Tonje Marita Bjerkan Heggeset

Genome Sequence of Thermotolerant Bacillus methanolicus: Features and Regulation Related to Methylotrophy and Production of l-Lysine and l-Glutamate from Methanol

ABSTRACT Bacillus methanolicus can utilize methanol as its sole carbon and energy source, and the scientific interest in this thermotolerant bacterium has focused largely on exploring its potential as a biocatalyst for the conversion of methanol into l-lysine and l-glutamate. We present here the genome sequences of the important B. methanolicus model strain MGA3 (ATCC 53907) and the alternative wild-type strain PB1 (NCIMB13113). The physiological diversity of these two strains was demonstrated by a comparative fed-batch methanol cultivation displaying highly different methanol consumption and respiration profiles, as well as major differences in their l-glutamate production levels (406 mmol liter−1 and 11 mmol liter−1, respectively). Both genomes are small (ca 3.4 Mbp) compared to those of other related bacilli, and MGA3 has two plasmids (pBM19 and pBM69), while PB1 has only one (pBM20). In particular, we focus here on genes representing biochemical pathways for methanol oxidation and concomitant formaldehyde assimilation and dissimilation, the important phosphoenol pyruvate/pyruvate anaplerotic node, the tricarboxylic acid cycle including the glyoxylate pathway, and the biosynthetic pathways for l-lysine and l-glutamate. Several unique findings were made, including the discovery of three different methanol dehydrogenase genes in each of the two B. methanolicus strains, and the genomic analyses were accompanied by gene expression studies. Our results provide new insight into a number of peculiar physiological and metabolic traits of B. methanolicus and open up possibilities for system-level metabolic engineering of this bacterium for the production of amino acids and other useful compounds from methanol.

Characterization of Three New Azotobacter vinelandii Alginate Lyases, One of Which Is Involved in Cyst Germination

Alginates are polysaccharides composed of 1-4-linked beta-D-mannuronic acid and alpha-L-guluronic acid. The polymer can be degraded by alginate lyases, which cleave the polysaccharide using a beta-elimination reaction. Two such lyases have previously been identified in the soil bacterium Azotobacter vinelandii, as follows: the periplasmic AlgL and the secreted bifunctional mannuronan C-5 epimerase and alginate lyase AlgE7. In this work, we describe the properties of three new lyases from this bacterium, AlyA1, AlyA2, and AlyA3, all of which belong to the PL7 family of polysaccharide lyases. One of the enzymes, AlyA3, also contains a C-terminal module similar to those of proteins secreted by a type I secretion system, and its activity is stimulated by Ca(2+). All three enzymes preferably cleave the bond between guluronic acid and mannuronic acid, resulting in a guluronic acid residue at the new reducing end, but AlyA3 also degrades the other three possible bonds in alginate. Strains containing interrupted versions of alyA1, alyA3, and algE7 were constructed, and their phenotypes were analyzed. Genetically pure alyA2 mutants were not obtained, suggesting that this gene product may be important for the bacterium during vegetative growth. After centrifugation, cultures from the algE7 mutants form a large pellet containing alginate, indicating that AlgE7 is involved in the release of alginate from the cells. Upon encountering adverse growth conditions, A. vinelandii will form a resting stage called cyst. Alginate is a necessary part of the protective cyst coat, and we show here that strains lacking alyA3 germinate poorly compared to wild-type cells.

Overexpression of Wild-Type Aspartokinase Increases l-Lysine Production in the Thermotolerant Methylotrophic Bacterium Bacillus methanolicus

ABSTRACT Aspartokinase (AK) controls the carbon flow into the aspartate pathway for the biosynthesis of the amino acids l-methionine, l-threonine, l-isoleucine, and l-lysine. We report here the cloning of four genes (asd, encoding aspartate semialdehyde dehydrogenase; dapA, encoding dihydrodipicolinate synthase; dapG, encoding AKI; and yclM, encoding AKIII) of the aspartate pathway in Bacillus methanolicus MGA3. Together with the known AKII gene lysC, dapG and yclM form a set of three AK genes in this organism. Overexpression of dapG, lysC, and yclM increased l-lysine production in wild-type B. methanolicus strain MGA3 2-, 10-, and 60-fold (corresponding to 11 g/liter), respectively, without negatively affecting the specific growth rate. The production levels of l-methionine (less than 0.5 g/liter) and l-threonine (less than 0.1 g/liter) were low in all recombinant strains. The AK proteins were purified, and biochemical analyses demonstrated that they have similar Vmax values (between 47 and 58 μmol/min/mg protein) and Km values for l-aspartate (between 1.9 and 5.0 mM). AKI and AKII were allosterically inhibited by meso-diaminopimelate (50% inhibitory concentration [IC50], 0.1 mM) and by l-lysine (IC50, 0.3 mM), respectively. AKIII was inhibited by l-threonine (IC50, 4 mM) and by l-lysine (IC50, 5 mM), and this enzyme was synergistically inhibited in the presence of both of these amino acids at low concentrations. The correlation between the impact on l-lysine production in vivo and the biochemical properties in vitro of the individual AK proteins is discussed. This is the first example of improving l-lysine production by metabolic engineering of B. methanolicus and also the first documentation of considerably increasing l-lysine production by overexpression of a wild-type AK.

Methylotrophic Bacillus methanolicus Encodes Two Chromosomal and One Plasmid Born NAD+ Dependent Methanol Dehydrogenase Paralogs with Different Catalytic and Biochemical Properties

Bacillus methanolicus can utilize methanol as the sole carbon source for growth and it encodes an NAD+-dependent methanol dehydrogenase (Mdh), catalyzing the oxidation of methanol to formaldehyde. Recently, the genomes of the B. methanolicus strains MGA3 (ATCC53907) and PB1 (NCIMB13113) were sequenced and found to harbor three different putative Mdh encoding genes, each belonging to the type III Fe-NAD+-dependent alcohol dehydrogenases. In each strain, two of these genes are encoded on the chromosome and one on a plasmid; only one chromosomal act gene encoding the previously described activator protein ACT was found. The six Mdhs and the ACT proteins were produced recombinantly in Escherichia coli, purified, and characterized. All Mdhs required NAD+ as cosubstrate, were catalytically stimulated by ACT, exhibited a broad and different substrate specificity range and displayed both dehydrogenase and reductase activities. All Mdhs catalyzed the oxidation of methanol; however the catalytic activity for methanol was considerably lower than for most other alcohols tested, suggesting that these enzymes represent a novel class of alcohol dehydrogenases. The kinetic constants for the Mdhs were comparable when acting as pure enzymes, but together with ACT the differences were more pronounced. Quantitative PCR experiments revealed major differences with respect to transcriptional regulation of the paralogous genes. Taken together our data indicate that the repertoire of methanol oxidizing enzymes in thermotolerant bacilli is larger than expected with complex mechanisms involved in their regulation.

Complete genome sequence of *Bacillus methanolicus* MGA3, a thermotolerant amino acid producing methylotroph

Bacillus methanolicus MGA3 was isolated from freshwater marsh soil and characterised as a thermotolerant and methylotrophic L-glutamate producer. The complete genome consists of a circular chromosome and the two plasmids pBM19 and pBM69. It includes genomic information about C1 metabolism and amino acid biosynthetic pathways.

Comparison of Different Strategies for Selection/Adaptation of Mixed Microbial Cultures Able to Ferment Crude Glycerol Derived from Second-Generation Biodiesel

Objective of this study was the selection and adaptation of mixed microbial cultures (MMCs), able to ferment crude glycerol generated from animal fat-based biodiesel and produce building-blocks and green chemicals. Various adaptation strategies have been investigated for the enrichment of suitable and stable MMC, trying to overcome inhibition problems and enhance substrate degradation efficiency, as well as generation of soluble fermentation products. Repeated transfers in small batches and fed-batch conditions have been applied, comparing the use of different inoculum, growth media, and Kinetic Control. The adaptation of activated sludge inoculum was performed successfully and continued unhindered for several months. The best results showed a substrate degradation efficiency of almost 100% (about 10 g/L glycerol in 21 h) and different dominant metabolic products were obtained, depending on the selection strategy (mainly 1,3-propanediol, ethanol, or butyrate). On the other hand, anaerobic sludge exhibited inactivation after a few transfers. To circumvent this problem, fed-batch mode was used as an alternative adaptation strategy, which led to effective substrate degradation and high 1,3-propanediol and butyrate production. Changes in microbial composition were monitored by means of Next Generation Sequencing, revealing a dominance of glycerol consuming species, such as Clostridium, Klebsiella, and Escherichia.

Combinatorial Mutagenesis and Selection of Improved Signal Sequences and Their Application for High-Level Production of Translocated Heterologous Proteins in Escherichia coli

ABSTRACT We previously designed the consensus signal peptide (CSP) and demonstrated that it can be used to strongly stimulate heterologous protein production in Escherichia coli. A comparative study using CSP and two bacterial signal sequences, pelB and ompA, showed that the effect of signal sequences on both expression level and translocation efficiency can be highly protein specific. We report here the generation of CSP mutant libraries by a combinatorial mutagenesis approach. Degenerated CSP oligonucleotides were cloned in frame with the 5′ end of the bla gene, encoding the mature periplasmic β-lactamase released from its native signal sequence. This novel design allows for a direct selection of improved signal sequences that positively affect the expression level and/or translocation efficiency of β-lactamase, based on the ampicillin tolerance level of the E. coli host cells. By using this strategy, 61 different CSP mutants with up to 8-fold-increased ampicillin tolerance level and up to 5.5-fold-increased β-lactamase expression level were isolated and characterized genetically. A subset of the CSP mutants was then tested with the alternative reporter gene phoA, encoding periplasmic alkaline phosphatase (AP), resulting in an up to 8-fold-increased production level of active AP protein in E. coli. Moreover, it was demonstrated that the CSP mutants can improve the production of the medically important human interferon α2b under high-cell-density cultivations. Our results show that there is a clear potential for improving bacterial signal sequences by using combinatorial mutagenesis, and bioinformatics analyses indicated that the beneficial mutations could not be rationally predicted.

Functional Characterization of Key Enzymes involved in l-Glutamate Synthesis and Degradation in the Thermotolerant and Methylotrophic Bacterium Bacillus methanolicus

ABSTRACT Bacillus methanolicus wild-type strain MGA3 secretes 59 g/liter−1 of l-glutamate in fed-batch methanol cultivations at 50°C. We recently sequenced the MGA3 genome, and we here characterize key enzymes involved in l-glutamate synthesis and degradation. One glutamate dehydrogenase (GDH) that is encoded by yweB and two glutamate synthases (GOGATs) that are encoded by the gltAB operon and by gltA2 were found, in contrast to Bacillus subtilis, which has two different GDHs and only one GOGAT. B. methanolicus has a glutamine synthetase (GS) that is encoded by glnA and a 2-oxoglutarate dehydrogenase (OGDH) that is encoded by the odhAB operon. The yweB, gltA, gltB, and gltA2 gene products were purified and characterized biochemically in vitro. YweB has a low Km value for ammonium (10 mM) and a high Km value for l-glutamate (250 mM), and the V max value is 7-fold higher for l-glutamate synthesis than for the degradation reaction. GltA and GltA2 displayed similar Km values (1 to 1.4 mM) and V max values (4 U/mg) for both l-glutamate and 2-oxoglutarate as the substrates, and GltB had no effect on the catalytic activities of these enzymes in vitro. Complementation assays indicated that GltA and not GltA2 is dependent on GltB for GOGAT activity in vivo. To our knowledge, this is the first report describing the presence of two active GOGATs in a bacterium. In vivo experiments indicated that OGDH activity and, to some degree, GOGAT activity play important roles in regulating l-glutamate production in this organism.

Genome-Based Genetic Tool Development for Bacillus methanolicus: Theta- and Rolling Circle-Replicating Plasmids for Inducible Gene Expression and Application to Methanol-Based Cadaverine Production

Bacillus methanolicus is a thermophilic methylotroph able to overproduce amino acids from methanol, a substrate not used for human or animal nutrition. Based on our previous RNA-seq analysis a mannitol inducible promoter and a putative mannitol activator gene mtlR were identified. The mannitol inducible promoter was applied for controlled gene expression using fluorescent reporter proteins and a flow cytometry analysis, and improved by changing the -35 promoter region and by co-expression of the mtlR regulator gene. For independent complementary gene expression control, the heterologous xylose-inducible system from B. megaterium was employed and a two-plasmid gene expression system was developed. Four different replicons for expression vectors were compared with respect to their copy number and stability. As an application example, methanol-based production of cadaverine was shown to be improved from 6.5 to 10.2 g/L when a heterologous lysine decarboxylase gene cadA was expressed from a theta-replicating rather than a rolling-circle replicating vector. The current work on inducible promoter systems and compatible theta- or rolling circle-replicating vectors is an important extension of the poorly developed B. methanolicus genetic toolbox, valuable for genetic engineering and further exploration of this bacterium.

Draft genome sequence of the docosahexaenoic acid producing thraustochytrid Aurantiochytrium sp. T66

Thraustochytrids are unicellular, marine protists, and there is a growing industrial interest in these organisms, particularly because some species, including strains belonging to the genus Aurantiochytrium, accumulate high levels of docosahexaenoic acid (DHA). Here, we report the draft genome sequence of Aurantiochytrium sp. T66 (ATCC PRA-276), with a size of 43 Mbp, and 11,683 predicted protein-coding sequences. The data has been deposited at DDBJ/EMBL/Genbank under the accession LNGJ00000000. The genome sequence will contribute new insight into DHA biosynthesis and regulation, providing a basis for metabolic engineering of thraustochytrids.