Marcel Bayen

The 1.700 DNA of Chlorella pyrenoidosa: Heterogeneity and complexity

Abstract In the present work, it has been shown that Chlorella 1.700 DNA renatures rapidly to its native density after thermal denaturation. The kinetic complexity of this satellite DNA has been determined as 6.6·10 7 daltons and 5.3·10 7 daltons after correction for the low GC content. The melting curves have failed to show any intramolecular heterogeneity in base composition. These data allow us to suggest that 1.700 DNA is probably mitochondrial in origin.

5-Methylcytosine in Chlorella pyrenoidosa DNAs

Abstract The presence of 5-methylcytosine in Chlorella pyrenoidosa (strain 211 8 b ) DNAs 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.

The chloroplastic DNA of Chlorella pyrenoidosa (Emerson Strain): Heterogeneity and complexity

The heterogeneity and the complexity of Emerson strain Chlorella pyrenoidosa chloroplastic DNA have been investigated by means of thermal denaturation and renaturation kinetics, and the results have been compared with those of the strain 211/8b of the same alga.The thermal denaturation properties are very close to those of the other strain: the Tm of 65° C in 0.1 standard saline citrate, the maximal hyperchromicity of 41%, and the dispersion coefficient Δ 2/3 of 6.65° C. The first derivated curves of the melting profiles show also five components.Denatured chloroplastic DNA renatures rapidly. Two fractions are found; their kinetic complexities have been estimated: 1.5×107 daltons for the fast renaturing fraction; 2×108 daltons for the slow renaturing one (after correction for the low d (G+C) content of the chloroplastic DNA: 1.24×108 daltons). The unique nucleotide sequence is present in about 19 copies per chloroplastic genome.This report confirms the homogeneity of the chloroplastic genome of algae.

Incorporation of radioactive thymidine into the nucleic acids of Chlorella pyrenoidosa (211/8b)

Summary [me-14C] thymidine and [2-14C] thymidine are incorporated into nuclear, chloroplastic, and 1.701 DNA of Chlorella pyrenoidosa strain 211/8b. When the cultures are supplied with [2-14C] thymidine the radioactivity is essentially found to be associated with thymidine. The labelling of the other bases is not significant. The use of [me-14C] thymidine reveals that the major part of the radioactivity is still associated with thymidine, but that a small amount of the radioactivity is incorporated into 5-methylcytosine only for nuclear DNA. Furthermore the RNA does not present any radioactivity with [2-14C] thymidine. These results suggest that radioactive thymidine is specifically incorporated into DNA, but there are particular systems which are able to degrade a small amount of the precursor. On the other hand the low specific activity of the 1.710 DNA labelled arises the problem of its origin within the cell.

Heterogeneity of Chlorella pyrenoidosa nuclear DNA (Strain 211/8b and strain emerson)

Abstract The heterogeneity of Chlorella pyrenoidosa nuclear DNAs has been investigated by means of thermal denaturation. From our results, it appears that the guanine + cytosine (G+C) contents calculated from the melting temperature (Tm) are different for both strains, being 51.3% for strain 211/8b and 61.8% for the Emerson strain. On the other hand, their Δ 2 3 values are identical and their first derivative profiles of the melting curves very similar. In addition, these first derivative curves indicate the presence, in the nuclear DNA, of two main components and at least four minor ones. These components reflect an essentially intramolecular heterogeneity of Chlorella nuclear DNA. Finally, the reassociation experiments show that some components, richer in adenine + thymine (A+T) than the average A+T content, exhibit a quicker renaturation than the others. The significance of this result is discussed.

Determination physico-chimique de la ploidie chez l'algue unicellulaire Chlorella pyrenoidosa (souche 211/8b)

Abstract Physico-chemical determination of the ploidy of the unicellular alga, Chlorella pyrenoidosa (strain 211/8b) The ploidy of the unicellular green alga Chlorella pyrenoidosa (strain 211/8b) has been determined by means of renaturation kinetics. The nuclear DNA is made up from fast, intermediate and slow renaturing sequences, which represent respectively about 5, 15 and 80% of the DNA. These observations are consistent with the findings in other eukaryotic nuclear DNAs. Nevertheless, the relative importance of the repeated sequences is much lower than that observed in Chlamydomonas reinhardi [16] and in higher plants [18–20], but slightly higher than that obtained in Chlorella vulgaris [17]. The kinetic complexity of the main fraction of the Cl. pyrenoidosa nuclear DNA is found to be 2.94 · 10 10 daltons (mean value of five independant experiments) assuming a value of 2.1 · 10 8 daltons for Cl. pyrenoidosa chloroplastic DNA. When compared with the analytical complexity of this fraction (80% of the nuclear DNA analytical complexity, that is 2.02 · 10 10 daltons), one can assume that the slow renaturing fraction of the nuclear DNA is constituted by a unique nucleotide sequence. This result thus suggests that Cl. pyrenoidosa (strain 211/8b) is an haploid organism. The possible existence of an haploid genome in the nuclei of the algae from Chlorella genus and the apparent absence of sexuality might explain the high discrepancy observed in the G + C content of the Chlorella nuclear DNAs.

Incorporation of radioactive pyrimidic precursors into the deoxyribonucleic acids of Chlorella pyrenoidosa, strain Emerson

Summary (2-14C) thymidine is incorporated into the nuclear and chloroplastic DNAs of Chlorella pyrenoidosa, strain Emerson , (me-14C) thymidine is incorporated into chloroplastic DNA, but never into nuclear DNA. The radioactivity is still found to be associated with thymine. When the cultures are supplied with (2-14C) uracil, both DNAs are radioactive and the radioactivity is found to be associated with thymine, cytosine, and, for the nuclear DNA, 5-methylcytosine. (2-14C) deoxycytidine is also incorporated into both DNAs, but their specific activity is very low. These results suggest that thymidine is specifically incorporated into chloroplastic DNA of the strain Emerson . In the case of the nuclear DNA, they suggest the existence of a thymidine demethylation mechanism, probably localized in the cytoplasm.

Interaction of polyribonucleotides with nuclear and chloroplastic DNA of Chlorella pyrenoidosa

Abstract 1. The interaction of various polyribonucleotides with denatured chloroplastic and nuclear DNAs of Chlorella pyrenoidosa has been investigated. 2. Poly(U) and, to a lesser extent, poly(G) and poly(I,G) bind to the denatured chloroplastic DNA, whereas no interaction is detected with poly(A) and poly(C). 3. Poly(G) and poly(I,G) interact with denatured nuclear DNA. Poly(C) binds also, but only at acidic pH. Neither poly(A) nor poly(U) appear to form complexes. 4. Sheared nuclear and chloroplastic DNAs interact with poly(I,G) to give asymmetrical broad peaks slightly displaced towards greater densities in the CsCl gradient. 5. One may conclude that chloroplastic DNA contains dA · T-rich clusters and to a lesser extent dC · G-rich clusters, whereas nuclear DNA contains dC · dG-rich clusters but no detectable amount of dA · dT-rich clusters. Lack of DNA strands separation after interaction of polyribonucleotides suggests that the clusters enriched in one nucleotide are equally distributed between the two strands.

Interaction des ions argent avec l'adn nucleaire de Chlorella pyrenoidosa: en|Interaction of Ag+ ions with Chorella pyrenoidosa nuclear DNA

Abstract When centrifuged in Ag+-C s 2SO4 gradients, Chlorella pyrenoidosa (strain 211/8b and strain Emerson) nuclear DNA exhibits a satellite, the density of which is heavier than that of the main band. The presence and the buoyant density of this satellite strongly depend on pH and molar ratio : Ag+/DNA-P (rf) of the DNA solution. The binding of Ag+ ions on the heavy satellite occurs from the lower rf. It is quite unaffected by previous chelating agent treatment of the DNA solutions or by purification on hydroxyapatite columns. After removing of bound Ag+ ions, the densities of the heavy satellite DNA and main band DNA return to the native DNA density in CsCl gradient.