Induction of nitrite reductase in synchronized cultures of Chlorella pyrenoidosa

Regulation of accumulation and turnover of an inducible glutamate dehydrogenase in synchronous cultures of Chlorella

Earlier studies indicated that the gene of an ammonium-inducible glutamate dehydrogenase (GDH) was inducible throughout the cell cycle and was expressible shortly after replication early in the S-phase in synchronous Chlorella cells growing at a rate of 13% per h in the absence of inducer. In the present study, synchronous cells cultured at the same growth rate in the continuous presence of inducer accumulated this enzyme in a linear manner, with a positive rate change observed late instead of early in the S-phase. At a growth rate of 26% per h, the positive rate change appeared to be displaced to 1.5 h before the S-phase in the next cell cycle. With 2'-deoxyadenosine, an in vivo inhibitor of deoxyribonucleic acid (DNA) synthesis, the magnitude of the positive rate change was shown to be proportional to the relative increase in DNA in the previous cell cycle. Collectively, these data support the idea that expression of newly replicated genes of this enzyme can be delayed into the subsequent cell cycle in cells in the continuous presence of inducer. Studies with cycloheximide indicated that the inducible GDH and another GDH isozyme were stable in fully induced cells in the absence of protein synthesis. However, after ammonium was removed from the culture medium, the activity of the inducible GDH decreased rapidly in vivo, with a half-time of 5 to 10 min at 38.5 degrees C, whereas the rate of accumulation of the other GDH isozyme did not change. Addition of cycloheximide, at the time of inducer removal, prevented this loss in activity of the inducible GDH. The inability to rescue the activity of the inducible GDH, by readdition of ammonium during the deinduction period, indicates that this enzyme probably underwent irreversible inactivation and/or proteolytic degradation.

The capacity for arylsulfatase synthesis in synchronous and synchronized cultures of Chlamydomonas reinhardti

The green algae Chlamydomonas reinhardti synthesizes arylsulfatase (arylsulfate sulfohydrolase EC 3.1.6.1) by derepression when the concentration of SO4-2-minus in the growth medium is less than about 5-10-minus 5 M. The following observations indicate that the arylsulfatase enzyme is stable while its mRNA was unstable: (1) The increase in enzyme activity stopped and remained constant after addition of cycloheximide to derepressed cells. (2) After readdition of SO4-2-minus the increase in enzyme activity continued at a lower rate whereafter it remained constant. (3) No decay of radioactivity was observed after readdition of SO4 2-minus in labelled enzyme protein isolated from pulse-labelled --S cells. The maximum rate of arylsulfatase synthesis. Measurements of this capacity in cells taken at different developmental stages from a selection synchronous and from a light-dark synchronized culture showed that: (1) Arylsulfatase was derepressible at all stages of the life cycle. (2) The same periodic capacity patterns were found, both with the synchronized and the synchronous cells. Furthermore, the rate of accummulation of RNA and protein changed in the same periodic manner during the life cycle as did the enzyme capacity.

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.

[Polarographic measurement of nitrite reduction by chlorella in monochromatic light]

In green plant cells nitrite is reduced by two systems, one dependent on photosynthesis and the other upon respiration. Using a polarographic method for continuous measurement of nitrite uptake, the relationship between light driven and respiration linked nitrite reduction of Chlorella cells was studied.Photosynthetic nitrite reduction is characterized by a pronounced increase in the velocity of nitrite uptake upon illumination. After the light is turned off the velocity immediately returns to the preillumination value. Photosynthetic nitrite reduction of Chlorella is separated from respiration linked nitrite reduction by illumination with red light under anaerobic conditions; it is stimulated by CO2 and is inhibited by DCMU, findings which confirm earlier observations.In white light a special blue light stimulation of nitrite uptake is overlapped by photosynthetic nitrite reduction. In contrast to photosynthetic nitrite reduction this type of light stimulation is characterized by a lag period of about I min from the onset of illumination; it continues about 10 min when the light is turned off. It is separated from photosynthetic nitrite reduction by irradiation of the algae with low intensities of short wavelength light (<500 nm). Blue light stimulation of nitrite uptake of Chlorella is strongly dependent on the developmental stage of the cells. It is observed with young cells (autospores) of synchronized algae only.There is no evidence for any connection between blue light stimulation of nitrite uptake and photosynthesis. From the sensitivity of this process towards anaerobic conditions and antimycin A it is concluded to be a stimulation of respiration linked nitrite reduction.Under conditions of low exogenous nitrite concentration a temporary inhibition of steady state dark nitrite reduction appears immediately after the light is turned off. From several observations it is concluded that the inhibition already exists during the preceding illumination and decreases the rate of total nitrite uptake in the light. This process is suppressed by inhibition of respiration as well as by the inhibitor of photosynthesis, DCMU.If nitrate is the source of nitrogen an excretion of nitrite is found following illumination. The kinetics of this process agree with those observed for the light induced inhibition of steady state dark nitrite reduction immediately after illumination.

UPTAKE OF NITRATE AND NITRITE BY DITYLUM BRIGHTWELLII-KINETICS AND MECHANISMS(1) (2)

Ditylum brightwellii grown on NO2 - as a nitrogen source took up and assimilated NO2 - only in the light, apparently via a photosynthetic nitrite reductase. Assimilation was inhibited by dichlorophenyldimethylurea (DCMU), KCN, partially by 2,4 dinitrophenol, and by NO3 -. Kinetics of inhibition of NO2 - assimilation by NO3 - appeared to be "competitive." D. brightwellii cells grown on NO2 - took up NO3 - in both light and dark and in both cases the uptake was inhibited by p-chloromercuribenzoate, but not by DCMU, KCN, or by NO2 -. Most of the NO3 - taken up in the dark was recovered unchanged from the cells. However only 40% of NO3 - taken up in light was recovered from the cells and no NO2 - was found. This suggests that a photosynthetic nitrate reduction mechanism was active in these cells. DCMU inhibited the light-induced NO3 - reduction. This mechanism of NO3 - reduction is distinct from that involving NADH nitrate reductase in D. brightwellii since the concentration of the latter enzyme is very low in cells grown on NO2 -. Saturation kinetics were observed for NO2 - and NO3 - uptake. Half-saturation concentrations (Ks values) were 4 and 2 μM, respectively. These values are compared with those obtained for NO2 - and NO3 - assimilation by other unicellular algae. The comparisons show lower Ks values in oceanic species compared with tide-pool or freshwater algae and they support the idea that Ks values for NO3 - assimilation may provide a key to understanding species succession when this is due to declining: nitrate concentrations in the sea.

Timing of enzyme synthesis during outgrowth of spores of Bacillus cereus. II. Relationship between ordered enzyme synthesis and deoxyribonucleic acid replication

Experiments were carried out to determine whether, during outgrowth of bacterial spores, deoxyribonucleic acid (DNA) replication provided the basis by which ordered transcription was controlled. During outgrowth, significant DNA synthesis does not occur until just prior to the onset of cell division. However, incorporation of radioactive DNA precursors into DNA is observed within 5 to 10 min after the initiation of germination. By employing a thymine-requiring auxotroph and (3)H-bromodeoxyuridine, this incorporation appears to be a result of DNA replication and not repair synthesis. For the following reasons it was concluded that, during outgrowth, transcriptional processes were not ordered by DNA replication. (i) In a thymine auxotroph, thymine addition did not alter the periodicity of induced alpha-glucosidase and histidase synthesis during outgrowth. (ii) DNA synthesis was inhibited 80% by 5-fluoro-2'-deoxyuridine (FUdR), and, after a 5-min lag, completely by mitomycin C, but these inhibitors exerted a differential effect on induced histidase synthesis. Enzyme synthesis was insensitive to FUdR but was inhibited by mitomycin C, presumably as a result of cross-linking of the complementary DNA strands.

Kinetic studies of the induction of nitrate reductase and cytochrome c reductase in the fungus Aspergillus nidulans

In an earlier paper (Cove, 1966) it was reported that the kinetics of appearance of nitrate reductase (NADPH-nitrate oxidoreductase, EC 1.6.6.3) on the addition of nitrate to a growing culture of Aspergillus nidulans were different in certain respects from those found for many Escherichia coli enzymes. When urea is used as an initial nitrogen source, a further difference is found: enzyme synthesis is no longer continuous. This interruption of synthesis does not appear to be due to synchronous cell division in the culture, nor to be due to accumulation of ammonia. Fluctuations in the intracellular concentration of nitrate, though appearing to be partly responsible for the discontinuity of enzyme syntheses, cannot account for all the observations. Two related hypotheses are put forward to explain this discontinuity of synthesis; each suggests that nitrate reductase is intimately concerned with its own synthesis. One possibility is that the enzyme when it is not in the form of a complex with nitrate is a co-repressor of its own synthesis, and the other that the enzyme is its own repressor.

[Studies on light-dependent nitrite reduction in Chlorella]

1. The dependence of nitrite reduction activity in Chlorella fusca SHIHIRA et KRAUSS, as measured by uptake of nitrite, on the developmental stage, light intensity and CO2 concentration was investigated. 2. There is a quantitative relation between uptake of nitrite-nitrogen and increase of organic cell-nitrogen. 3. The kinetics of nitrite reduction in light are linear with time at high CO2 concentrations (3% in air) but biphasic at low CO2 concentrations (0 to 0.03%). In the steady state phase nitrite uptake is stimulated by an increase in the CO2 concentration from 0 to 0.03%. The initial phase and the steady state phase exhibit a different temperature dependence. 4. Nitrite uptake in the steady state phase is completely inhibited by iodoacetamide at concentrations which inhibit the photosynthetic CO2 reduction (5×10(-5) M). At lower iodoacetamide concentrations (1 to 2×10(-5) M) the linear time course of nitrite uptake at high CO2 concentrations is changed in the same manner as it is when the CO2 concentration is lowered. It is concluded that under special conditions (e.g. low CO2 concentration) the rate of steady state nitrite reduction in light is limited by the supply of carbon skeleton (i.e. N acceptors in the synthesis of organic nitrogen compounds) from the photosynthetic CO2 reduction cycle. 5. Pre-illumination in the presence of nitrite followed by a dark period depresses the initial phase of nitrite uptake in light at low CO2 concentrations. Pretreatment with nitrite in the dark lowers the nitrite uptake of the cells in the initial phase by exactly the same amount of nitrite which has been taken up during the pre-darkening period. It is concluded that there is a correlation between the initial phase of nitrite reduction in light and the respiration-linked nitrite reduction in the dark.