SUMMARYThe presence of an incomplete tricarboxylic acid cycle in Anabaenu vuriabilis and Anacystis nidulans is described. These blue-green algae lack both a-oxoglutarate dehydrogenase and succinyl-CoA synthetase. Succinyl-CoA was formed in extracts of A. variabilis by 3-ketoacyl CoA-transferase using acetoacetyl-CoA as CoA donor. The activities of the other tricarboxylic cycle enzymes were measured in extracts prepared from autotrophic organisms and from those grown in the presence of acetate. No alterations in activity indicative of enzyme repression or de-repression were observed. These results are discussed in relation to possible reasons for autotrophic behaviour.

The behaviour, during the multicellular phase of the life-cycle, of amoebae of Dictyostelium discoideum grown in different media is described. Amoebal populations were marked by growth-temperature-sensitive genetic lesions which do not interfere with developmental phenomena. The fate of cell populations was determined by measuring the relative number of mutant and wild-type cells at various stages in the life-cycle. Cells sort out during development in such a way that they may be ordered in a sequence in which those given early in the following list preferentially appear in the spore population when mixed with those given later in the list: cells grown in axenic medium + 86 mm glucose and harvested when in the exponential phase of growth; cells grown in axenic medium and harvested when in the exponential phase of growth; cells grown on bacteria and harvested when in the exponential phase of growth; cells grown in axenic medium + 86 mM glucose and harvested when in the stationary phase of growth. Chemotactic aggregation and grex migration are not essential for sorting-out to occur but, in the normal life-cycle, the cells of a grex formed from amoebae grown in different media have sorted out anteroposteriorly. The relationship between this sorting out behaviour and the mechanism of pattern formation in fruiting-body morphogenesis is discussed. Differences in density of the amoebae cannot account for the sorting out predispositions we observe.

S U M M A R YThe respiratory electron transport system involved in NADPH (2.0 nmoles/ min./mg.) and NADH (1.0 nmoles/min./mg.) oxidation in the dark that was operative in extracts of light-grown Anabaena variabilis has been examined.NADPH oxidation was inhibited 50 % by cyanide (2 x I O -~ M), rotenone (IO-* M); antimycin A, amytal and azide were markedly less inhibitory. NADPH : ferrocytochrome c oxidoreductase, NADPH : rnenadione oxidoreductase, ferrocytochrome c : oxygen oxidoreductase and succinic dehydrogenase were detected. A phosphorylation (0.4 nmoles/min./mg.), associated with NADPH oxidation, was measured, NADPH could be replaced by NADP and isocitrate. This phosphorylation was absolutely dependent upon oxygen and was inhibited 25 yo by carbonyl cyanide p-trifluoromethoxyphenylhydrazone (5 p~) .

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DNA from two blue-green algae was isolated and characterized. The buoyant densities, thermal denaturation and re-naturation, thermal melting values, base compositions, sedi-mentation coefficients, and molecular weights were determined. Blue-green algal DNA renatured extensively and at a comparable rate to that of bacterial DNA. The similarities among the kinds of DNA from bacteria and blue-green algae were interpreted to reflect a close relationship. A popular hypothesis to explain the origin of chloroplasts is that they arose as the result of symbiosis between blue-green algae and eucaryotic cells. Indeed, Ris and Plaut (28) pointed out that the morphological appearance of DNA in blue-green algae cells seemed remarkably similar to the DNA observed in chloroplasts. Many other articles have appeared which support the endosymbiotic hypothesis (4, 9, 12, 23, 29, 34). Further evidence in support of the hypothesis would be information showing a considerable degree of relatedness in the physi-cochemical properties of blue-green algal DNA compared to the DNA of the chloroplast. Whereas extensive knowledge is available on the physicochemical properties of a variety of chloroplastic DNA (17, 19, 35, 38), information on blue-green algal DNA is very limited (2, 10, 14, 20). Thus, the purpose of this communication is to examine in detail the properties of DNA from two species of blue-green algae, so that more meaningful comparisons of these kinds of DNA could be made with chloroplastic DNA. As a further measure of the significance to be attached to relatedness in physicochemical properties, the blue-green algal and chloroplastic DNA were also compared to bacterial DNA. MATERIALS AND METHODS Axenic cultures of Anacystis niditlans (625 high temperature strain) and Lyngbya sp. (Indiana University Culture Collection No. 487) were grown at 25-30 C in 1-1 Erlenmeyer flasks containing 400 ml of liquid media (1) with constant supply of air. The cells were harvested by filtration and checked for absence of bacterial contamination before extracting DNA. The cells were homogenized and treated with lysozyme (6). DNA was extracted by the procedure of Marmur (24) using only one ethanol precipitation. At this stage the DNA was I Present address: treated with /8-amylase according to Ritossa and Spiegelman (30) and further purified with chloroform. DNA (15-20 ,ug/ml) in 0.15 M NaCl, 15 mM SSC2 was thermally denatured and renatured according to Marmur and Doty (25). Spontaneous or "snap back" renaturation was carried out by the procedure described by Dawid and Wolsten-holme (7). CsCl density gradient centrifugation was done according to Schildkraut et al. (32) in the Spinco Model E analytical ultra-centrifuge. Micrococcus lysodeikticus (p = 1.731 g/cm3) and E. coli (p = 1.710 g/cm') DNA species were used as markers. Alkaline CsCl was prepared by the method of Flamm et al. (11) by adding 100 jul 1 N NaOH to 3.3 ml of solution containing 10 to 30 ,ig of DNA in 10 mm tris-HCl buffer, pH 8.5. CsCl was then added to establish a proper initial den...