The derepression of delta-aminolevulinate synthetase in yeast

Cloning of the δ-aminolevulinic acid synthase structural gene in yeast

HEM1, the structural gene for δ-aminolevulinic acid synthase, has been isolated on recombinant plasmids. A yeast genomic pool constructed in the E. coli - yeast shuttle vector YEp13 was used to clone the HEM1 gene by complementation. A leu2 hem1 yeast mutants was transformed with the yeast genomic pool and hybrid YEp13 plasmids carrying the HEM1 gene were cloned by their ability to complement both the leu2 and hem1 mutations in the recipient strain. The yeast transformants, bearing the HEM1-containing plasmids pYe(HEM1), showed a 24-28 fold increase in δ-aminolevulinic acid synthase activity and in the intracellular content of δ-aminolevulinic acid (5-8 fold) as compared to wild type strains, suggesting that the p(HEM1) gene is being expressed as a catalytically active enzyme which can be transported into the mitochondria. However, the transformant strains did not present higher-than-normal content of heme or cytochromes either in glucose or in glycerol media, indicating that the production of δ-aminolevulinic acid is not the rate-limiting step in heme biosynthesis in yeast.

Isolation and characterization of the KlHEM1 gene in Kluyveromyces lactis

The KlHEM1 gene from Kluyveromyces lactis encodes a functional 5-aminolevulinate synthase (deltaALA synthase), as confirmed by complementation of a hem1 mutant Saccharomyces cerevisiae strain, homology search, and detection of a 2.3 kb transcript. The gene is highly homologous to the ScHEM1 gene, and the sequence of the promoter region contains a complex combination of putative regulatory signals. Some of them are related to phospholipid biosynthesis, glycolytic metabolism, and regulation by carbon source. Transcription of KlHEM1 increased significantly in response to limited oxygen, and only slightly with the change from repressed (glucose) to derepressed conditions (glycerol). The deltaALA synthase from K. lactis contains, in the amino-terminal region, two heme-responsive elements that are not present in the protein from Saccharomyces cerevisiae.

Purification of delta-aminolevulinate dehydratase from genetically engineered yeast

Saccharomyces cerevisiae transformed with a multicopy plasmid carrying the yeast structural gene HEM2, which codes for delta-aminolevulinate dehydratase, was enriched 20-fold in the enzyme. Beginning with cell-free extracts of transformed cells, the dehydratase was purified 193-fold to near-homogeneity. This represents a 3900-fold purification relative to the enzyme activity in normal, untransformed yeast cells. The specific activity of the purified enzyme was 16.2 mumol h-1 per mg protein at pH 9.4 and 37.5 degrees C. In most respects the yeast enzyme resembles mammalian enzymes. It is a homo-octamer with an apparent Mr of 275,000, as determined by centrifugation in glycerol density gradients, and under denaturing conditions behaved as a single subunit of Mr congruent to 37,000. The enzyme requires reduced thiol compounds to maintain full activity, and maximum activity was obtained in the presence of 1.0 mM-Zn2+. It is sensitive to inhibition by the heavy metal ions Pb2+ and Cu2+. The enzyme exhibits Michaelis-Menten kinetics and has an apparent Km of 0.359 mM. Like dehydratases from animal tissues, the yeast enzyme is rather thermostable. During the purification process an enhancement in total delta-aminolevulinate dehydratase activity suggested the possibility that removal of an inhibitor of the enzyme could be occurring.

An amphitropic cAMP-binding protein in yeast mitochondria. 1. Synergistic control of the intramitochondrial location by calcium and phospholipid

A cAMP-binding protein is found to be integrated into the inner mitochondrial membrane of the yeast Saccharomyces cerevisiae under normal conditions. It resists solubilization by high salt and chaotropic agents. The protein is, however, converted to a soluble form which then resides in the intermembrane space, when isolated mitochondria are incubated with low concentrations of calcium. Phospholipids or diacylglycerol (or analogues) dramatically increases the efficiency of receptor release from the inner membrane, whereas these compounds alone are ineffective. Also, cAMP does not effect or enhance liberation from the membrane of the cAMP-binding protein. Photoaffinity labeling with 8-N3-[32P]cAMP followed by mitochondrial subfractionation and sodium dodecyl sulfate-polyacrylamide gel electrophoresis does not reveal differences in the apparent molecular weight between the membrane-bound and the soluble form of the cAMP receptor. The two forms differ, however, in their partitioning behavior in Triton X-114 as well as in their protease resistance, indicating that the release from the membrane is accompanied by a change in lipophilicity and conformation of the receptor protein. Evidence is presented that a change of the intramitochondrial location of the yeast cAMP-binding protein also occurs in vivo and leads to the activation of a mitochondrial cAMP-dependent protein kinase. The cAMP-binding protein is the first example of a mitochondrial protein with amphitropic character; i.e., it has the property to occur in two different locations, as a membrane-embedded and a soluble form.

Constitutive expression of the yeast HEM1 gene is actually a composite of activation and repression

We show that HEM1 (encoding 5-aminolevulinate synthase) expression, while constitutive under all steady-state growth conditions tested, is activated by the HAP2-HAP3 global activation system that controls expression of apocytochromes. This finding creates a paradox because apocytochrome activation by HAP2-HAP3 is highly regulated, subject to induction by heme, and subject to further derepression by a shift from glucose medium to one containing a nonfermentable carbon source. We clarify this issue by showing that HEM1 is subject to two additional layers of control that mask regulatory changes. First is a second activation system acting at a site close to the HAP2-HAP3 target sequence that keeps HEM1 turned on under conditions of heme deficiency. Second is a regulated negative control site downstream of the upstream activation site that counteracts derepression in medium containing a nonfermentable carbon source. Thus, transcription of the constitutive gene is actually a composite of opposing regulatory sites. This complex regulatory arrangement may exist to allow HEM1 to be coordinated transiently with apocytochromes for transition to respiratory growth. Conversely, it may reflect the alteration of HEM1 from a regulated to a constitutive gene over evolution.

Mechanism of thiamine-induced respiratory deficiency in Saccharomyces carlsbergensis

Cells of Saccharomyces carlsbergensis 4228 grown aerobically with added thiamine (1 microgram . ml-1) in a vitamin B6-free medium contained no detectable heme precursors, such as delta-aminolevulinate, coproporphyrin III, or protoporphyrin IX. The deficiency in heme precursors in the thiamine-grown cells was accompanied by previously reported phenomena, i.e., growth depression, vitamin B6 deficiency, and respiratory deficiency due to a marked decrease in the activities of heme-containing enzymes and cytochrome level (I. Nakamura et al., FEBS Lett. 62: 354-358, 1976). It has been reported that all of the effects of thiamine are abolished by adding pyridoxine to the medium. delta-Aminolevulinate was found to have quite similar effects to those of pyridoxine, except that growth was partially improved by delta-aminolevulinate, whereas it was fully restored by pyridoxine. Incubation of the thiamine-grown cells with delta-aminolevulinate resulted in the appearance of the heme precursors and the heme-containing enzymes. Consistent with the lowered amount of vitamin B6, the thiamine-grown cells had a lowered activity of delta-aminolevulinate synthase, a pyridoxal phosphate-dependent enzyme. Not only the holoenzyme activity but also the apoenzyme activity was very low in these cells. These results indicate that the thiamine-induced vitamin B6 deficiency brings about the decrease in delta-aminolevulinate synthase activity, which leads to heme deficiency and therefore to respiratory deficiency.

Molecular events during the release of delta-aminolevulinate dehydratase from catabolite repression

Transfer of exponential-phase cells of Saccharomyces cerevisiae, previously grown in 2% glucose, to a derepression medium resulted in a prompt increase in the level of delta-aminolevulinate dehydratase, the rate-limiting enzyme of heme biosynthesis under these conditions. This derepression exhibited a lag of 35 min at 23 degrees C and required the participation of both RNA and protein syntheses. Dissection of the molecular events during this lag period disclosed that RNA synthesis, rnal gene function (messenger RNA transport from nucleus to cytosol), and initiation of protein synthesis were completed within less than 10, 18, and 24 min, respectively. The potential regulation of derepression by mitochondrial gene products and mitochondrial function was probed by means of a series of isogenic, respiration-deficient (rho-, pet-, and mit-) mutants; no such regulation was found.

Derepression in Saccharomyces cerevisiae can be dissociated from cellular proliferation and deoxyribonucleic acid synthesis

A method has been developed that permits precise control of release from catabolite repression in Saccharomyces cerevisiae. It consists of transferring cells growing exponentially on 5% glucose to derepression medium at high cell density. Derepression then proceeds with reproducible kinetics and is complete within 6 to 7.5 h for various intra- and extramitochondrial markers, in the absence of any substantial increase in cellular dry weight or protein. Nuclear (and mitochondrial) deoxyribonucleic acid synthesis can be interrupted in certain thermosensitive (cdc) mutants at the nonpermissive temperature; a shift to this temperature before the onset of derepression has no effect on its outcome.