Control of de novo isocitrate lyase synthesis in Chlorella

Regulation of expression of the cucumber isocitrate lyase gene in cotyledons upon seed germination and by sucrose

A 6.5 kb cucumber genomic DNA fragment containing the icl gene was introduced into Nicotiana plumbaginifolia and shown to direct isocitrate lyase (ICL) mRNA synthesis in transgenic seedlings upon germination, in a temporally regulated manner. Two putative icl promoter fragments, of 2900 and 572 bp, were subsequently linked to the GUS reporter gene and introduced into N. plumbaginifolia. Both constructs directed GUS expression after transgenic seed germination, and although the 572 bp fragment gave only 1% of the activity of the 2900 bp fragment, it directed expression in the same cotyledon-specific and temporally regulated pattern. Seedlings were transferred to darkness after 18 days growth in the light, to induce a starvation response. The 2900 bp construct was activated by starvation and repressed by exogenous sucrose, whereas the 572 bp construct was not starvation-responsive. To localize the region of the 2900 bp promoter fragment which is responsible for regulation by sucrose, further deletions were made, linked to GUS, and assayed in a cucumber protoplast transient assay system. Constructs with promoters of 2900, 2142 and 1663 bp were activated by starvation and repressed by sucrose, but promoters of 1142 and 572 bp showed no such response. We conclude that the icl gene promoter contains at least two distinct cis-acting elements, one required for the response to sucrose and the other which participates in expression upon seed germination.

Control of isocitrate lyase synthesis in Chlorella fusca var. vacuolata. The basal activity of the enzyme and the kinetics of induction

1. Isocitrate lyase activity was measured in non-induced Chlorella fusca var. vacuolata cells. 2. During exponential autotrophic growth about 1-2 molecules of the enzyme per cell were present. 3. In light-limited cultures the amount of the enzyme increased to 10-20 molecules/cell. 4. When autotrophic cultures were placed in the dark, the basal activity of isocitrate lyase increased after a 2h lag so that after 8h in the dark there was a 500-fold increase in activity. 5. When isocitrate lyase was induced (by addition of acetate and removal of illumination) in autotrophic cultures which had been growing exponentially, the full induced rate of enzyme synthesis was obtained after 70-80min. 6. When light-limited autotrophic cultures were induced, the rate of isocitrate lyase synthesis was maximal after only 40-50min. 7. These data are consistent with a catabolite-repression control co-ordinated with photosynthetic activity,which may be independent of the specific inducing effect of acetate.

Induction potential for glyoxylate cycle enzymes during the cell cycle of Euglena gracilis

In light/dark synchronized cultures of Euglena gracilis Klebs Z the enzymes malate synthase, isocitrate lyase and acetate thiokinase were induced upon addition of acetate at all stages of the cell cycle. Cycloheximide and p-fluorophenylalanine inhibited the development of enzyme activity, showing that induction was dependent on protein synthesis. The maximum rate of induction for all three enzymes was constant for much of the cell cycle but doubles in a single step during the period of DNA replication. Although these data indicate that enzyme potential was regulated by gene dosage and that the structural gene for each enzyme was continuously available for transcription during the cell-cycle it was not possible by using inhibitors of RNA synthesis, to demonstrate concurrent transcription during enzyme induction.

Metabolic control of urea catabolism in Chlamydomonas reinhardi and Chlorella pyrenoidosa

In the unicellular green alga Chlamydomonas reinhardi (strain y-1), synthesis of the enzymes required for urea hydrolysis is under substrate induction control by urea and under end product repression control by ammonia. Hydrolysis of urea if effected by the sequential action of the discrete enzymes urea carboxylase and allophanate lyase, collectively called urea amidolyase. The carboxylase converts urea to allophanate in a reaction requiring biotin, adenosine 5'-triphosphate, and Mg2+. The lyase hydrolzyes allophanate to ammonium ions and bicarbonate. Neither activity is present in more than trace amounts when cultures are grown with ammonia or urea plus ammonia, or when they are starved for nitrogen for 8 h. Urea in the absence of ammonia induces both activities 10 to 100 times the basal levels. Addition of ammonia to an induced culture causes complete cessation of carboxylase accumulation and an 80% depression of lyase accumulation. Ammonia does not reduce urea uptake by repressed cells, so it does not prevent induction by the mechanism of inducer exclusion. The unicellular green alga Chlorella pyrenoidosa (strain 3 Emerson) also has discrete carboxylase and lyase enzymes, but only the carboxylase exhibits metabolic control.