Ribitol and D-arabitol catabolism in Escherichia coli

A Vibrio-based microbial platform for accelerated lignocellulosic sugar conversion

Background

Owing to increasing concerns about climate change and the depletion of fossil fuels, the development of efficient microbial processes for biochemical production from lignocellulosic biomass has been a key issue. Because process efficiency is greatly affected by the inherent metabolic activities of host microorganisms, it is essential to utilize a microorganism that can rapidly convert biomass-derived sugars. Here, we report a novel Vibrio-based microbial platform that can rapidly and simultaneously consume three major lignocellulosic sugars (i.e., glucose, xylose, and arabinose) faster than any previously reported microorganisms.

Results

The xylose isomerase pathway was constructed in Vibrio sp. dhg, which naturally displays high metabolic activities on glucose and arabinose but lacks xylose catabolism. Subsequent adaptive laboratory evolution significantly improved xylose catabolism of initial strain and led to unprecedently high growth and sugar uptake rate (0.67 h⁻¹ and 2.15 g g_{dry cell weight}⁻¹ h⁻¹, respectively). Furthermore, we achieved co-consumption of the three sugars by deletion of PtsG and introduction of GalP. We validated its superior performance and applicability by demonstrating efficient lactate production with high productivity (1.15 g/L/h) and titer (83 g/L).

Conclusions

In this study, we developed a Vibrio-based microbial platform with rapid and simultaneous utilization of the three major sugars from lignocellulosic biomass by applying an integrated approach of rational and evolutionary engineering. We believe that the developed strain can be broadly utilized to accelerate the production of diverse biochemicals from lignocellulosic biomass.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13068-022-02157-3.

Arabitol metabolism of Corynebacterium glutamicum and its regulation by AtlR

Expression profiling of Corynebacterium glutamicum in comparison to a derivative deficient in the transcriptional regulator AtlR (previously known as SucR or MtlR) revealed eight genes showing more than 4-fold higher mRNA levels in the mutant. Four of these genes are located in the direct vicinity of the atlR gene, i.e., xylB, rbtT, mtlD, and sixA, annotated as encoding xylulokinase, the ribitol transporter, mannitol 2-dehydrogenase, and phosphohistidine phosphatase, respectively. Transcriptional analysis indicated that atlR and the four genes are organized as atlR-xylB and rbtT-mtlD-sixA operons. Growth experiments with C. glutamicum and C. glutamicum ΔatlR, ΔxylB, ΔrbtT, ΔmtlD, and ΔsixA derivatives with sugar alcohols revealed that (i) wild-type C. glutamicum grows on D-arabitol but not on other sugar alcohols, (ii) growth in the presence of D-arabitol allows subsequent growth on D-mannitol, (iii) D-arabitol is cometabolized with glucose and preferentially utilized over D-mannitol, (iv) RbtT and XylB are involved in D-arabitol but not in D-mannitol metabolism, (v) MtlD is required for D-arabitol and D-mannitol metabolism, and (vi) SixA is not required for growth on any of the substrates tested. Furthermore, we show that MtlD confers D-arabitol and D-mannitol dehydrogenase activities, that the levels of these and also xylulokinase activities are generally high in the C. glutamicum ΔatlR mutant, whereas in the parental strain, they were high when cells were grown in the presence of D-arabitol and very low when cells were grown in its absence. Our results show that the XylB, RbtT, and MtlD proteins allow the growth of C. glutamicum on D-arabitol and that D-arabitol metabolism is subject to arabitol-dependent derepression by AtlR.

Raman spectroscopy of xylitol uptake and metabolism in Gram-positive and Gram-negative bacteria

Visible-wavelength Raman spectroscopy was used to investigate the uptake and metabolism of the five-carbon sugar alcohol xylitol by Gram-positive viridans group streptococcus and the two extensively used strains of Gram-negative Escherichia coli, E. coli C and E. coli K-12. E. coli C, but not E. coli K-12, contains a complete xylitol operon, and the viridans group streptococcus contains an incomplete xylitol operon used to metabolize the xylitol. Raman spectra from xylitol-exposed viridans group streptococcus exhibited significant changes that persisted even in progeny grown from the xylitol-exposed mother cells in a xylitol-free medium for 24 h. This behavior was not observed in the E. coli K-12. In both viridans group streptococcus and the E. coli C derivative HF4714, the metabolic intermediates are stably formed to create an anomaly in bacterial normal survival. The uptake of xylitol by Gram-positive and Gram-negative pathogens occurs even in the presence of other high-calorie sugars, and its stable integration within the bacterial cell wall may discontinue bacterial multiplication. This could be a contributing factor for the known efficacy of xylitol when taken as a prophylactic measure to prevent or reduce occurrences of persistent infection. Specifically, these bacteria are causative agents for several important diseases of children such as pneumonia, otitis media, meningitis, and dental caries. If properly explored, such an inexpensive and harmless sugar-alcohol, alone or used in conjunction with fluoride, would pave the way to an alternative preventive therapy for these childhood diseases when the causative pathogens have become resistant to modern medicines such as antibiotics and vaccine immunotherapy.

Isolation of an operon involved in xylitol metabolism from a xylitol-utilizing Pantoea ananatis mutant

An operon involved in cryptic xylitol metabolism of Pantoea ananatis was cloned by transposon tagging. A xylitol negative mutant with a transposon insertion in the xylitol 4-dehydrogenase gene (xdh) was isolated and genomic DNA around the transposon was sequenced. Consequently, six consecutive genes, xytB-G are located downstream of xdh in the same strand. These seven genes are cotranscribed as a single transcript in a P. ananatis xylitol-utilizing mutant, suggesting that they comprise an operon. In addition to xdh, xytF also encodes oxidoreductase that is a member of the short-chain dehydrogenase/reductase family. Recombinant Escherichia coli that heterologously expresses the Xdh protein converts xylitol to xylulose as expected. On the other hand, the recombinant XytF protein has activity with l-arabitol but not with xylitol. XytB, xytD and xytE have significant sequence similarities to genes encoding the substrate-binding, ATP-binding and permease subunits, respectively, of ATP-binding cassette transporters. Although the physiological role of the operon remains unknown, the operon appears to be involved in uptake and metabolism of a various sugar alcohols. A gene encoding a DeoR-type transcriptional regulator, xytR, is located upstream of the operon in the opposite strand and a single nucleotide substitution that could cause a nonsense mutation is present in the xytR gene of the xylitol-utilizing mutant. This result suggests that the product of xytR negatively controls expression of the operon like other DeoR regulators.

Advances in selectable marker genes for plant transformation

Plant transformation systems for creating transgenics require separate process for introducing cloned DNA into living plant cells. Identification or selection of those cells that have integrated DNA into appropriate plant genome is a vital step to regenerate fully developed plants from the transformed cells. Selectable marker genes are pivotal for the development of plant transformation technologies because marker genes allow researchers to identify or isolate the cells that are expressing the cloned DNA, to monitor and select the transformed progeny. As only a very small portion of cells are transformed in most experiments, the chances of recovering transgenic lines without selection are usually low. Since the selectable marker gene is expected to function in a range of cell types it is usually constructed as a chimeric gene using regulatory sequences that ensure constitutive expression throughout the plant. Advent of recombinant DNA technology and progress in plant molecular biology had led to a desire to introduce several genes into single transgenic plant line, necessitating the development of various types of selectable markers. This review article describes the developments made in the recent past on plant transformation systems using different selection methods adding a note on their importance as marker genes in transgenic crop plants.

Production ofD-arabitol by a metabolic engineered strain ofBacillus subtilis

A novel method for D-arabitol production with a metabolically engineered Bacillus subtilis strain is described. A known transketolase-deficient and D-ribose-producing mutant of B. subtilis (ATCC 31094) was further modified by disruption of its rpi (D-ribose phosphate isomerase) gene to create a D-ribulose- and D-xylulose-producing B. subtilis strain. Expression of the D-arabitol phosphate dehydrogenase gene of Enterococcus avium in the D-ribulose- and D-xylulose-producing strain resulted in a strain of B. subtilis capable of converting D-glucose to D-arabitol with a high yield (38%) and little by-product formation.

Arabitol dehydrogenase as a selectable marker for rice

Arabitol dehydrogenase has been adapted for use as a plant selectable marker. Arabitol is a five-carbon sugar alcohol that can be used by E. coli strain C, but not by the laboratory K12 strains. The enzyme converts the non-plant-metabolizable sugar arabitol into xylulose, which is metabolized by plant cells. Rice was transformed with a plant-expression-optimized synthetic gene using Biolistic-mediated transformation. Selection on 2.75% arabitol and 0.25% sucrose yielded a transformation efficiency (9.3%) equal to that obtained with hygromycin (9.2%). Molecular analyses showed that the atlD gene was integrated into the rice genome of selected plants and was inherited in a Mendelian manner. This study indicates that arabitol could serve as an effective means of plant selection.

Biochemical and genetic characterization of a novel enzyme of pentitol metabolism: D-arabitol-phosphate dehydrogenase

An enzyme with a specificity that has not been described previously, D-arabitol-phosphate dehydrogenase (APDH), has been purified from cell lysate of Enterococcus avium. SDS/PAGE indicated that the enzyme had a molecular mass of 41+/-2 kDa, whereas a molecular mass of 160+/-5 kDa was observed under non-denaturing conditions, implying that the APDH may exist as a tetramer with identical subunits. Purified APDH was found to have a narrow substrate specificity, converting only D-arabitol 1-phosphate and D-arabitol 5-phosphate into xylulose 5-phosphate and ribulose 5-phosphate, respectively, in the oxidative reaction. Both NAD(+) and NADP(+) were accepted as cofactors. Based on the partial protein sequences, the APDH gene was cloned. Homology comparisons place APDH within the medium-range dehydrogenase family. Unlike most members of this family, APDH requires Mn(2+) but no Zn(2+) for enzymic activity. The DNA sequence surrounding the gene suggests that it belongs to an operon that also contains several components of phosphotransferase system. Both biochemical evidence and protein sequence homology comparisons indicate that similar enzymes are widespread among the Gram-positive bacteria. Their apparent biological role is to participate in arabitol catabolism via the 'arabitol phosphate route', similar to the ribitol and xylitol catabolic routes described previously.

Genes for D-arabinitol and ribitol catabolism from Klebsiella pneumoniae

The enzymes for catabolism of the pentitols D-arabinitol (Dal) and ribitol (Rbt) and the corresponding genes from Klebsiella pneumoniae (dal and rbt) and Escherichia coli (atl and rtl) have been used intensively in experimental evolutionary studies. Four dal and four rbt genes from the chromosome of K. pneumoniae 1033-5P14 were cloned and sequenced. These genes are clustered in two adjacent but divergently transcribed operons and separated by two convergently transcribed repressor genes, dalR and rbtR. Each operon encodes an NAD-dependent pentose dehydrogenase (dalD and rbtD), and ATP-dependent pentulose kinase (dalK and rbtK) and a pentose-specific ion symporter (dalT and rbtT). Although the biochemical reactions which they catalyse are highly similar, the enzymes showed interesting deviations. Thus, DalR (313 aa) and RbtR (270 aa) belong to different repressor families, and DalD (455 aa) and RbtD (248 aa), which are active as a monomer or as tetramers, respectively, belong to different dehydrogenase families. Of the two kinases (19.3% identity), DalK (487 aa) belongs to the subfamily of short D-xylulokinases and RbtK (D-ribulokinase; 535 aa) to the subfamily of long kinases. The repressor, dehydrogenase and kinase genes did not show extensive similarity beyond local motifs. This contrasts with the ion symporters (86.6% identity) and their genes (82.7% identity). Due to their unusually high similarity, parts of dalT and rbtT have previously been claimed erroneously to correspond to 'inverted repeats' and possible remnants of a 'metabolic transposon' comprising the dal and rbt genes. Other characteristic structures, e.g. a secondary att lambda site and chi-like sites, as well as the conservation of this gene group in E. coli C are also discussed.

Substrate recognition domains as revealed by active hybrids between the D-arabinitol and ribitol transporters from Klebsiella pneumoniae

Two new genes, dalT and rbtT, have been cloned from the dal operon for D-arabinitol and the rbt operon for ribitol uptake and degradation, respectively, in Klebsiella pneumoniae 1033-5P14, derivative KAY2026. Each gene codes for a specific transporter which, based on sequence data, belongs to a large family of carbohydrate transporters which constitutes 12 transmembrane helices. DalT and RbtT show an unusually high similarity (86.2% identical residues for totals of 425 and 427 amino acids, respectively). This allowed the construction of DalT'-Rbt"T and RbtT'-Dal'T crossover hybrids by using a natural restriction site overlapping Met202. This site is located within the large cytoplasmic loop which connects the putative helices 6 and 7 and in particular the amino- and the carboxy-terminal halves of the transporters. Both hybrids have close to normal transport activities but essentially the substrate specificities and kinetic properties of the amino-terminal half. This result localizes essential substrate binding and recognition sites to the amino-terminal halves of the proteins in this important class of carbohydrate transporters.

Molecular cloning of the Escherichia coli B L-fucose-D-arabinose gene cluster

To metabolize the uncommon pentose D-arabinose, enteric bacteria often recruit the enzymes of the L-fucose pathway by a regulatory mutation. However, Escherichia coli B can grow on D-arabinose without the requirement of a mutation, using some of the L-fucose enzymes and a D-ribulokinase that is distinct from the L-fuculokinase of the L-fucose pathway. To study this naturally occurring D-arabinose pathway, we cloned and partially characterized the E. coli B L-fucose-D-arabinose gene cluster and compared it with the L-fucose gene cluster of E. coli K-12. The order of the fucA, -P, -I, and -K genes was the same in the two E. coli strains. However, the E. coli B gene cluster contained a 5.2-kb segment located between the fucA and fucP genes that was not present in E. coli K-12. This segment carried the darK gene, which encodes the D-ribulokinase needed for growth on D-arabinose by E. coli B. The darK gene was not homologous with any of the L-fucose genes or with chromosomal DNA from other D-arabinose-utilizing bacteria. D-Ribulokinase and L-fuculokinase were purified to apparent homogeneity and partially characterized. The molecular weights, substrate specificities, and kinetic parameters of these two enzymes were very dissimilar, which together with DNA hybridization analysis, suggested that these enzymes are not related. D-Arabinose metabolism by E. coli B appears to be the result of acquisitive evolution, but the source of the darK gene has not been determined.

D-arabitol metabolism in Candida albicans: studies of the biosynthetic pathway and the gene that encodes NAD-dependent D-arabitol dehydrogenase

Candida albicans produces large amounts of the pentitol D-arabitol in culture and in infected mammalian hosts, but the functional and pathogenic significance of D-arabitol in C. albicans is not known. In this study, we sought to elucidate the pathway by which C. albicans synthesizes D-arabitol and to identify and characterize key enzymes in this pathway. C. albicans B311 produced D-[14C-1]arabitol from [14C-2]glucose; this finding implies on structural grounds that D-ribulose-5-PO4 from the pentose pathway is the major metabolic precursor of D-arabitol. NAD- or NADP-dependent pentitol dehydrogenases catalyze the final steps in D-arabitol biosynthesis in other fungi; therefore, lysates of C. albicans B311 were tested for enzymes of this class and were found to contain a previously unknown NAD-dependent D-arabitol dehydrogenase (ArDH). The ArDH structural gene was cloned by constructing a new D-arabitol utilization pathway in Escherichia coli. The C. albicans ArDH gene expressed in E. coli and Saccharomyces cerevisiae an enzyme that catalyzes the reaction D-arabitol + NAD <-->D-ribulose + NADH; this gene was present as a single copy per haploid genome, and its deduced peptide sequence was homologous with sequences of several members of the short-chain dehydrogenase family of enzymes. These results suggest that (i) C. albicans synthesizes D-arabitol by dephosphorylating and reducing the pentose pathway intermediate D-ribulose-5-PO4 and (ii) ArDH catalyzes the final step in this pathway.

Ribitol dehydrogenase of Klebsiella aerogenes. Sequence of the structural gene

The ribitol dehydrogenase gene was cloned from wild-type Klebsiella aerogenes and also from a transducing phage lambda prbt which expresses the rbt operon constitutively. The coding sequence for 249 amino acids is separated from the following D-ribulokinase gene by 31 base pairs containing three stop codons, one of which overlaps the ribosome binding site for D-ribulokinase. Three residues in the amino acid sequence differ from that predicted from the DNA sequence: Asp-212 for Asn-212 is probably a protein sequencing error, but -Ala-Val- for -Ser-Ser- at 146-147 appears to be a 'neutral mutation' that may have arisen during prolonged chemostat selection of a strain that superproduces the enzyme from which the protein sequence was determined.

Inducible xylitol dehydrogenases in enteric bacteria

Morganella morganii ATCC 25829, Providencia stuartii ATCC 25827, Serratia marcescens ATCC 13880, and Erwinia sp. strain 4D2P were found to induce a xylitol dehydrogenase when grown on a xylitol-containing medium. The xylitol dehydrogenases were partially purified from the four strains, and those from M. morganii ATCC 25829, P. stuartii ATCC 25827, and S. marcescens ATCC 13880 were all found to oxidize xylitol to D-xylulose. These three enzymes had KmS for xylitol of 7.1 to 16.4 mM and molecular weights ranging from 130,000 to 155,000. In contrast, the xylitol dehydrogenase from Erwinia sp. strain 4D2P oxidized xylitol at the C-4 position to produce L-xylulose, had a Km for xylitol of 72 mM, and had a molecular weight of 102,000.

Characterization of xylitol-utilizing mutants of Erwinia uredovora

Of the four pentitols ribitol, xylitol, D-arabitol, and L-arabitol, Erwinia uredovora was able to utilize only D-arabitol as a carbon and energy source. Although attempts to isolate ribitol- or L-arabitol-utilizing mutants were unsuccessful, mutants able to grow on xylitol were isolated at a frequency of 9 X 10(-8). Xylitol-positive mutants constitutively synthesized both a novel NAD-dependent xylitol-4-dehydrogenase, which oxidized xylitol to L-xylulose, and an L-xylulokinase. The xylitol dehydrogenase had a Km for xylitol of 48 mM and showed best activity with xylitol and D-threitol as substrates. However, D-threitol was not a growth substrate for E. uredovora, and its presence did not induce either dehydrogenase or kinase activity. Attempts to determine the origin of the xylitol catabolic enzymes were unsuccessful; neither enzyme was induced on any growth substrate or in the presence of any polyol tested. Analysis of xylitol-negative mutants isolated after Tn5 mutagenesis suggested that the xylitol dehydrogenase and the L-xylulokinase structural genes were components of two separate operons but were under common regulatory control.

Directed evolution of a second xylitol catabolic pathway in Klebsiella pneumoniae

Klebsiella pneumoniae PRL-R3 has inducible catabolic pathways for the degradation of ribitol and D-arabitol but cannot utilize xylitol as a growth substrate. A mutation in the rbtB regulatory gene of the ribitol operon permits the constitutive synthesis of the ribitol catabolic enzymes and allows growth on xylitol. The evolved xylitol catabolic pathway consists of an induced D-arabitol permease system that also transports xylitol, a constitutively synthesized ribitol dehydrogenase that oxidizes xylitol at the C-2 position to produce D-xylulose, and an induced D-xylulokinase from either the D-arabitol or D-xylose catabolic pathway. To investigate the potential of K. pneumoniae to evolve a different xylitol catabolic pathway, strains were constructed which were unable to synthesize ribitol dehydrogenase or either type of D-xylulokinase but constitutively synthesized the D-arabitol permease system. These strains had an inducible L-xylulokinase; therefore, the evolution of an enzyme which oxidized xylitol at the C-4 position to L-xylulose would establish a new xylitol catabolic pathway. Four independent xylitol-utilizing mutants were isolated, each of which had evolved a xylitol-4-dehydrogenase activity. The four dehydrogenases appeared to be identical because they comigrated during nondenaturing polyacrylamide gel electrophoresis. This novel xylitol dehydrogenase was constitutively synthesized, whereas L-xylulokinase remained inducible. Transductional analysis showed that the evolved dehydrogenase was not an altered ribitol or D-arabitol dehydrogenase and that the evolved dehydrogenase structural gene was not linked to the pentitol gene cluster. This evolved dehydrogenase had the highest activity with xylitol as a substrate, a Km for xylitol of 1.4 M, and a molecular weight of 43,000.

Selection of Escherichia coli K12 1EA mutants with increased synthesis of ribitol dehydrogenase

Selection of an interspecific hybrid Escherichia coli K12 1EA in a chemostat on xylitol yielded a stable mutant synthesizing a four-fold amount of ribitol dehydrogenase (EC 1.1.1.56). Subsequent cultivation of the mutant under increased selection pressure resulted in an accumulation of a mutant with 12-fold higher level of ribitol dehydrogenase relative to the parent strain 1EA. A selection during which a UV-mutagenized population of the 1EA mutant was cultivated in a chemostat on xylitol was accompanied by monitoring the activities of ribitol dehydrogenase and D-arabinitol dehydrogenase (EC 1.1.1.11) of two adjacent catabolite operons. A several-fold increase in the activity of the two enzymes was followed by further increase in the activity of ribitol dehydrogenase and a concomitant drop in the activity of D-arabinitol dehydrogenase. The two hyperproducing strains are compared with the parent mutant as to the rate of synthesis of the two dehydrogenases and growth parameters under the conditions of batch cultivation.

Genotypic exclusion: a novel relationship between the ribitol-arabitol and galactitol genes of E. coli

Genetic studies indicate that the E. coli C chromosomal genes which are responsible for catabolism of the pentitol sugars, ribitol and D-arabitol, are not present in the closely related E. coli K12 strains (Reiner 1975). Molecular studies of these tightly linked genes reveal that they are surrounded by 1.4 kilobase inverted repeats of imperfect homology (Link and Reiner 1982). Here we report that E. coli C lacks genes for catabolism of the hexitol sugar galactitol, genes which are present in E. coli K12. Furthermore, the ribitol-arabitol and galactitol genes, which show no mutual homology, are mutually exclusive when exchanged (by homologous recombination) between E. coli C and K12. Physical characterization of lambda specialized transducing phages carrying the ribitol-arabitol or galactitol genes demonstrates that this exclusion results because these genes have identical locations in their respective chromosomes. This novel type of allelic relationship between nonhomologous genes has not been previously described in prokaryotes. Analysis of the catabolic capabilities of a collection of natural E. coli strains suggests that this exclusion relationship extends to strains in the natural E. coli population. We suggest an insertion/deletion model to account for the origins of this unusual gene arrangement.

[Microbial utilization of mixed substrates]

Decomposition of substrates by heterotrophic microorganisms is accomplished in natural biotopes such as in soil and in waters, on or in macroorganisms but also in laboratory and industrial biotopes. The interest of man in these processes is manifold. Starting with the division of substrates into three groups of simple substrates, complex and mixed substrates with or without solid particles their qualitative and quantitative occurrence in nature and their significance in biotechnology will be discussed. In the decomposition of these substrates their utilization by pure cultures or mixed populations is to be exactly distinguished. Simple growth curves, di- or polyauxy, sequences of decomposition of simple substrates of a mixed substrate, population changes and successions are only some of the phenomena occurring in this process. The pathways of catabolism are subjected to manifold regulations on the three levels of stoichiometric regulation, the regulation of enzyme activity and the regulation of enzyme synthesis. In natural biotopes there is hardly a constant substrate supply over a longer period. That's why certain mechanisms of regulation are permanently acting. Thus the "normal" physiological state for microorganisms is characterized by permanent transition situations--called "transients". These reactions are also applied to many biotechnological processes.

Purification and properties of D-ribulokinase and D-xylulokinase from Klebsiella aerogenes

The D-ribulokinase and D-xylulokinase of Klebsiella aerogenes were purified to homogeneity from Escherichia coli K12 construct strains that synthesized these enzymes constitutively. The D-ribulokinase, which is encoded in the ribitol operon, is active as a dimer of 60 000 subunit mol.wt., whereas the D-xylulokinase, which is encoded in the D-arabitol operon, is active as a dimer of 54 000 subunit mol.wt. The amino acid compositions and N-terminal sequences of both pentulokinases are reported. The Kapp. values of the enzymes for their D-pentulose substrates were determined, and the D-ribulokinase was shown to have a low-affinity side-specificity for ribitol and D-arabitol. These results are discussed in the context of the evolution of the Klebsiella aerogenes pentitol operons.

Tandem and inverted repeats of arginine genes in Escherichia coli: structural and evolutionary considerations

Duplications of arg genes produced in the Rec+ and in the recA genetic backgrounds are shown by heteroduplex analysis to be strictly tandem at the level of resolution of this technique. The formation of these particular rearrangements therefore does not require the inclusion of transposons or other sequences of an appreciable size in their final structure. Duplications of short segments (about 2,000 nucleotides) appear unexpectedly stable when compared with duplications of longer segments (about 10,000 nucleotides). One of the structures analyzed displays two inversely repeated argE genes rearranged into an artificial divergent operon. The bearing of this observation on the origin of bipolar operons, of "mirror-image" map symmetries and on the production of inverted repeats in general, is discussed.

A unique pattern of toxic synthesis in pentitol catabolism: implications for evolution

All of our Escherichia coli C mutants blocked in the first step of D-arabitol catabolism (D-arabitol dehydrogenase) became unable to grow in the presence of D-arabitol. We have shown that this sensitivity is eliminated by a defect in the second enzyme of the pathway (D-xylulokinase), leading to a pattern of toxicity and its relief which has not been previously reported. We have found a similar pattern of toxicity and its relief in the closely related ribitol pathway. The evolutionary significance of these findings is discussed.

Acquisition of ability to utilize Xylitol: disadvantages of a constitutive catabolic pathway in Escherichia coli

Ribitol+ strains of Escherichia coli acquire the ability to utilize xylitol by mutating to constitutive production of the coordinately controlled ribitol catabolic enzymes ribitol dehydrogenase (RDH) and D-ribulokinase (DRK). Such strains concomitantly acquire toxicity to galacitol and L-arabitol, and to D-arabitol if they are unable to utilize it for growth. Strains selected for resistance to these polyols have DRK structural gene mutations or other mutations that eliminate the constitutive production of DRK, consistent with the view that DRK phosphorylates those polyols to toxic substances. Ribitol+ strains selected for growth on 8 mM xylitol fail to grow on 30 mM xylitol. A product of ribitol and xylitol catabolism represses synthesis of RDH, an enzyme required for growth on xylitol. At 30 mM xylitol, greater than 99% of RDH synthesis is repressed. Strains that grow on 8 mM xylitol can mutate to grow on 30 mM xylitol. Such mutants, relieved of this repression, overproduce RDH, resulting in good growth on the poor substrate, xylitol, but poor growth on the normal substrate, ribitol.