Development of chromatin-bound and soluble DNA polymerase activities during germination of Pisum sativum L

Cell cycling with the SEB: a personal view

This review, written from a personal perspective, traces firstly the development of plant cell cycle research from the 1970s onwards, with some focus on the work of the author and of Dr Dennis Francis. Secondly there is a discussion of the support for and discussion of plant cell cycle research in the SEB, especially through the activities of the Cell Cycle Group within the Society's Cell Biology Section. In the main part of the review, selected aspects of DNA replication that have of been of special interest to the author are discussed. These are DNA polymerases and associated proteins, pre-replication events, regulation of enzymes and other proteins, nature and activation of DNA replication origins, and DNA endoreduplication. For all these topics, there is mention of the author's own work, followed by a brief synthesis of current understanding and a look to possible future developments.

DNA-polymerase activity detected in situ. Relationship between incorporation of 3H-deoxytriphosphates and 3H-actinomycin D binding during the cell cycle in antheridial filaments of Chara vulgaris L. Effect of auxin and cytokinins

DNA-polymerase activity during the cell cycle (S + G2 + M + C type) in antheridial filaments cells of Chara vulgaris was studied using the autoradiographic method. Incorporation of 3H-deoxytriphosphates (3H-dTPs) during the whole of interphase indicates, that the cell cycle is not accompanied by distinct changes in enzyme activity. Incorporation of 3H-dTPs was also observed in spermatids and in early stages of spermatogenesis. Intensity of 3H-dTPs incorporation during interphase and spermatogenesis is similar to the intensity of 3H-actinomycin D (3H-AMD) binding. Auxin (IAA) and kinetin stimulate both 3H-AMD binding and 3H-dTPs incorporation; benzyladenine does not affect any of these processes. The in situ autoradiographic method of detecting DNA-polymerase activity reveals availability of DNA template for the enzyme rather than DNA polymerase activity itself.

Activities of DNA polymerases and RNA polymerases detected in situ in growing and differentiating cells of root cortex

Activities of DNA polymerases and RNA polymerases were studied by autoradiographic methods in growing and differentiating root cortex cells of Zea mays - a species in which endomitosis occurs - and Tulipa kaufmanniana - in which this process does not occur. In Tulipa kaufmanniana, the highest activity of DNA polymerase appears in the nuclei of meristematic zone during the S phase of the cell cycle. In Zea mays, endomitotic replication of DNA occurs in all growth and differentiation zones and the activity of DNA polymerase in the nuclei is similar to that in the meristematic zone. In both species, nuclear RNA synthesis, measured with 3H uridine incorporation, is highest in the meristematic zone and declines steadily with development. Activity of nuclear RNA polymerase is present in all developmental zones in both species and is similar to that in the meristematic zone. 3H uridine incorporation into nucleoli decreases markedly in both species, whereas the activity of nucleolar RNA polymerase remains at a high level in all root segments in Zea mays and decreases slightly in Tulipa kaufmanniana. It is argued that the differences between the incorporation of 3H uridine and that or 3H UMP may be caused by a reduction of the pool of endogenous ribonucleoside triphosphates. Marked activities of DNA polymerase and RNA polymerase in cytoplasm are possibly related to the growth and division of plastids and mitochondria.

Deoxyribonucleotide synthesis and DNA polymerase activity in plant cells (Vicia faba and Glycine max)

Enzymes of deoxyribonucleotide and DNA biosynthesis, which are little known in plants, were studied in root tips of germinating broad beans (Vicia faba) and in fast-growing cultures of soybean cells (Glycine max). The plant cells contain a ribonucleoside 5'-diphosphate reductase which is detected in vitro only during a limited period of growth, viz. 30--32 h after inhibition of Vicia seeds, and between the second and third day after inoculation of soybean cultures. In both species ribonucleotide reductase activity precedes maximum DNA synthesis. The reductases could be precipitated with ammonium sulfate but were not purified further due to the extremely low enzyme content of the plant extracts. Therefore the reductive pathway of deoxyribotide formation was also established in Vicia root tips by efficient labeling of the plant DNA with a ribonucleoside, [5-3H]cytidine, which reaches a maximum at the same time as the reductase activity measured in vitro. Cycloheximide inhibits this process, indicating the need for de novo enzyme induction. In contrast, DNA polymerase is present in the tissue throughout the entire development and rises only 2-fold in activity during the S phase. The soluble polymerases were partially characterized in both legume species and were found very similar to the DNA polymerase of pea seedlings. Ribonucleotide reductase is more likely a limiting component of DNA formation during the plant cell cycle than DNA polymerase.

Partial purification and characterization of the soluble DNA polymerase (polymerase-α) from seedlings of Pisum sativum L

Soluble DNA polymerase has been extracted from pea seedlings and partially purified by chromatography on columns of DEAE-cellulose or DEAE-Sephadex. The enzyme elutes from DEAE-cellulose as a single peak, but is fractionated into three peaks, SI, SIa and SII by DEAE-Sephadex chromatography. SIa and SII may be derived from SI by freeze-thaw treatment or by treatment with (NH4)2SO4. The ion and pH requirements and the sensitivity to N-ethyl maleimide of the pea seedling soluble DNA polymerase are similar to those of the DNA polymerase-α from vertebrates.

Chromatin-bound DNA polymerase from higher plants : A DNA polymerase-β-like enzyme

Chromatin-bound DNA polymerase has been extracted from pea (Pisum sativum L.) seedlings, and partially purified by solubilization from chromatin followed by chromatography on columns of either DEAE-cellulose or DEAE-Sephadex. The enzyme elutes from DEAE-cellulose as a single peak, but is fractionated into two peaks, CI and CII, by DEAE-Sephadex chromatography. If the enzyme is stored at-15°C for several days prior to chromatography, a third peak, CIII, derived from CII, is obtained. The polymerase is devoid of nuclease activity, and is relatively insensitive to N-ethyl-maleimide. These features, taken with the ion requirements and with data obtained from other plant species, lead to the suggestion that the chromatin-bound DNA polymerase of higher plants is similar to the DNA polymerase-β from vertebrates.

Correlation between deoxyribonuclease activity and DNA replication in the embryonic axes of germinating peas (Pisum sativum L.)

Crude chromatin preparations from pea seedlings contain calcium-dependent deoxyribonuclease activity, at least part of which is endonucleolytic. During germination, there is a dramatic increase in chromatin-bound deoxyribonuclease activity in the embryonic axis immediately prior to the onset of DNA replication. Evidence has been obtained for the presence of an inhibitor of deoxyribonuclease activity in chromatin preparations from embryonic axes not undergoing DNA replication.