De Novo Biosynthesis of Deoxyribonucleic Acid Polymerase during Wheat Embryo Germination

Maize DNA polymerase alpha is phosphorylated by a PCNA-associated cyclin/Cdk complex: effect of benzyladenine

The activity of maize DNA polymerases 1 and 2 (delta and alpha-type enzymes, respectively) is stimulated during germination if embryo axes are imbibed in the presence of benzyladenine. In vivo, DNA pol 2 is a phosphorotein that appears to be maximally phosphorylated previous to the S phase start time (by 12 h of germination, Coello and Vázquez-Ramos 1995a). We find that, in vitro, a PCNA-associated cyclin/kinase activity isolated from maize axes acquires an increasing capacity to phosphorylate DNA pol 2 as germination advances; moreover, the PCNA-associated kinase isolated from BA-treated maize axes germinated at 3 h phosphorylates DNA pol 2 at the same level observed in samples of axes germinated for 13 h in the absence of exogenous BA. PCNA-associated kinase activity from BA-treated axes germinated at 13 h maximal using DNA pol 2 as substrate. However, there is no modification in DNA polymerase activity as a consequence of protein phosphorylation. Results are discussed in terms of their significance for cell cycle regulation during seed germination.

Comparison among DNA polymerases 1, 2 and 3 from maize embryo axes. A DNA primase activity copurifies with DNA polymerase 2

Three DNA polymerase activities, named 1, 2 and 3 were purified from maize embryo axes and were compared in terms of ion requirements, optimal pH, temperature and KCl for activity, response to specific inhibitors and use of templates. All three enzymes require a divalent cation for activity, but main differences were observed in sensitivity to inhibitors and template usage: while DNA polymerases 1 and 2 were inhibited by N-ethyl maleimide and aphidicolin, inhibitors of replicative-type enzymes, DNA polymerase 3 was only marginally or not affected at all. In contrast, DNA polymerase 3 was highly inhibited by very low concentrations of ddTTP, an inhibitor of repair-type enzymes, and a 100-fold higher concentration of the drug was needed to inhibit DNA polymerases 1 and 2. Additionally, DNA polymerases 1 and 2 used equally or more efficiently the synthetic template polydA-oligodT, as compared to activated DNA, while polymerase 3 used it very poorly. Whereas DNA polymerases 1 and 2 shared properties of replicative-type enzymes, DNA polymerase 3 could be a repair-type enzyme. Moreover, a DNA primase activity copurified with the 8000-fold purified DNA polymerase 2, strengthening the suggestion that polymerase 2 is a replicative enzyme, of the alpha-type. This DNA primase activity was also partially characterized. The results are discussed in terms of relevant data about other plant DNA polymerases and primases reported in the literature.

Nucleic acid and protein synthesis and loss of vigour in germinating wheat embryos

A study has been made of the RNA and protein synthesising systems of wheat embryos isolated from seed lots having high viability but differing in vigour. The rate of RNA and protein synthesis in wheat embryos during the early hours of germination is related to the vigour of the seed lot. The imposition of a stress factor, in the nature of a sub-optimal germination temperature, during germination of isolated wheat embryos magnifies the differences in rates of protein and RNA synthesis between high and low vigour seed. Using cell-free protein synthesising systems it has been demonstrated that an important difference between high and low vigour embryos lies in the relative levels of messenger RNA in the embryo. High vigour embryos contain relatively higher levels of poly A(+)-RNA (i.e. potential mRNA species) than lower vigour embryos and furthermore the level of poly A(+)-RNA in high vigour embryos increases during early germination whilst in lower vigour embryos the level decreases. The difference in poly A(+)-RNA levels accounts, at least partially, for the differences in rates of protein synthesis observed between embryos from high and low vigour wheat seed during early germination at both optimal and sub-optimal germination temperatures.

The major DNA polymerase in cultured plant cells: Partial purification and correlation with cell multiplication

A DNA polymerase activity was isolated from cells of Oryza sativa L. grown in suspension culture. Molecular mass (∼ 180,000), optimal requirements for pH (neutral), Mg(2+) (5-10 mM), Mn(2+) (1 mM), template preference (activated DNA), lack of activity with native or denatured DNA, and sensitivity to N-ethylmaleimide and ionic strength are similar to those of the vertebrate α-polymerase. Like DNA polymerase α, the DNA polymerase described in this work is the most abundant in proliferating cells of Oryza sativa L., Parthenocissus tricuspidata (Siebold et Zucc.) Planchon, Acer pseudoplatanus L., and Medicago sativa L. and responds to changes in the rate of cell multiplication. We therefore postulate that this α-like DNA polymerase is the replicating enzyme of plant cells.

Multiple deoxyribonucleic acid polymerases from quiescent wheat embryos. Purification and characterization of three enzymes from the soluble cytoplasm and one from purified mitochondria

Three DNA polymerases (A, B and C) have been purified from the soluble cytoplasm of ungerminated embryos. Mainly on the basis of chromatographic, template-specificity and salt-inhibition evidence, we have characterized the three enzymes. Other physico-chemical and enzymic properties are described. From purified mitochondria we have purified a DNA polymerase that behaves like DNA polymerase B on chromatographic and template-specificity criteria. Only highly purified enzyme B from the soluble cytoplasm showed an exonuclease activity able to degrade 3'- or 5'-labelled polydeoxyribonucleotides, as well as a 'proof-reading' capacity.

Factors affecting the onset of deoxyribonucleic acid synthesis during wheat embryo germination. Study of the changes in DNA polymerases A, B and C and the pool of DNA precursors

DNA synthesis starts about 12 h after water imbibition in wheat embryos. We have determined that noticeable amounts of labelled thymidine are found inside the embryo only after 6 hr of germination. DNA polymerase C from ungerminated wheat embryos decreased markedly in activity during the first hours of germination, whereas the activities of DNA polymerases A and B increased, having a maximum at about 15 h or germination. Serological evidence has suggested a clear antigenic relationship between DNA polymerases A and C. Although the pool of ATP increases rapidly after water imbibition, the increase in the pool of dNTP species was much slower.

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.

Potential for the use of germinated wheat and soybeans to enhance human nutrition

Wheat and soybeans are the major agricultural exports of the United States. The U.S. sells more of each crop than any other nation. Soybeans are the main staple in China, but the U.S. sells more soybeans than China grows. For hundreds of millions of other people, wheat is the main staple. And yet, most Americans eat whole grains of neither wheat nor soybeans. In the United States, many nutrients of wheat and soybeans are lost in processing or are fed to animals. A highly significant share of the wheat nutrients are lost from the main foodstream when the germ and bran (with aleurone) portions are separated. Whole soybeans are carefully processed for food by only a handful of Americans.

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.