The hexokinase gene is required for transcriptional regulation of the glucose transporter gene RAG1 in Kluyveromyces lactis

Hexokinase as a sugar sensor in higher plants

The mechanisms by which higher plants recognize and respond to sugars are largely unknown. Here, we present evidence that the first enzyme in the hexose assimilation pathway, hexokinase (HXK), acts as a sensor for plant sugar responses. Transgenic Arabidopsis plants expressing antisense hexokinase (AtHXK) genes are sugar hyposensitive, whereas plants overexpressing AtHXK are sugar hypersensitive. The transgenic plants exhibited a wide spectrum of altered sugar responses in seedling development and in gene activation and repression. Furthermore, overexpressing the yeast sugar sensor YHXK2 caused a dominant negative effect by elevating HXK catalytic activity but reducing sugar sensitivity in transgenic plants. The result suggests that HXK is a dual-function enzyme with a distinct regulatory function not interchangeable between plants and yeast.

Glucose uptake in Kluyveromyces lactis: role of the HGT1 gene in glucose transport

A gene for high-affinity glucose transport, HGT1, has been isolated from the lactose-assimilating yeast Kluyveromyces lactis. Disruption strains showed much-reduced uptake of glucose at low concentrations and growth was particularly affected in low-glucose medium. The HGT1 nucleotide sequence implies that it encodes a typical transmembrane protein with 12 hydrophobic domains and with 26 to 31% amino acid identity with the Hxtp family of glucose transport elements in Saccharomyces cerevisiae. Expression is constitutive (in contrast to RAG1, the major gene for low-affinity glucose uptake in K. lactis) and is controlled by several genes also known to affect expression of RAG1. These include RAG5 (which codes for the single hexokinase of K. lactis), which is required for HGT1 transcription, and RAG4, which has a negative effect. The double mutant deltahgt1deltarag1 showed further reduced glucose uptake but still grew quite well on 2% glucose and was not completely impaired even on 0.1% glucose.

Cloning and Biochemical Characterisation of an Aspergillus Niger Glucokinase. Evidence for the Presence of Separate Glucokinase and Hexokinase Enzymes

The Aspergillus niger glucokinase gene glkA has been cloned using a probe generated by polymerase chain reaction with degenerate oligonucleotides. The DNA sequence of the gene was determined, and the deduced amino acid sequence shows significant similarity to other eukaryotic hexokinase and glucokinase proteins, in particular to the Saccharomyces cerevisiae glucokinase protein. The encoded protein was purified from a multicopy glkA transformant, and extensively characterised. The protein has a molecular mass of 54536 Da and a pI of 5.2. The enzyme has high affinity for glucose (K(m) 0.063 mM at pH 7.5) and a relatively low affinity for fructose (K(m) 120 mM at pH 7.5), and in vivo fructose phosphorylation by glucokinase is consequently negligible. The configurations at C1 and C4 of the substrate appear to be essential for substrate specificity. The A. niger glucokinase shows non-competitive inhibition by ADP towards ATP and uncompetitive inhibition by ADP towards glucose. The kcal (turnover number) decreases rapidly below pH 7.5 (56% at pH 7.0 and 17% at pH 6.5) and this may have important implications for the in vivo regulation of activity. In addition, proof is provided for the presence of a second hexosephosphorylating enzyme in A. niger. This enzyme is probably a hexokinase, since unlike glucokinase, this activity is inhibited by trehalose 6-phosphate.

Characterisation of the Aspergillus nidulans frA1 mutant: hexose phosphorylation and apparent lack of involvement of hexokinase in glucose repression

Hexose phosphorylation was studied in Aspergillus nidulans wild-type and in a fructose non-utilising mutant (frA). The data indicate the presence of at least one hexokinase and one glucokinase in wild-type A. nidulans, while the frA1 mutant lacks hexokinase activity. The A. nidulans gene encoding hexokinase was isolated by complementation of the frA1 mutation. The absence of hexokinase activity in the frA1 mutant did not interfere with glucose repression of the enzymes involved in alcohol and L-arabinose catabolism. This suggest that, unlike the situation in yeast where mutation of hexokinase PII abolishes glucose repression, the A. nidulans hexokinase might not be involved in glucose repression.

Reoxidation of the NADPH produced by the pentose phosphate pathway is necessary for the utilization of glucose by Kluyveromyces lactis rag2 mutants

Kluyveromyces lactis mutants defective in the glycolytic enzyme phosphoglucose isomerase are able to grow in glucose media and to produce ethanol, but they depend on a functional respiratory chain and do not grow in glucose-antimycin media. We postulate that this is due to the necessity of reoxidizing, in the mitochondria, the NADPH produced by the pentose phosphate pathway, which may be highly active in these mutants in order to bypass the blockade in the phosphoglucose isomerase step. This oxidation would be mediated by a cytoplasmic-side mitochondrial NAD(P)H dehydrogenase that would pass the electrons to ubiquinone. Data supporting this hypothesis are provided.

RAG3 gene and transcriptional regulation of the pyruvate decarboxylase gene in Kluyveromyces lactis

The RAG3 gene has been cloned from a Kluyveromyces lactis genomic library by complementation of the rag3 mutation, which shows impaired fermentative growth on glucose in the presence of respiratory inhibitors. From the nucleotide sequence of the cloned DNA, which contained an open reading frame of 765 codons, the predicted protein is 49.5% identical to the Pdc2 protein of Saccharomyces cerevisiae, a regulator of pyruvate decarboxylase in this yeast. Measurement of the pyruvate decarboxylase activity in the original rag3-1 mutant and in the null mutant confirmed that the RAG3 gene is involved in pyruvate decarboxylase synthesis in K. lactis. The effect is exerted at the mRNA level of the pyruvate decarboxylase structural gene KIPDCA. Despite analogies between the RAG3 gene of K. lactis and the PDC2 gene of S. cerevisiae, these genes were unable to reciprocally complement.

FOG1 and FOG2 genes, required for the transcriptional activation of glucose-repressible genes of Kluyveromyces lactis, are homologous to GAL83 and SNF1 of saccharomyces cerevisiae

The fog1 and fog2 mutants of the yeast Kluyveromyces lactis were identified by inability to grow on a number of both fermentable and non-fermentable carbon sources. Genetic and physiological evidences suggest a role for FOG1 and FOG2 in the regulation of glucose-repressible gene expression in response to a glucose limitation. The regulatory effect appears to be at the transcriptional level, at least for beta-galactosidase. Both genes have been cloned by complementation and sequenced. FOG1 is a unique gene homologous to GAL83, SIP1 and SIP2, a family of regulatory genes affecting glucose repression of the GAL system in Saccharomyces cerevisiae. However, major differences exist between fog1 and gal83 mutants. FOG2 is structurally and functionally homologous to SNF1 of S. cerevisiae and shares with SNF1 a role also in sporulation.

Nucleic acid-binding metabolic enzymes: living fossils of stereochemical interactions?

Recently, a series of intriguing observations expanded the list of a number of metabolic enzymes known to be associated with various forms of nucleic acids, including single- and double-stranded DNA, cognate and noncognate RNAs, and specific tRNAs. There is no clear reason why such a phenomenon should take place in contemporary cell physiology, or, further, why such a property has evolved at all. Sixteen known cases are presented in an attempt to delineate any common features of these enzymes. Apart from their ancient nature, as judged by their wide distribution and their participation in fundamental biochemical pathways, it appears that these enzymes do not share any structural or functional characteristics. Given that most of these proteins require nucleotide-based cofactors for their activity, it is proposed that they may represent genuine molecular fossils of the transition from an RNA to a protein world. Their nucleic acid-binding properties are in keeping with previously proposed hypotheses regarding the origins and evolution of nucleotide-based cofactors. The mode of interaction between these proteins and their nucleic acid substrates remains unclear, but it may represent an extended form of stereochemical interactions that have been proposed for the origins of the genetic code.

Molecular and biochemical characterization of the hexokinase from the starch-utilizing yeast Schwanniomyces occidentalis

Hexose-phosphorylating enzymes from the starch-utilizing yeast Schwanniomyces occidentalis were purified and two isoenzymes separated. The substrate pattern characterized one of these as a hexokinase phosphorylating glucose and fructose and the other as a glucokinase unable to phosphorylate fructose. The purified Schw. occidentalis hexokinase had a KM value of 0.98 mM for glucose and 9.3 mM for fructose. The hexokinase gene was cloned by cross hybridization with a probe from the Saccharomyces cerevisiae HXK2 gene. Deletion of Schw. occidentalis hexokinase by gene replacement yielded a mutant unable to grow on fructose as sole carbon source, but still growing on glucose. Deletion mutants of Schw. occidentalis hexokinase prevented glucose repression of invertase and maltase. Growth deficiencies and the defect of glucose repression of a S. cerevisiae hexokinase null mutant could be restored by heterologous expression of the Schw. occidentalis hexokinase. Moreover, the results clearly showed the existence of a separate glucokinase in Schw. occidentalis.

The respiratory system of Kluyveromyces lactis escapes from HAP2 control

A functional homolog of the Saccharomyces cerevisiae HAP2 gene, coding for one element of a transcriptional activator complex, was cloned from the yeast Kluyveromyces lactis and its nucleotide sequence was determined. Inactivation of the gene had no significant effect on respiration-dependent growth, suggesting that the HAP2/3/4 complex has no major control over the formation of the mitochondrial respiratory system in K. lactis.

Isolation and preliminary characterization of Pichia pinus mutants insensitive to glucose repression

A new method for the isolation of glucose repression-insensitive mutants in the methylotrophic yeast Pichia pinus was developed. The method is based on screening of small suspension samples derived from 2-deoxyglucose-resistant colonies for alcohol oxidase activity. Alcohol oxidase activity was evaluated by determination of formaldehyde excreted by cells. Mutants with glucose non-repressible alcohol oxidase and catalase synthesis were obtained. All mutants grew poorly on D-xylose compared to the wild type, whereas growth on L-arabinose was similar to the wild type. Changes in the glucose transport system were suggested to be responsible for altered growth characteristics and defective glucose repression.

Glucose repression in Streptomyces coelicolor A3(2): a likely regulatory role for glucose kinase

The glucose kinase gene (glkA-ORF3) of Streptomyces coelicolor A3(2) plays an essential role in glucose utilisation and in glucose repression of a variety of genes involved in the utilisation of alternative carbon sources. These genes include dagA, which encodes an extracellular agarase that permits agar utilisation. Suppressor mutants of glkA-ORF3 deletion strains capable of utilising glucose (Glc+) arise at a frequency of about 10(-5) on prolonged incubation. The Glc+ phenotype of the mutants is reversible (at a frequency of about 10(-3) and reflects either the activation of a normally silent glucose kinase gene or the modification of an existing sugar kinase. Although the level of glucose kinase activity in the Glc+ supressor mutants is similar to that in the glkA+ parental strain, glucose repression of dagA remains defective. Expression of the glucose kinase gene of Zymomonas mobilis in glkA-ORF3 mutants restored glucose utilisation, but not glucose repression of dagA. Over-expression of glkA-ORF3 on a high-copy-number plasmid failed to restore glucose repression of dagA in glkA-ORF3 mutants and led to loss of glucose repression of dagA in a glkA+ strain. These results suggest that glucose phosphorylation itself is not sufficient for glucose repression and that glkA-ORF3 plays a specific regulatory role in triggering glucose repression in S. coelicolor A3(2).