Biomedical subjects
D A Walker
Publications and source records attributed to D A Walker.
Thermal uncoupling in chloroplasts.
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Mixed monolayers of phospholipids and cholesterol.
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Polarity in gene araB of the l-Arabinose operon in Escherichia coli B/r.
A series of mutations are described which map in the araB gene of the l-arabinose operon and exert a polar effect on gene araA, the structural gene for the l-arabinose isomerase. Ten of the 20 araB point mutants examined exhibited absolute polarity and may represent insertions of genetic material into the araB gene. The remaining 10 point mutants exhibit strong polarity (less than 10% of the normal wild-type inducible level of isomerase) and may represent a class of externally suppressible polar mutations other than amber or ochre. Seven of the 12 araB deletion mutants examined, or 58%, exhibit polarity, suggesting that a shift in the reading frame has been generated in the polycistronic message for the l-arabinose operon. The remaining, presumably in-phase, deletion mutants exhibit hyperinducible levels of isomerase, an effect that is eliminated when an araB(+) gene is introduced in the trans position. The hyperinducibility effect is discussed in terms of a model for self-catabolite repression, originally proposed by Katz and Englesberg.
Inhibition, by sulphate, of the oxygen evolution associated with photosynthetic carbon assimilation.
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Photosynthesis by isolated chloroplasts. Reversal of orthophosphate inhibition by Calvin-cycle intermediates.
1. The orthophosphate inhibition of photosynthesis by isolated spinach chloroplasts can be reversed by 3-phosphoglycerate, dihydroxyacetone phosphate, glyceraldehyde 3-phosphate, fructose 6-phosphate and fructose 1,6-diphosphate. 2. Metabolically related compounds such as ribulose 1,5-diphosphate, glucose 6-phosphate, 6-phosphogluconate and phosphoenolpyruvate are ineffective. 3. The kinetics of reversal are characteristic of the intermediate used, but, in each instance, the onset of oxygen evolution is accompanied by a carbon dioxide fixation and except with 3-phosphoglycerate the stoicheiometry is close to unity. 4. The nature of orthophosphate inhibition and its reversal is discussed in relation to metabolic control of photosynthesis.
The isolation of spinach chloroplasts in pyrophosphate media.
A simplified procedure (involving disruption in sorbitol-pyrophosphate mixtures) permits the separation of spinach chloroplasts which retain the ability to catalyze the photosynthetic assimilation of carbon dioxide and its associated oxygen evolution.
Photosynthesis by isolated chloroplasts, simultaneous measurement of carbon assimilation and oxygen evolution.
Photosynthetic carbon assimilation by isolated chloroplasts and its associated oxygen evolution were measured simultaneously in a single simplified reaction mixture. Each showed an initial lag prior to the attainment of the maximal rate and the photosynthetic quotient was unity during the period of illumination. Following illumination an oxygen uptake was observed in the dark which was not accompanied by any net release of newly fixed carbon dioxide.
The egg-drop syndrome and related conditions--a review and discussion.
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Oxygen evolution by isolated chloroplasts with carbon dioxide as the hydrogen acceptor. A requirement for orthophosphate or pyrophosphate.
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The relation of oxygen evolution to carbon assimilation with isolated chloroplasts.
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Photosynthetic induction phenomena in spinach chloroplasts in relation to the nature of the isolating medium.
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Some effects of inorganic phosphate on O2 evolution by isolated chloroplasts.
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Temperature and photosynthesis. I. Some effects of temperature on carbon dioxide fixation by isolated chloroplasts.
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Physical studies of phospholipids. II. Monolayer studies of some synthetic 2,3-diacyl-DL-phosphatidylethanolamines and phosphatidylcholines containing trans double bonds.
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An illuminated constant-temperature water bath for the study of photochemical reactions in biological systems.
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Some effects of sugars and sugar phosphates on carbon dioxide fixation by isolated chloroplasts.
1. Carbon dioxide fixation by isolated pea chloroplasts was stimulated by the addition of intermediates of the Calvin photosynthesis cycle and by some related compounds. 2. Ribose 5-phosphate and fructose 1,6-diphosphate consistently produced the largest effects; free sugars such as erythrose and sedoheptulose and acids such as glycollate and glyoxylate were largely ineffective or even inhibitory. 3. Small effects were produced by fructose and ribose but not by their isomers, glucose and xylose. 4. Maximal rates in the presence of ribose 5-phosphate varied between 10 and 50mumoles of carbon dioxide fixed/mg. of chlorophyll/hr.
Calvin-cycle intermediates in relation to induction phenomena in photosynthetic carbon dioxide fixation by isolated chloroplasts.
1. Induction periods in carbon dioxide fixation by isolated pea chloroplasts were shortened by small quantities of Calvin-cycle intermediates. The additional fixation was larger than that which would have followed direct stoicheiometric conversion into ribulose 1,5-diphosphate. 2. When chloroplasts were illuminated in the absence of added substrates (other than carbon dioxide) soluble products were formed in the medium that stimulated fixation by fresh chloroplasts. 3. The induction periods were lengthened by washing the chloroplasts. Addition of catalytic quantities of Calvin-cycle intermediates then decreased the induction periods to their previous values. 4. The induction period was extended by a decrease in temperature but was largely unaffected by a decrease in light-intensity that was sufficient to decrease the maximum rate. 5. It is concluded that the lag periods are a consequence of the loss of Calvin-cycle intermediates, such as sugar phosphates, through the intact chloroplast envelope and that these losses can be made good by new synthesis from carbon dioxide in the reactions of the Calvin cycle.