The C4-dicarboxylic acid pathway of photosynthesis. Identification of intermediates and products and quantitative evidence for the route of carbon flow

1. When leaves with the C(4)-dicarboxylic acid pathway of photosynthesis are exposed to (14)CO(2) the major labelled compounds formed, in order of labelling, are dicarboxylic acids, 3-phosphoglycerate, bexose phosphates and sucrose. During the present studies several quantitatively minor intermediates were identified and their labelling behaviour is described. 2. The pattern of labelling of dihydroxyacetone phosphate, fructose 1,6-diphosphate and ribulose di- and mono-phosphates during radiotracer pulse-chase experiments was consistent with their operation as intermediates in the pathway of carbon dioxide fixation. 3. Serine, glycine, alanine and glutamate had labelling patterns typical of products secondary to the main flow of carbon. 4. The mechanism of the transfer of label from C-4 of dicarboxylic acids to C-1 of 3-phosphoglycerate was also examined. Evidence consistent with pyruvate being derived from C-1, C-2 and C-3 of oxaloacetate, and for a relationship between ribulose 1,5-diphosphate and the acceptor for the C-4 carboxyl group, was obtained. 5. Evidence is provided that, under steady-state conditions, essentially all the label incorporated from (14)CO(2) into C-1 of 3 phosphoglycerate enters via C-4 of the dicarboxylic acids. These and other studies indicated that the route via dicarboxylic acids is essentially the sole route for entry of carbon into 3-phosphoglycerate.     

Nitrite oxidase and nitrate reductase in Nitrobacter agilis

Nitrite oxidase and nitrate reductase in Nitrobacter agilis were shown to be separate enzymes. The best separation of the two systems was achieved by ammonium sulphate fractionation. The effects of various compounds, including antimycin A, 2-n-heptyl-4-hydroxyquinoline N-oxide and chlorate, also clearly distinguish between the two enzyme reactions. The relationship between the two opposing reactions in Nitrobacter is discussed.     

The inhibition of pepsin-catalysed reactions by products and product analogues. Kinetic evidence for ordered release of products

1. The inhibition of pepsin-catalysed hydrolysis of N-acetyl-l-phenylalanyl-l-phenylalanylglycine by products and product analogues was studied. 2. The non-competitive nature of the inhibition by the product N-acetyl-l-phenylalanine confirms an ordered release of products, and points to a common mechanism (involving an amino-enzyme) for pepsin-catalysed transpeptidation and hydrolysis reactions. 3. N-Acetyl-l-phenylalanine ethyl ester is also a non-competitive inhibitor, but here the inhibition is of the ;dead-end' type. No ethanol is detectable in reaction mixtures, indicating that this ester cannot act as an amino group acceptor in a transpeptidation process. 4. The same is true for N-methanesulphonyl-l-phenylalanine methyl and methyl thiol esters. No methanethiol is liberated when the methyl thiol ester is present as an inhibitor of the hydrolytic reaction, and the hope that such a thiol ester would effectively trap the amino-enzyme was not fulfilled.     

Chromosome Replication in Salmonella typhimurium

The replication of the Salmonella typhimurium chromosome was studied. As with E. coli 15T(-), replication was sequential. After amino acid starvation, replication proceeded from a unique and heritable region of the chromosome. 5-Bromouracil, when substituted for thymine, did not disturb the sequence of replication nor did it initiate extra replication cycles. By labeling the origin and the terminus of the chromosome with (3)H- and (14)C-thymine, respectively, it was possible to determine that the rate of chain elongation decreases as the growth rate decreases. No gap in the replication cycle could be observed.     

Investigation of Reported Aflatoxin Production by Fungi Outside the Aspergillus flavus Group

A screening study of 121 fungus isolates, representing 29 species, for aflatoxin synthesis demonstrated this property only in Aspergillus flavus and A. parasiticus. Eight of the organisms found negative were isolates reported by other investigators to produce aflatoxin. Since similar negative reports have come from several other workers, it is concluded that only the A. flavus group of Aspergillus can presently be certified as sources of these toxins. Reasons for possible false-positive findings are discussed along with precautionary measures and differential analytical procedures useful in aflatoxin screening studies.     

Chromosomes of Syrphidae

In this report the karyotypes of 54 species of the tribe Milesiini and of four species of the tribe Myoleptini are described in detail with illustrations and idiograms. These species belong in the genera Lejota, Myolepta, Blera, Calliprobola, Criorhina, Hadromyia, Milesia, Somula, Sphecomyia, Spilomyia, Syritta, Temnostoma, Tropidia and Xylota. Six species have 2n = 8 chromosomes, 35 have 2n = 10 (including Xylota nemorum which has about 20 extra microchromosomes in some specimens), 15 have 2n = 12, one has 2n = 14, and Somula decora has 2n = 10 large chromosomes plus about eight microchromosomes. The mean total complement length (TCL) for 347 complements analysed in these tribes was 53.7 but there is great variation between TCL's of complements analysed even from a single fly. Karyotypes of species of Myolepta in the Myoleptini resemble in certain respects those of species of Tropidia in the Milesiini. Our observations support Currran's transfer of Lejota cyanea to the Milesiini. The 2n = 12 karyotypes of species of Blera, Criorhina, Lejota, Milesia, and to a lesser extent Sphecomyia, have some features in common. Spilomyia species have rather distinct 2n = 10 karyotypes. Certain species in Calliprobola, Syritta and Hadromyia are karyologically similar to some species of the genus Xylota in which species studied fall into fairly distinct karyological groups. These observations provide clear evidence of the accumulation of karyotypic variations in the origin of species in these two tribes.     

Genetic and biochemical analysis of the isoenzymes concerned in the first reaction of aromatic biosynthesis in Escherichia coli

Mutant strains of Escherichia coli K-12 were isolated possessing mutations which affected the tyrosine-inhibitable 3-deoxy-d-arabinoheptulosonic acid-7-phosphate (DAHP) synthetase, the phenylalanine-inhibitable DAHP synthetase, or the tryptophan-repressible DAHP synthetase. The mutations causing the loss of each of these activities have been mapped and are widely separated from each other on the E. coli chromosome. Chromatography on diethylaminoethyl cellulose columns allowed the recognition of four peaks of activity.