from the same strand and organized in two functional cassettes: the replication region and a putative mobilization region. In the replication region, two ORFs specifying proteins homologous to others found in some classes of rolling circle-replicating plasmids were encountered (copG and repB). In fact, single-stranded DNA was detected as a replication intermediate of pBM02. copG and repB, together with some upstream sequences, formed part of the minimal replication unit of the plasmid. Interestingly, pBM02 shared a 212 bp stretch with plasmids of the pWV01 type, in which the whole single-strand origin of replication is included. In the mobilization region, an ORF coding for a mobilization-like protein was present, preceded by a putative oriT sequence homologous to that of plasmid pMV158. The replicon of pBM02 is of the wide-host range type, and functions in both Gram-positive and Gram-negative bacteria, including Lactobacillus casei, Lactobacillus plantarum, Bacillus subtilis, and Escherichia coli.

Hydrophobic amino acid residues in the acceptor binding site are main determinants for reaction mechanism and specificity of cyclodextrin-glycosyltransferase

Cyclodextrin-glycosyltransferases (CGTases) (EC ) preferably catalyze transglycosylation reactions with glucosyl residues as acceptor, whereas the homologous alpha-amylases catalyze hydrolysis reactions using water as acceptor. This difference in reaction specificity is most likely caused by the acceptor binding site. To investigate this in detail we altered the acceptor site residues Lys-232, Phe-183, Phe-259, and Glu-264 of Bacillus circulans strain 251 CGTase using site-directed mutagenesis. Lys-232 is of general importance for catalysis, which appears to result mainly from stabilization of the conformation of the loop containing the catalytic nucleophile Asp-229 and His-233, a residue that has been implied in transition state stabilization. Glu-264 contributes to the disproportionation reaction only, where it is involved in initial binding of the (maltose) acceptor. Phe-183 and Phe-259 play important and distinct roles in the transglycosylation reactions catalyzed by CGTase. Mutation of Phe-183 affects especially the cyclization and coupling reactions, whereas Phe-259 is most important for the cyclization and disproportionation reactions. Moreover, the hydrophobisity of Phe-183 and Phe-259 limits the hydrolyzing activity of the enzyme. Hydrolysis can be enhanced by making these residues more polar, which concomitantly results in a lower transglycosylation activity. A double mutant was constructed that yielded an enzyme preferring hydrolysis over cyclization (15:1), whereas the wild type favors cyclization over hydrolysis (90:1).

Transcriptional pattern of genes coding for the proteolytic system of Lactococcus lactis and evidence for coordinated regulation of key enzymes by peptide supply

The transcription of 16 genes encoding 12 peptidases (pepC, pepN, pepX, pepP, pepA, pepF2, pepDA1, pepDA2, pepQ, pepT, pepM, and pepO1), P(I) and P(III) proteinases (prtP1 and prtP3), and three transport systems (dtpT, dtpP, and opp-pepO1) of Lactococcus lactis MG1363 was analyzed in response to different environmental factors. Promoter fusions with luciferase reporter genes and/or mRNA analysis were used to study the effects of sugar sources, growth at 37 degrees C, and peptide supply on the transcription of these genes. Only transcription of the pepP gene is modulated by the source of sugar. The presence of potential catabolite-responsive element (CRE) boxes in its promoter region suggests that expression of this gene is directly controlled by catabolic repression. Elevated temperature had no significant effect on the level of transcription of these genes. prtP1, prtP3, pepC, pepN, pepX, and the opp-pepO1 operon are the most highly expressed genes in chemically defined medium, and their expression is repressed 5- to 150-fold by addition of peptide sources such as Casitone in the medium. Moreover, the transcription of prtP1, prtP3, pepC, pepN, and the opp-pepO1 operon is repressed two- to eight-fold by the dipeptides leucylproline and prolylleucine. The transcription of pepDA2 might also be repressed by the peptide sources, but this effect is not observed on the regulation of dtpT, pepP, pepA, pepF2, pepDA1, pepQ, pepT, pepM, and the dtpP operon. The significance of these results with respect to the functions of different components of the proteolytic system in L. lactis are discussed.

The role of arginine 47 in the cyclization and coupling reactions of cyclodextrin glycosyltransferase from Bacillus circulans strain 251 implications for product inhibition and product specificity

Cyclodextrin glycosyltransferase (CGTase) (EC 2.4.1.19) is used for the industrial production of cyclodextrins. Its application, however, is hampered by the limited cyclodextrin product specificity and the strong inhibitory effect of cyclodextrins on CGTase activity. Recent structural studies have identified Arg47 in the Bacillus circulans strain 251 CGTase as an active-site residue interacting with cyclodextrins, but not with linear oligosaccharides. Arg47 thus may specifically affect CGTase reactions with cyclic substrates or products. Here we show that mutations in Arg47 (to Leu or Gln) indeed have a negative effect on the cyclization and coupling activities; Arg47 specifically stabilizes the oligosaccharide chain in the transition state for these reactions. As a result, the mutant proteins display a shift in product specificity towards formation of larger cyclodextrins. As expected, both mutants also showed lower affinities for cyclodextrins in the coupling reaction, and a reduced competitive (product) inhibition of the disproportionation reaction by cyclodextrins. Both mutants also provide valuable information about the processes taking place during cyclodextrin production assays. Mutant Arg47-->Leu displayed an increased hydrolyzing activity, causing accumulation of increasing amounts of short oligosaccharides in the reaction mixture, which resulted in lower final amounts of cyclodextrins produced from starch. Interestingly, mutant Arg47-->Gln displayed an increased ratio of cyclization/coupling and a decreased hydrolyzing activity. Due to the decreased coupling activity, which especially affects the production of larger cyclodextrins, this CGTase variant produced the various cyclodextrins in a stable ratio in time. This feature is very promising for the industrial application of CGTase enzymes with improved product specificity.

The raw starch binding domain of cyclodextrin glycosyltransferase from Bacillus circulans strain 251

The E-domain of cyclodextrin glycosyltransferase (CGTase) (EC 2.4.1.19) from Bacillus circulans strain 251 is a putative raw starch binding domain. Analysis of the maltose-dependent CGTase crystal structure revealed that each enzyme molecule contained three maltose molecules, situated at contact points between protein molecules. Two of these maltoses were bound to specific sites in the E-domain, the third maltose was bound at the C-domain. To delineate the roles in raw starch binding and cyclization reaction kinetics of the two maltose binding sites in the E-domain, we replaced Trp-616 and Trp-662 of maltose binding site 1 and Tyr-633 of maltose binding site 2 by alanines using site-directed mutagenesis. Purified mutant CGTases were characterized with respect to raw starch binding and cyclization reaction kinetics on both soluble and raw starch. The results show that maltose binding site 1 is most important for raw starch binding, whereas maltose binding site 2 is involved in guiding linear starch chains into the active site. beta-Cyclodextrin causes product inhibition by interfering with catalysis in the active site and the function of maltose binding site 2 in the E-domain. CGTase mutants in the E-domain maltose binding site 1 could no longer be crystallized as maltose-dependent monomers. Instead, the W616A mutant CGTase protein was successfully crystallized as a carbohydrate-independent dimer; its structure has been refined to 2.2 A resolution. The three-dimensional structure shows that, within the error limits, neither the absence of carbohydrates nor the W616A mutation caused significant further conformational changes. The modified starch binding and cyclization kinetic properties observed with the mutant CGTase proteins thus can be directly related to the amino acid replacements.

Identical transcriptional control of the divergently transcribed prtP and prtM genes that are required for proteinase production in lactococcus lactis SK11

We have investigated transcriptional regulation of the divergently transcribed genes required for proteinase production (prtP and prtM) of Lactococcus lactis SK11. Their promoters partially overlap and are arranged in a face-to-face configuration. The medium-dependent activities of both prtP and prtM promoters were analyzed by quantitative primer extension studies and beta-glucuronidase assays with L. lactis MG1363 cells harboring transcriptional gene fusions of each promoter with the promoterless beta-glucuronidase gene (gusA) from Escherichia coli. High-level production of prtP- or prtM-specific mRNAs was found after the growth of cells in media with low peptide concentrations, while increases in peptide concentrations resulted in an approximately eightfold decrease in mRNA production. Furthermore, prtP and prtM promoters exhibited similar efficiencies under different growth conditions. Deletion analysis of the prt promoter region showed that all the information needed for full activity and regulation of the prtP and prtM promoters is retained within a 90-bp region which includes both transcription initiation sites. An inverted repeat sequence positioned around the prtP and prtM transcription initiation sites was disrupted by either deletion or insertion of a small DNA sequence to analyze their effects on the activities of both prtP and prtM promoters. The mutations affected the activities of these promoters only marginally at low peptide concentrations but resulted in 1.5- to 5-fold derepression at high peptide concentrations. These results indicate that the expression of both prtM and prtP genes is controlled in an identical manner via a control mechanism capable of repressing transcription initiation at high peptide concentrations.

Site-directed mutations in tyrosine 195 of cyclodextrin glycosyltransferase from Bacillus circulans strain 251 affect activity and product specificity

Tyrosine 195 is located in the center of the active site cleft of cyclodextrin glycosyltransferase (EC 2.4.1.19) from Bacillus circulans strain 251. Alignment of amino acid sequences of CGTases and alpha-amylases, and the analysis of the binding mode of the substrate analogue acarbose in the active site cleft [Strokopytov, B., et al. (1995) Biochemistry 34, (in press)], suggested that Tyr195 plays an important role in cyclization of oligosaccharides. Tyr195 therefore was replaced with Phe (Y195F), Trp (Y195W), Leu (Y195L), and Gly (Y195G). Mutant proteins were purified and crystallized, and their X-ray structures were determined at 2.5-2.6 angstrum resolution, allowing a detailed comparison of their biochemical properties and three-dimensional structures with those of the wild-type CGTase protein. The mutant proteins possessed significantly reduced cyclodextrin forming and coupling activities but were not negatively affected in the disproportionation and saccharifying reactions. Also under production process conditions, after a 45 h incubation with a 10% starch solution, the Y195W, Y195L, and Y195G mutants showed a lower overall conversion of starch into cyclodextrins. These mutants produced a considerable amount of linear maltooligosaccharides. The presence of aromatic amino acids (Tyr or Phe) at the Tyr195 position thus appears to be of crucial importance for an efficient cyclization reaction, virtually preventing the formation of linear products. Mass spectrometry of the Y195L reaction mixture, but not that of the other mutants and the wild type, revealed a shift toward the synthesis (in low yields) of larger products, especially of beta- and gamma- (but no alpha-) cyclodextrins and minor amounts of delta-, epsilon-, zeta- and eta-cyclodextrins.(ABSTRACT TRUNCATED AT 250 WORDS)

Identification and characterization of the alpha-acetolactate synthase gene from Lactococcus lactis subsp. lactis biovar diacetylactis

The conversion of 3-13C-labelled pyruvate in an acetoin-producing clone from a Lactococcus lactis subsp. lactis biovar diacetylactis strain DSM 20384 plasmid bank in Escherichia coli was studied by 13C nuclear magnetic resonance analysis. The results showed that alpha-acetolactate was the first metabolic product formed from pyruvate, whereas acetoin appeared at a much slower rate and reached only low concentrations. This alpha-acetolactate production shows that the cells express the gene for alpha-acetolactate synthase (als). Nucleotide sequence analysis identified an open reading frame encoding a protein of 554 amino acids. The deduced amino acid sequence exhibits extensive similarities to those of known alpha-acetolactate synthases from both prokaryotes and eukaryotes. The als gene is expressed on a monocistronic transcriptional unit, which is transcribed from a promoter located just upstream of the coding region.