Effects of psoralen on replicon size and mean rate of DNA synthesis in partially synchronized cells of Pisum sativum L

We have examined by fibre autoradiography the spacing of replicons in pea root meristems during synchronized entry into S phase from arrest at the G1/S boundary. Pretreatment with the DNA cross-linking agent, psoralen, produces a marked shortening of replicon spacing, suggesting that premature arrest of the replication fork results in the recruitment of additional initiation points within a given replicon family. This is discussed in relation to models for the control of DNA replication.

Phosphoenolpyruvate carboxylase from pennywort (Umbilicus rupestris). Changes in properties after exposure to water stress

Umbilicus rupestris (pennywort) switches from C3 photosynthesis to an incomplete form of crassulacean acid metabolism (referred to as 'CAM-idling') when exposed to water stress (drought). This switch is accompanied by an increase in the activity of phosphoenolpyruvate carboxylase. This enzyme also shows several changes in properties, including a marked decrease in sensitivity to acid pH, a lower Km for phosphoenolpyruvate, very much decreased sensitivity to the allosteric inhibitor malate, and increased responsiveness to the allosteric effector glucose 6-phosphate. The Mr of the enzyme remains unchanged, at approx. 185 000. These changes in properties of phosphoenolpyruvate carboxylase are discussed in relation to the roles of the enzyme in C3 and in CAM plants.

Molecular weights of the major DNA polymerases in a higher plant, Pisum sativum L. (PEA)

The molecular weights of the soluble (alpha-like) and chromatin-bound (beta-like) DNA polymerases of pea have been determined. The bulk of the soluble activity consists of molecular species of ca 101,500 and ca 140,000 molecular weights. Smaller (49,000) and larger (182,000 and 234,000) species are also observed in some preparations. The chromatin-bound DNA polymerase exhibits a molecular weight of ca 50,000, although a larger (ca 88,000) species is also detected under conditions which favour aggregate formation.

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.

Development of nuclease activity in cotyledons of Pisum sativum L

The RNA content of pea cotyledons shows little change during the first five days of germination at 22°C. From day five onwards there is a rapid net degradation of RNA, which continues until day thirteen. The DNA content of the cotyledons increases slightly during the first nine days of germination, after which there is a net decrease. Acid and alkaline ribonuclease activities increase markedly between day one and day five, and then decline between day five and day nine. There is a second increase in the activities of both enzymes from day nine onwards. Soluble deoxyribonuclease activity exhibits a single peak, seven days after the onset of germination. The first increase in acid ribonuclease activity is only partially inhibited by cycloheximide at concentrations which severely inhibit protein synthesis.

Iso-enzymes of acid ribonuclease in cotyledons of Pisum sativum L

Acid ribonuclease from cotyledons of Pisum sativum is very stable, with a temperature optimum of 65° C. It has a molecular weight of 17,500 and there is evidence that a fragment, with a molecular weight of 3,100, retains enzyme activity. Acid ribonuclease from the cotyledons of five-day old seedlings may be fractionated into two iso-enzymes, I and II, by CM-cellulose chromatography. The increase in activity of iso-enzyme I is not inhibited by cycloheximide, whereas the increase in iso-enzyme II activity is strongly inhibited by cycloheximide. Cotyledons of 9-day old seedlings contain only iso-enzyme I, whilst cotyledons of 15-day old seedlings contain three iso-enzymes, I, IIa and III.

Onset of nucleic acid synthesis during germination of Pisum sativum L

Measurments of total nucleic acid content of the embryonic axis indicated that massive net synthesis of both DNA and RNA was initiated at approximately 30 h after the onset of germination. The onset of net nucleic acid synthesis was marked by an increase in the rate of incorporation of [(3)H]thymidine into DNA, and of [(3)H]orotic acid and [(3)H]uridine into both DNA and RNA. rRNA was usually more heavily labelled than tRNA, but was not preferentially accumulated, suggesting a grater rate of turnover of rRNA than tRNA. Some incorporation of precursors occurred prior to the onset of net nucleic acid synthesis, particularly into RNA. This was taken to represent nucleic acid turnover. There was no evidence that the "scavenging" pathways for nucleotide biosynthesis were more important than the "normal" pathways in contributing precursors for net nucleic acid synthesis.

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

Chromatin-bound and soluble DNA-dependent DNA polymerases were assayed in the embryonic axes of germinating peas (Pisum sativum L.). Activities of both enzymes increased markedly during germination, prior to the onset of net DNA synthesis. The activity of the soluble polymerase showed a better temporal correlation with net DNA synthesis than did the activity of the chromatin-bound polymerase. The activity of the soluble polymerase greatly exceeded the activity of the chromatin-bound polymerase, particularly during the period of DNA replication.

Metabolically labile DNA in aseptically grown seedlings of Pisum sativum L

Pea seedlings (Pisum sativum L.) were grown aseptically in a sterile flexible film animal isolator. Radioactivity from [Me-(3)H]thymidine, supplied to the roots of the seedlings, was specifically incorporated into pea DNA. There was no incorporation into RNA or into bacterial DNA; neither was there any non-specific binding of label to carbohydrate. Much of the radioactivity incorporated into the DNA was lost during a cold chase. The loss could not be accounted for by net DNA synthesis and cell division, and is therefore taken as strong evidence for DNA turnover. The significance of these results is discussed.

Stimulation of DNA Synthesis by Prolactin in Rabbit Mammary Tissue

The capacity of mammary gland tissue slices to incorporate [3H]thymidine into DNA was studied at intervals after intraductal injection of prolactin. Prolactin markedly increased the rate of incorporation of thymidine into DNA (as extracted by HCIO4 or by sodium dodecyl sulphate). The increase in incorporation rate occurred within 6 h of prolactin injection. Autoradiography of mammary tissue, after administration of [8H]thymidine in vivo, showed that the increased rate of incorporation of thymidine was caused by an increase in the number of alveolar cells entering S-phase. These results are discussed in relation to current theories concerning functional differentiation of mammary tissue.