SATELLITE DNA COMPONENTS IN EUGLENA GRACILIS CELLS LACKING CHLOROPLASTS

Ribosomal RNA genes in Euglena gracilis mitochondrial DNA: fragmented genes in a seemingly fragmented genome

Because relatively little information is available about mtDNA in the euglenid protozoa, distant relatives of the kinetoplastid protozoa, we investigated mitochondrial genome structure and expression in Euglena gracilis. We found that isolated E. gracilis mtDNA comprises a heterodisperse collection of short molecules (modal size approximately 4 kbp) and that the mitochondrial large subunit (LSU) and small subunit (SSU) rRNAs are each split into two pieces. For the two halves of the SSU rRNA, we identified separate, non-contiguous coding modules that are flanked by a complex array of (primarily direct) A + T-rich repeats. The potential secondary structure of the bipartite SSU rRNA displays the expected conserved elements implicated in ribosome function. Label from [α-(32)P]GTP was incorporated in the presence of guanylyltransferase into each of the separate SSU and LSU rRNA fragments, confirming that these RNAs are primary transcripts, separately expressed from non-contiguous rRNA modules. In addition to authentic genes for SSU rRNA, we discovered numerous short fragments of protein-coding and rRNA genes dispersed throughout the E. gracilis mitochondrial genome. We propose that antisense transcripts of gene fragments of this type could have been the evolutionary precursors of the guide RNAs that mediate U insertion/deletion editing in the kinetoplastid relatives of the euglenids. To the extent that E. gracilis mtDNA is a representative euglenid mitochondrial genome, it differs radically in structure and organization from that of its kinetoplastid relatives, instead more closely resembling the mitochondrial genome of dinoflagellates in many of its features, an apparent evolutionary convergence.

Capacity for RNA synthesis in 70S ribosome-deficient plastids of heat-bleached rye leaves

In the leaves of rye seedlings (Secale cereale L.) grown at an elevated temperature of 32°C the formation of plastidic 70S ribosomes is specifically prevented. The resulting plastid ribosome-deficient leaves, which are chlorotic in light, represent a system for the identification of translation products of the 80S ribosomes among the chloroplastic proteins. Searching for the primary heat-sensitive event causing the 70S ribosome-deficiency, the thermostability of the chloroplastic capacity for RNA synthesis was investigated. The RNA polymerase activity of isolated normal chloroplasts from 22°-grown rye leaves was not inactivated in vitro at temperatures between 30° and 40°C. The ribosome-deficient plastids purified from bleached 32°-grown leaf parts contained significant RNA polymerase activity which was, however, lower than in functional chloroplasts. After application of [(3)H]uridine to intact leaf tissues [(3)H]uridine incorporation was found in ribosome-deficient plastids of 32°C-grown leaves. The amount of incorporation was similar to that in the control chloroplasts from 22°C-grown leaves. According to these results, it is unlikely that the non-permissive temperature (32°C) causes a general inactivation of the chloroplastic RNA synthesis in rye leaves.

The presence of DNA in ribosome-deficient plastids of heat-bleached rye leaves

In leaves of rye seedlings (Secale cereale L.) grown at 32 degrees C the formation of plastid (70-S) ribosomes is specifically prevented. The resulting plastid-ribosome-deficient leaves can be used as a suitable system to identify chloroplast proteins which are translation products of cytosolic (80-S) ribosomes. The ribosome deficiency in plastids is accompanied by a bleaching of the leaves in light. In experiments aimed at finding the primary heat-sensitive event leading to ribosome deficiency the DNA of rye chloroplasts has been identified. Its properties are similar to those of chloroplast DNAs from other higher plants. The ribosome-deficient plastids isolated from heat-bleached rye leaves contained a DNA species which was indistinguishable from that of chloroplasts with regard to buoyant density in CsCl equilibrium gradients, reassociation properties and fragment patterns obtained upon cleavage by restriction endonucleases. Its quantity was comparable to that of chloroplast DNA of green leaves grown at a permissive temperature (22 degrees C). These results suggest that, unlike the effect in heat-bleached Euglena strains, lack of chloroplast DNA cannot be considered as the reason for the primary effect of high temperature in rye leaves but steps in the biosynthetic pathway of plastid ribosomes themselves must be affected more directly.

5-Methylcytosine in Chlorelle pyrenoidosa DNAs

The presence of 5-methylcytosine in Chlorella pyrenoidosa (strain 211/8b) DNA's has been investigated by means of paper chromatography and thermal chromatography on hydroxyapatite. It has been shown that nuclear DNA contains 3.5 mol% 5-methylcytosine whereas no significant amount of this base can be detected in chloroplast DNA. The thermal chromatography of nuclear DNA labelled from [6-3H]- or [Me-14C] methionine lead us to conclude that the 5-methylcytosine content is directly proportional to the G + C content of the various DNA fractions. The existence of methylated sequences in DNA is postulated and the biological function of the 5-methylcytosine is discussed.

Characterization of cytoplasmic and nuclear genomes in the colorless alga Polytoma. II. General characterization of organelle nucleic acids

Polytoma obtusum has a main band DNA (alpha) with a buoyant density in CsC1 of rho = 1.711 g/ml and a light DNA satellite (beta) with rho = 1.682 g/ml. beta-DNA was substantially enriched in a fraction containing small leucoplast fragments and some mitochondria, which was obtained in a pellet sedimenting between 3,000 g and 5,000 g. A crude mitochondrial pellet was also obtained by sedimenting at 12,000 g to recover particulates remaining in the supernate after 10 min at 5,000 g. This fraction contained a third DNA component (gamma) with rho = 1.714 g/ml. We have concluded that the leucoplasts of P. obtusum contain the beta-DNA (1.6882) and the mitochondria possess the gamma-component (1.714). Two distinct classess of ribosomes were isolated and separated by sucrose density gradients, a major 79S species and a minor species at 75S. The major species possessed the 25S and 18S ribosomal RNA (rRNA), characteristic of cytoplasmic ribosomes, and these particles co-sedimented in sucrose gradients with the 79S cytoplasmic ribosomes of Chlamydomonas reinhardtii. The minor species was present in about 2% of the total ribosomal population but showed an eight-to-ninefold enrichment in the leucoplast pellet, suggesting that it was of organelle origin. These 73S particles had RNA components migrating very closely with the 18S and 25S species of the 79S ribosomes, but the base composition of the rRNA from these two classes of ribosomes was significantly different; the rRNA from the 79S ribosomes had a G+C mole ratio of 50.0%, while the rRNA from the 73S class had a ratio of 47.5%. By comparison, chloroplast ribosomes of C. reinhardtii were found to sediment at 70S and contain rRNA molecules of 23S and 16S, with a G + C content of 51.0%. These findings support the concept that the Polytoma leucoplast possesses characteristic genetic and protein-forming systems.

Biochemical differentiation of plastids and other organelles in rye leaves with a high-temperature-induced deficiency of plastid ribosomes

1. In developing rye (Secale cereale L.) leaves the formation of plastidic ribosomes was selectively prevented in light as well as in darkness, when the seedlings were grown at an elevated temperature of 32° instead of 22° where normal development ocurred. Plastid ribosome deficient parts of lightgrown leaves were chlorotic at 32°. - 2. At both temperatures the leaves contained under all conditions (light or dark, on H2O or nutrient solution) equal or very similar amounts of total amino nitrogen. In light, the contents of total protein and dry weight were lower at 32° than at 22°, especially when the plants were grown on nutrient solution. - 3. Mitochondrial marker enzymes had normal or even higher activities in 32°-grown leaves. Respiration rates were similar for segments of leaves grown on water in light either at 32° or at 22° but by 20-30% lower for 32°-grown plants when they had been raised in darkness or on nutrient solution. In contrast to 22°-grown tissue, respiration of 32°-grown leaf segments was rather insensitive to KCN. Comparative inhibitor studies indicated the presence of both the cyanide-sensitive and the cyanide-insensitive pathway of respiration in 32°-grown leaves. - 4. Leaf microbody marker enzymes were present in leaves grown at 32°. From chlorotic parts of 32°-light-grown leaves a typical microbody fraction was isolated on sucrose densitygradients. - 5. Leaves of seedlings grown at 32° contained only very low levels of ribulosediphosphate carboxylase activity and of fraction I protein. Photosynthetic (14)CO2-fixation of such leaves was only a few per cent of that observed in normal leaves, and no photosynthetic oxygen evolution was observed in chlorotic leaf segments. However, ten other soluble enzymes which are exclusively or partially localized in chloroplasts reached high activities under all conditions at 32° (Table 4). - 6. From chlorotic parts of 32°-light-grown leaves as well as from etiolated 32°-grown leaves a fraction of intact plastids was isolated and purified by sucrose gradient centrifugation which contained several soluble chloroplast enzymes. From the results we conclude that cytoplasmic protein synthesis must contribute a functional chloroplast envelope including the mechanism for the recognition and uptake of chloroplast proteins which are synthesized on cytoplasmic ribosomes.

The mitochondrial genome of Euglena gracilis

Mitochondrial DNA from Euglena gracilis has been investigated in its chemical and physical properties. Its G + C content is equal to 25%; its buoyant density in a CsCl density gradient (1.690 g/cm3) is higher, by 5 mg/cm3, than expected for a bacterial DNA having the same base composition. The buoyant densities of denatured and renatured DNA are higher than that of native DNA by 10-12 mg/cm3 and 6 mg/cm3, respectively. The melting temperature, Tm, is 77 degrees C in standard saline citrate; the first derivative of the melting curve shows a striking multimodality. Degradation of the DNA by micrococcal nuclease indicates that about 40% of the DNA is formed by stretches lower than 10% in G + C. In all its properties the mitochondrial DNA from Euglena gracilis is strikingly similar to that of Saccharomyces cerevisiae.

Circular chloroplast DNA from Euglena gracilis

Chloroplast DNA of the protozoan flagellate, Euglena gracilis, exists as circular molecules, 40 mum in contour length, as shown by electron microscopy and buoyant density analyses.

Effects of streptomycin on plastids in dividing Euglena

The plastids of dividing Euglena cells growing in the light in the presence of streptomycin decreased in length after a lag period of seven generations. The typical structure of the chloroplast was lost after a similar lag period. This loss of structure did not follow a regular pattern. After 11 generations the plastids resembled normal proplastids of dark-grown cells. Initial chlorophyll loss of treated cells was slow, but after 3 generations the rate of loss was about 0.5/generation, indicating a cessation of synthesis and a dilution among the progeny.

A multiple origin for plastids and mitochondria

The impressive homologies between mitochondria and plastids, on the one hand, and procaryotic organisms, on the other, make it almost certain that these important cellular organelles had their origin as independent organisms. The vast number of symbiotic relationships of all degrees of evolutionary antiquity which have been found in contemporary organisms point to the ease with which such relationships can be established.

Multiple amounts of DNA related to the size of chloroplasts : I. An autoradiographic study

Plants containing genetically small or large plastids appear in some euploid and trisomic types of Beta vulgaris.Small tissue samples of the first 6-10 leaves of nearly 30 euploid plants from 7 different generations were incubated in a solution containing (3)H-thymidine in dark/light cycles, for up to 72 hr. For semiquantitative autoradiography the chloroplasts were then prepared on the slides with various kinds of isolation media, and fixed with OsO4, glutaraldehyde, formaldehyde, or ethanol. The specifity of incorporation was tested by observing the extractability of label after differential treatment with acid or nucleases.Chloroplasts in leaves 2-11 cm long preferentially incorporate (3)H-thymidine. The silver grains over plastids appear to be in clusters (centres). A relationship between the number of grains and also between the number of centres on the one hand, and the chloroplast size on the other could be found.It is concluded that chloroplasts occur in various degrees of polyenergide organization, as has been described, for example, for blue-green algae. Regarding the presence and degree of polyploidy-the other form of genetic "polyvalency" -the experiments provided no information. A remarkable variation in chloroplast size (and number of labelled centres) was observed, not only between different plants or between different leaves of a plants, but also within small tissue samples.

A mutagenic effect of visible light mediated by endogenous pigments in Euglena gracilis

Mutant cells lackng chlorophyll, chloroplasts, and chloroplast DNA were produced by irradiating Euglena gracilis in aerobic conditions with visible or red light (greater than 610 nanometers) of an intensity equivalent to that of direct sunlight. The photosensitizer is apparently the endogenous chlorophyll present in the chloroplasts. These mutants are comparable to those induced by ultraviolet light, x-rays, heat, or streptomycin. Our findings indicate that visible light can serve as a mutagenic agent in the absence of exogenous photosensitizers, thus directly effecting the course of evolution of organisms containing chlorophyll.

Replication of chloroplast DNA of tobacco

An experimental method has been designed for determining the relative rates of replication of the chloroplast and nuclear DNA's of Nicotiana tabacum. By this method chloroplast DNA in week-old seedlings is being replicated several times faster than nuclear DNA.