Expression Vector for Zymomonas mobilis

Complete sequences of conjugal helper plasmids pRK2013 and pEVS104

We present the complete sequences of two commonly used conjugal helper plasmids: pRK2013 and pEVS104. These sequences will enable engineering of custom helper plasmids, for example, with different antibiotic markers or origins of replication. We provide both sequence information and plasmid maps to aid future engineering efforts.

Zymomonas mobilis as a model system for production of biofuels and biochemicals

Zymomonas mobilis is a natural ethanologen with many desirable industrial biocatalyst characteristics. In this review, we will discuss work to develop Z. mobilis as a model system for biofuel production from the perspectives of substrate utilization, development for industrial robustness, potential product spectrum, strain evaluation and fermentation strategies. This review also encompasses perspectives related to classical genetic tools and emerging technologies in this context.

pZMO7-Derived shuttle vectors for heterologous protein expression and proteomic applications in the ethanol-producing bacterium Zymomonas mobilis

BACKGROUND

The ethanol-producing bacterium Zymomonas mobilis has attracted considerable scientific and commercial interest due to its exceptional physiological properties. Shuttle vectors derived from native plasmids have previously been successfully used for heterologous gene expression in this bacterium for a variety of purposes, most notably for metabolic engineering applications.

RESULTS

A quantitative PCR (qPCR) approach was used to determine the copy numbers of two endogenous double stranded DNA plasmids: pZMO1A (1,647 bp) and pZMO7 (pZA1003; 4,551 bp) within the NCIMB 11163 strain of Z. mobilis. Data indicated pZMO1A and pZMO7 were present at ca. 3-5 and ca. 1-2 copies per cell, respectively. A ca. 1,900 bp fragment from plasmid pZMO7 was used to construct two Escherichia coli - Z. mobilis shuttle vectors (pZ7C and pZ7-184). The intracellular stabilities and copy numbers of pZ7C and pZ7-184 were characterized within the NCIMB 11163, ATCC 29191 and (ATCC 10988-derived) CU1 Rif2 strains of Z. mobilis. Both shuttle vectors could be stably maintained within the ATCC 29191 strain (ca. 20-40 copies per cell), and the CU1 Rif2 strain (ca. 2-3 copies per cell), for more than 50 generations in the absence of an antibiotic selectable marker. A selectable marker was required for shuttle vector maintenance in the parental NCIMB 11163 strain; most probably due to competition for replication with the endogenous pZMO7 plasmid molecules. N-terminal glutathione S-transferase (GST)-fusions of four endogenous proteins, namely the acyl-carrier protein (AcpP); 2-dehydro-3-deoxyphosphooctonate aldolase (KdsA); DNA polymerase III chi subunit (HolC); and the RNA chaperone protein Hfq; were successfully expressed from pZ7C-derived shuttle vectors, and their protein-protein binding interactions were analyzed in Z. mobilis ATCC 29191. Using this approach, proteins that co-purified with AcpP and KdsA were identified.

CONCLUSIONS

We show that a shuttle vector-based protein affinity 'pull-down' approach can be used to probe protein interaction networks in Z. mobilis cells. Our results demonstrate that protein expression plasmids derived from pZMO7 have significant potential for use in future biological or biotechnological applications within Z. mobilis.

Sorbitol production using recombinant Zymomonas mobilis strain

A recombinant Zymomonas mobilis strain harboring the plasmid pHW20a-gfo for over-expression of glucose-fructose oxidoreductase (GFOR) was constructed. The specific activity of GFOR enzyme in the new recombinant strain was at least two folds greater than that in the wild strain. The maximum GFOR activity achieved in terms of the volumetric, and the cellular were 2.59 U ml(-1), and 0.70 U mg(-1), respectively, in the batch cultures. A significant improvement of the bioconversion process for the production of sorbitol and gluconic acid from glucose and fructose was made using divalent metal ions which drastically reduced the ethanol yield and significantly increased the yield of target product. Among several divalent metal ions evaluated, Zn(2+) was found to be most effective by inhibiting the Entner-Doudoroff pathway enzymes. The yield of the byproduct ethanol was reduced from 16.7 to 1.8 gl(-1) and the sorbitol yield was increased to almost 100% from 89%. The Ca(2+) enhanced the sorbitol yield and the formation of calcium gluconate salt made the separation of gluconate from the reaction system easier.

Over-expression of xylulokinase in a xylose-metabolising recombinant strain of Zymomonas mobilis

The broad host range vector pBBR1MCS-2 has been evaluated as an expression vector for Zymomonas mobilis. The transformation efficiency of this vector was 2 x 10(3) CFU per mug of DNA in a recombinant strain of Z. mobilis ZM4/AcR containing the plasmid pZB5. Stable replication for this expression vector was demonstrated for 50 generations. This vector was used to study xylose metabolism in acetate resistant Z. mobilis ZM4/AcR (pZB5) by over-expression of xylulokinase (XK), as previous studies had suggested that XK could be the rate-limiting enzyme for such strains. Based on the above vector, a recombinant plasmid pJX1 harboring xylB (expressing XK) under control of a native Z. mobilis promotor Ppdc was constructed. When this plasmid was introduced into ZM4/AcR (pZB5) a 3-fold higher XK expression was found compared to the control strain. However, fermentation studies with ZM4/AcR (pZB5, pJX1) on xylose medium did not result in any increase in rate of growth or xylose metabolism, suggesting that XK expression was not rate-limiting for ZM4/AcR (pZB5) and related strains.

Kinetics of Sugar Transport and Phosphorylation Influence Glucose and Fructose Cometabolism by Zymomonas mobilis

The competitive inhibition of fructokinase by glucose has been proposed as the mechanism by which Zymomonas mobilis preferentially consumes glucose from mixtures of glucose and fructose and accumulates fructose when growing on sucrose. In this study, incorporation of radioactive fructose into biomass was used as a measure of fructose catabolism. It was determined that the rate of fructose incorporation by Z. mobilis CP4 was somewhat lower in the presence of an equimolar concentration of glucose but that the inhibition of fructokinase by glucose was not nearly as severe in vivo as was predicted from in vitro studies. Interestingly, addition of glucose to a culture of Z. mobilis CP4-M2, a glucokinaseless mutant, resulted in an immediate and nearly complete inhibition of fructose incorporation. Furthermore, addition of nonmetabolizeable glucose analogs had a similar effect on fructose catabolism by the wild-type Z. mobilis CP4, and fructose uptake by Z. mobilis CP4-M2 was shown to be severely inhibited by equimolar amounts of glucose. These results suggest that competition for fructose transport plays an important role in preferential catabolism of glucose from sugar mixtures. Indeed, the apparent K(infm) values for sugar uptake by Z. mobilis CP4 were approximately 200 mM for fructose and 13 mM for glucose. Other experiments supported the conclusion that a single facilitated diffusion transport system, encoded by the glf gene, is solely responsible for the uptake of both glucose and fructose. The results are discussed with regard to the hypothesis that the kinetics of sugar transport and phosphorylation allow the preferential consumption of glucose and accumulation of fructose, making the fructose available for the enzyme glucose-fructose oxidoreductase, which forms sorbitol, an important osmoprotectant for Z. mobilis when growing in the presence of high sugar concentrations.

Construction of an integrative shuttle vector for Zymomonas mobilis

An integrative shuttle vector, pZMOCP1, was constructed by ligating EcoRV digests of the plasmid cloning vector pBluescript and pZMP1, a cryptic plasmid of Zymomonas mobilis PROIMI A1. The 7.2-kb plasmid pZMOCP1 replicated in Escherichia coli and could also be transferred from this host by electroporation to Z. mobilis ATCC 29191. The transformants were selected by ampicillin resistance. The integrative characteristic was detected by hybridization in situ. The vector was stably maintained in Z. mobilis after 200 generations without selective pressure.

Cloning, sequencing, and expression of the Zymomonas mobilis phosphoglycerate mutase gene (pgm) in Escherichia coli

Phosphoglycerate mutase is an essential glycolytic enzyme for Zymomonas mobilis, catalyzing the reversible interconversion of 3-phosphoglycerate and 2-phosphoglycerate. The pgm gene encoding this enzyme was cloned on a 5.2-kbp DNA fragment and expressed in Escherichia coli. Recombinants were identified by using antibodies directed against purified Z. mobilis phosphoglycerate mutase. The pgm gene contains a canonical ribosome-binding site, a biased pattern of codon usage, a long upstream untranslated region, and four promoters which share sequence homology. Interestingly, adhA and a D-specific 2-hydroxyacid dehydrogenase were found on the same DNA fragment and appear to form a cluster of genes which function in central metabolism. The translated sequence for Z. mobilis pgm was in full agreement with the 40 N-terminal amino acid residues determined by protein sequencing. The primary structure of the translated sequence is highly conserved (52 to 60% identity with other phosphoglycerate mutases) and also shares extensive homology with bisphosphoglycerate mutases (51 to 59% identity). Since Southern blots indicated the presence of only a single copy of pgm in the Z. mobilis chromosome, it is likely that the cloned pgm gene functions to provide both activities. Z. mobilis phosphoglycerate mutase is unusual in that it lacks the flexible tail and lysines at the carboxy terminus which are present in the enzyme isolated from all other organisms examined.

Mutational analysis of segmental stabilization of transcripts from the Zymomonas mobilis gap-pgk operon

In Zymomonas mobilis, the genes encoding glyceraldehyde-3-phosphate dehydrogenase and phosphoglycerate kinase are transcribed together from the gap-pgk operon. However, higher levels of the former enzyme are present in the cytoplasm because of increased stability of a 5' segment containing the gap coding region. This segment is bounded by an upstream untranslated region which can be folded into many stem-loop structures and a prominent intercistronic stem-loop. Mutations eliminating a proposed stem-loop in the untranslated region or the intercistronic stem-loop resulted in a decrease in the stability and pool size of the 5' gap segment. Site-specific mutations in the unpaired regions of both of these stems also altered the message pools. Elimination of the intercistronic stem appeared to reduce the endonucleolytic cleavage within the pgk coding region, increasing the stability and abundance of the full-length message. DNA encoding the prominent stem-loop at the 3' end of the message was shown to be a transcriptional terminator both in Z. mobilis and in Escherichia coli. This third stem-loop region (part of the transcriptional terminator) was required to stabilize the full-length gap-pgk message.

Zymomonas mobilis--science and industrial application

Zymomonas mobilis is undoubtedly one of the most unique bacterium within the microbial world. Known since 1912 under the names Termobacterium mobilis, Pseudomonas linderi, and Zymomonas mobilis, reviews on its uniqueness have been published in 1977 and 1988. The bacterium Zymomonas mobilis not only exhibits an extraordinarily uniqueness in its biochemistry, but also in its growth behavior, energy production, and response to culture conditions, as well as cultivation techniques used. This uniqueness caused great interest in the scientific, biotechnological, and industrial worlds. Its ability to couple and uncouple energy production in favor of product formation, to respond to physical and chemical environment manipulation, as well as its restricted product formation, makes it an ideal microorganism for microbial process development. This review explores the advances made since 1987, together with new developments in the pure scientific and applied commercial areas.

Molecular characterization of the Zymomonas mobilis enolase (eno) gene

The Zymomonas mobilis gene encoding enolase was cloned by genetic complementation of an Escherichia coli eno mutant. An enzyme assay and sodium dodecyl sulfate-polyacrylamide gel electrophoresis confirmed the overexpression of enolase in E. coli clones carrying the Z. mobilis eno gene. The eno gene is present in a single copy of the Z. mobilis genome. Nucleotide sequence analysis of the eno region revealed an open reading frame of 1,293 bp that encodes a protein of 428 amino acids with a predicted molecular weight of 45,813. Comparison of the sequence of Z. mobilis enolase with primary amino acid sequences for other enolases indicates that the enzyme is highly conserved. Unlike all of the previously studied glycolytic genes from Z. mobilis that possess canonical ribosome binding sites, the eno gene is preceded by a modest Shine-Dalgarno sequence. The transcription initiation site was mapped by primer extension and found to be located within a 115-bp sequence that is 55.7% identical to a highly conserved consensus sequence found within the regulatory regions of highly expressed Z. mobilis genes. Northern RNA blot analysis revealed that eno is encoded on a 1.45-kb transcript. The half-life of the eno mRNA was determined to be 17.7 +/- 1.7 min, indicating that it is unusually stable. The abundance of the eno message is proposed to account for enolase being the most prevalent protein in Z. mobilis.

Cloning, sequence analysis, and expression of the structural gene encoding glucose-fructose oxidoreductase from Zymomonas mobilis

The gene encoding glucose-fructose oxidoreductase (gfo) from Zymomonas mobilis was cloned in Escherichia coli and sequenced. An open reading frame of 439 amino acids encoded a protein of 49 kDa. A leader sequence of 52 amino acids preceded the N-terminal sequence of the enzyme, indicating cleavage of the precursor protein at an Ala-Ala site to give rise to an active form of the enzyme of 43 kDa. Processing of the glucose-fructose oxidoreductase leader sequence, although not complete, was demonstrated in an in vitro translation system. The two Z. mobilis promoters of the gfo gene show considerable homology to other highly expressed Z. mobilis genes (pdc, adhB, gap, and pgk) as well as to the E. coli consensus sequence. Although translation of the gfo gene was demonstrated in vitro in an E. coli S30 coupled transcription-translation system, a functional stable protein was not produced in the E. coli clone. However, the gfo gene cloned into a shuttle vector was shown to overexpress glucose-fructose oxidoreductase to levels of up to 6% of the soluble protein in Z. mobilis.

Gel electrophoretic analysis of Zymomonas mobilis glycolytic and fermentative enzymes: identification of alcohol dehydrogenase II as a stress protein

The 13 major enzymes which compose the glycolytic and fermentative pathways in Zymomonas mobilis are particularly abundant and represent one-half of the soluble protein in exponential-phase cells. One- and two-dimensional polyacrylamide gel electrophoresis maps were developed for 12 of these enzymes. Assignments were made by comigration with purified proteins, comparison with overexpressed genes in recombinant strains, and Western blots (immunoblots). Although most glycolytic enzymes appeared resistant to turnover and accumulated in stationary-phase cells, the protein levels of pyruvate kinase, alcohol dehydrogenase I, and glucokinase declined. Alcohol dehydrogenase II was identified as a major stress protein and was induced both by exposure to ethanol and by elevated temperature (45 degrees C). This enzyme, encoded by the adhB gene, is expressed from tandem promoters which share partial sequence identity with the Escherichia coli consensus sequence for heat shock proteins.

Cloning, characterization, and nucleotide sequence analysis of a Zymomonas mobilis phosphoglucose isomerase gene that is subject to carbon source-dependent regulation

The Zymomonas mobilis gene encoding phosphoglucose isomerase (pgi) was cloned by genetic complementation of an Escherichia coli pgi mutant. An enzyme assay and sodium dodecyl sulfate-polyacrylamide gel electrophoresis confirmed the presence of excess amounts of phosphoglucose isomerase in E. coli clones carrying the Z. mobilis pgi gene. The pgi gene is present in only one copy on the Z. mobilis genome. Nucleotide sequence analysis of the pgi region revealed an open reading frame of 1,524 bp preceded by a strong Shine-Dalgarno sequence. The pgi gene encodes a 507-amino-acid protein with a predicted molecular weight of 55,398. Z. mobilis phosphoglucose isomerase is between 38 and 43% identical to the enzyme from other species. Northern (RNA) blot analysis showed that the pgi transcript is 1.8 kb in length. The level of the pgi transcript was found to be influenced by the phase of growth and by the carbon and energy sources. Transcript levels increased with respect to total RNA during logarithmic growth and were threefold higher when grown on fructose than on glucose. These changes in transcript levels paralleled phosphoglucose isomerase activities in the cultures. Differential mRNA stability was not a factor, since the half-life of the pgi transcript was 6.3 min in glucose-grown cells and 6.0 min in fructose-grown cells. Thus, an increase in the rate of transcription appears to be at least partially responsible for the increased levels of phosphoglucose isomerase observed for Z. mobilis grown on fructose.

Expression of an L-alanine dehydrogenase gene in Zymomonas mobilis and excretion of L-alanine

An approach to broaden the product range of the ethanologenic, gram-negative bacterium Zymomonas mobilis by means of genetic engineering is presented. Gene alaD for L-alanine dehydrogenase (EC 1.4.1.1.) from Bacillus sphaericus was cloned and introduced into Z. mobilis. Under the control of the strong promoter of the pyruvate decarboxylase (pdc) gene, the enzyme was expressed up to a specific activity of nearly 1 mu mol . min -1 . mg of protein -1 in recombinant cells. As a results of this high L-alanine dehydrogenase activity, growing cells excreted up to 10 mmol of alanine per 280 mmol of glucose utilized into a mineral salts medium. By the addition of 85 mM NH4+ to the medium, growth of the recombinant cells stopped, and up to 41 mmol alanine was secreted. As alanine dehydrogenase competed with pyruvate decarboxylase (PDC) (EC 4.1.1.1.) for the same substrate (pyruvate), PDC activity was reduced by starvation for the essential PDC cofactor thiamine PPi. A thiamine auxotrophy mutant of Z. mobilis which carried the alaD gene was starved for 40 h in glucose-supplemented mineral salts medium and then shifted to mineral salts medium with 85 mM NH4+ and 280 mmol of glucose. The recombinants excreted up to 84 mmol of alanine (7.5 g/liter) over 25 h. Alanine excretion proceeded at an initial velocity of 238 nmol . min-1 . mg [dry weight]-1. Despite this high activity, the excretion rate seemed to be a limiting factor, as the intracellular concentration of alanine was as high as 260 mM at the beginning of the excretion phase and decreased to 80 to 90 mM over 24 h.

Segmental message stabilization as a mechanism for differential expression from the Zymomonas mobilis gap operon

In Zymomonas mobilis, three- to fourfold more glyceraldehyde-3-phosphate dehydrogenase protein than phosphoglycerate kinase is needed for glycolysis because of differences in catalytic efficiency. Consistent with this requirement, higher levels of glyceraldehyde-3-phosphate dehydrogenase were observed with two-dimensional polyacrylamide gel electrophoresis. The genes encoding these enzymes (gap and pgk, respectively) form a bicistronic operon, and some form of regulation is required to provide this differential expression. Two transcripts were observed in Northern RNA analyses with segments of gap as a probe: a more abundant 1.2-kb transcript that contained gap alone and a 2.7-kb transcript that contained both genes. Based on the relative amounts of these transcripts, the coding regions for glyceraldehyde-3-phosphate dehydrogenase were calculated to be fivefold more abundant than those for phosphoglycerate kinase. Assuming equal translational efficiency, this is sufficient to provide the observed differences in expression. Operon fusions with lacZ provided no evidence for intercistronic terminators or attenuation mechanisms. Both gap operon messages were very stable, with half-lives of approximately 16 min (1.2-kb transcript) and 7 min (2.7-kb transcript). Transcript mapping and turnover studies indicated that the shorter gap message was a stable degradation product of the full-length message. Thus differential expression of gap and pgk results primarily from increased translation of the more stable 5' segment of the transcript containing gap. The slow turnover of the messages encoding glyceraldehyde-3-phosphate dehydrogenase and phosphoglycerate kinase is proposed as a major feature contributing to the high level of expression of these essential enzymes.

Construction of expression vectors for the gram-negative bacterium Zymomonas mobilis

A set of vectors was constructed for the cloning and expression of heterologous genes in the Gram-negative bacterium Zymomonas mobilis under the control of the pdc promoter of Z. mobilis. The vectors pPTZ1, pPTZ3, and pPTZ4 are based on the cryptic Z. mobilis plasmid pZM02 and on parts of the Escherichia coli plasmids pKK223-3 and pBR322 together with the multiple cloning site of phage M13mp18. DNA fragments can be readily inserted immediately downstream from the pdc promoter at unique restriction sites for KpnI, XbaI and PstI in pPTZ1 and additionally for SmaI and BamHI in pPTZ3. In pPTZ4, the 5' terminal codons of pdc were deleted allowing the formation of gene fusions. Expression of a promoterless chloramphenicol acetyltransferase gene (cat) controlled by the pdc gene promoter resulted in enzyme activities of up to 5.5 U/mg total cell protein in Z. mobilis cells.

Genetic modification of Zymomonas mobilis

The bacterium Zymomonas mobilis is a potentially useful organism for the commercial production of ethanol as it is capable of more than double the rate of alcohol production by yeast. However, industrial application of this bacterium has been restricted in part due to the disadvantages of its limited substrate range (glucose, fructose and sucrose) and by-product formation. Progress in strain improvement and genetic manipulation of this ethanologen is reviewed. Methodologies for gaining reproducible gene transfer in Z. mobilis have recently been developed. Genetic modification has led to its growth on the additional substrates lactose and mannitol. Additionally, a range of by-product negative mutants have also been isolated. Further interest has focused on transfer of Z. mobilis genes to other fermentive organisms in order to gain enhanced product formation. Overall, these genetic approaches should lead to development of novel strains of Z. mobilis and other genera, capable of the use of starch, cellulose and xylan in a manner attractive for industrial ethanol production, besides facilitating over production of products from E. coli strains with enhanced capability to grow at high density.

Differential expression of gap and pgk genes within the gap operon of Zymomonas mobilis

In Zymomonas mobilis, the genes encoding glyceraldehyde-3-phosphate dehydrogenase (GAP) and phosphoglycerate kinase (PGK) are encoded in an operon that is transcribed from tandem promoters. The promoter-proximal gap gene is expressed at six- to ninefold higher levels than the pgk gene from chromosomal genes and from multiple copies of plasmid-borne genes. Two dominant transcripts were identified. The smaller, most abundant transcript contained primarily the gap message, whereas the larger, less abundant message contained both genes. The ratio of message levels for gap and pgk was calculated to be 5:1 and is sufficient to account for the observed differences in levels of GAP and PGK. The differences in message abundance are proposed to result from either transcriptional attenuation or preferential degradation of the 3' region encoding pgk. Increases in gene dosage were accompanied by one-third the expected increase in enzymatic activity on the basis of estimates of copy number, consistent with the presence of a limiting, positive regulatory factor. However, GAP and PGK expressions were not reduced from the chromosome in recombinants that contained multiple copies of the gap operon with inactive genes.

Isolation and characterization of a new plasmid from a Flavobacterium sp. which carries the genes for degradation of 2,4-dichlorophenoxyacetate

A Flavobacterium sp. (strain 50001), capable of degrading 2,4-dichlorophenoxyacetate (2,4-D), 2-methyl-4-chlorophenoxyacetate, and 2-chlorobenzoate and imparting resistance to mercury, harbored a degradative plasmid, pRC10. Cured strains of the Flavobacterium sp. lost the plasmid as well as the ability to degrade these chlorinated compounds. Comparison of this plasmid with the well-characterized 2,4-D-degradative plasmid pJP4 from Alcaligenes eutrophus showed regions of homology between the two plasmids. Restriction fragments of plasmid pRC10 which shared homology with the regions conferring 2,4-D-degradative genes (tfd) of plasmid pJP4 were cloned into a broad-host-range plasmid and studied in Pseudomonas putida. From the results obtained, the cloned DNA fragment expressed the genes for 2,4-D monooxygenase (tfdA) and 2,4-dichlorophenol hydroxylase (tfdB). In spite of the similarity in function, the size (45 kilobases) and restriction pattern of plasmid pRC10 were considerably different from those of pJP4 (80 kilobases). This may be due to the difference in the microbial background during evolution of the two plasmids.

Phosphoglycerate kinase gene from Zymomonas mobilis: cloning, sequencing, and localization within the gap operon

The Zymomonas mobilis gene encoding phosphoglycerate kinase (EC 2.7.3.2), pgk, has been cloned into Escherichia coli and sequenced. It consists of 336 amino acids, including the N-terminal methionine, with a molecular weight of 41,384. This promoterless gene is located 225 base pairs downstream from the gap gene and is part of the gap operon. Previous studies have shown that the specific activities of glyceraldehyde-3-phosphate dehydrogenase and phosphoglycerate kinase do not change coordinately in Z. mobilis, although the two enzymes appear to be under the control of a common promoter. The translated amino acid sequence for the Z. mobilis phosphoglycerate kinase is less conserved than those of eucaryotic genes. A comparison of known sequences for phosphoglycerate kinase revealed a high degree of conservation of structure with 102 amino acid positions being retained by all. In general, the amino acid positions at the boundaries of beta-sheet and alpha-helical regions and those connecting these regions were more highly conserved than the amino acid positions within regions of secondary structure.

Expression of Different Levels of Ethanologenic Enzymes from Zymomonas mobilis in Recombinant Strains of Escherichia coli

The expression of Zymomonas mobilis genes encoding pyruvate decarboxylase and alcohol dehydrogenase II in Escherichia coli converted this organism from the production of organic acids to the production of ethanol. Ethanol was produced during both anaerobic and aerobic growth. The extent to which these ethanologenic enzymes were expressed correlated with the extent of ethanol production. The replacement of organic acids with ethanol as a metabolic product during aerobic and anaerobic growth resulted in dramatic increases in final cell density, indicating that these acids (and the associated decline in pH) are more damaging than the production of ethanol. Of the plasmids examined, the best plasmid for growth and ethanol production expressed pyruvate decarboxylase and alcohol dehydrogenase II at levels of 6.5 and 2.5 IU/mg of total cell protein, respectively.

Glyceraldehyde-3-phosphate dehydrogenase gene from Zymomonas mobilis: cloning, sequencing, and identification of promoter region

The gene encoding glyceraldehyde-3-phosphate dehydrogenase was isolated from a library of Zymomonas mobilis DNA fragments by complementing a deficient strain of Escherichia coli. It contained tandem promoters which were recognized by E. coli but appeared to function less efficiently than the enteric lac promoter in E. coli. The open reading frame for this gene encoded 337 amino acids with an aggregate molecular weight of 36,099 (including the N-terminal methionine). The primary amino acid sequence for this gene had considerable functional homology and amino acid identity with other eucaryotic and bacterial genes. Based on this comparison, the gap gene from Z. mobilis appeared to be most closely related to that of the thermophilic bacteria and to the chloroplast isozymes. Comparison of this gene with other glycolytic enzymes from Z. mobilis revealed a conserved pattern of codon bias and several common features of gene structure. A tentative transcriptional consensus sequence is proposed for Z. mobilis based on comparison of the five known promoters for three glycolytic enzymes.

Gene expression in Zymomonas mobilis: promoter structure and identification of membrane anchor sequences forming functional lacZ' fusion proteins

We have described a procedure for the isolation of lacZ' fusion genes which contain anchor sequences conferring membrane association. This method was used to isolate fragments of DNA from Zymomonas mobilis which contain promoter activity and amino-terminal sequences. The sequences and transcriptional initiation sites of three of these were compared. Both Escherichia coli and Z. mobilis recognized similar regions of DNA for transcriptional initiation. Five to eight consecutive hydrophobic amino acids in the amino terminus served to anchor these hybrid proteins to the membrane in both E. coli and Z. mobilis. General features observed in the Z. mobilis fragments included partial sequence homology with the -35 region sequence of E. coli, repetitive and palindromic A + T-rich regions preceding and adjoining the -10 region, a sequence resembling the consensus sequence of E. coli in the -10 region, and a potential ribosomal-binding site (AGGA) 8 to 12 bases upstream from an in-frame start codon. The level of expression of fusion proteins was generally higher in E. coli than in Z. mobilis. This higher level of expression in E. coli may result from multiple sites of transcriptional initiation and higher plasmid copy number.

Cloning and sequencing of the alcohol dehydrogenase II gene from Zymomonas mobilis

The gene which encodes alcohol dehydrogenase II (adhB) from Zymomonas mobilis was cloned in Escherichia coli as a 1.4-kilobase DNA fragment by using a novel indicator plate which directly detects the expression of this activity by recombinant colonies. The DNA sequence for this clone contained an open reading frame encoding a polypeptide of 383 amino acids, with a molecular weight of 40,141. Although this protein exhibited very little homology with other known alcohol dehydrogenases, the predicted amino acid composition was in excellent agreement with that reported for the purified alcohol dehydrogenase II protein from Z. mobilis. In Z. mobilis, the adhB gene was transcribed from tandem promoters which were separated by 100 base pairs and ended with a transcriptional terminator (13-base-pair palindrome). In Escherichia coli, only one of the Z. mobilis promoters was used, despite apparent similarity to the enteric consensus promoter. The adhB gene was transcribed at low levels in E. coli from the P2 promoter of Z. mobilis but was expressed well in E. coli under control of the lac promoter (approximately 0.25% of the total cell protein).