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We have studied the 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) action on the different S states by oxygen, fluorescence and luminescence measurements. We show that no oxygen is evolved during a flash following the addition of DCMU to centers in their S3 state. This suggests that oxygen inhibition cannot be attributed solely to a blocking between Q and A. For all the photoinactive states, the only remaining pathway for the quencher reoxidation, in the presence of DCMU, appears to proceed through a back reaction. Therefore, the complete quencher regeneration still occurring when the fourth positive charge is formed in the presence of DCMU is also an indication of an action by DCMU at the donor side. The data well fit the model in which the oscillations of the fluorescence yield and their damping are attributed to a fast equilibrium between two forms of the centers: a photoactive and a photoinactive form, both of which are quenchers. The equilibrium constant depends on the number of positive charges stored and DCMU changes the characteristics of this equilibrium.

1. An apparatus and method are described with which the oxidation of labelled substrates to (14)CO(2) by cell suspensions may be examined. 2. The use of high-specific-radioactivity substrates at low concentration together with frequent quantitative collection of CO(2) permit a more detailed analysis of the appearance of various substrate carbon atoms as CO(2) than is possible by existing techniques. 3. Typical experiments with various cell types are reported, in which pathways of glucose oxidation are examined.

The extent of the electrical potential delta phi(SS) across the thylakoid membrane of Chlorella cells was estimated under steady state conditions. This has been achieved by comparing the absorption change which occurs after continuous light is switched off with a calibrated field indicating absorption change induced by flash light. Under saturating light conditions delta phi(SS) is in the order of 100 mV.

1. Growth conditions were devised which enabled Anacystis nidulans to be grown on high specific radioactivity (14)CO(2) (at over 98% isotopic abundance of (14)C) in a closed system with greater than 85% substrate utilization. 2. The DNA and RNA content of A. nidulans was estimated by various methods and compared with that of Chlorella pyrenoidosa. 3. A procedure was developed for the quantitative extraction and separation of the DNA and RNA from A. nidulans. 4. The fractionation and analysis of the DNA and RNA includes methods for the preparation of the uniformly (14)C-labelled deoxyribonucleoside monophosphates and the uniformly (14)C-labelled ribonucleoside monophosphates. 5. The base 6-methylaminopurine was identified as a component of the DNA. 6. The preparation and total yield of uniformly (14)C-labelled amino acids from A. nidulans are also briefly described. The results are discussed in relation to the use of other micro-organisms for the preparation of compounds labelled with (14)C.