Further Evidence for the Role of the Tricarboxylic Acid Cycle in Morphogenesis in Blastocladiella Emersonii

Blastocladiella emersonii is a primitive, arbuscu-late, aquatic Phycomycete with a determinate system of growth. It produces motile, uniflagel-late swarmers which can develop into either (1) a thallus bearing an apical, thin-walled, colorless, papillate spore sac or (2) a thallus bearing an apical, thick-walled, brown, pitted, resistant sporangium. In the presence of 10-2 M bicarbonate salts, resistant sporangium plants are formed; in their absence, thin-walled plants are formed instead. Extensive and generally consistent in-direct evidence, derived from growth studies, led to the hypothesis that bicarbonate probably tended to decrease the rate of a biotin dependent oxidation of a-ketoglutarate and to increase the rate of CO2 fixation by way of oxalacetate or malate; this in turn was assumed to cause some degree of piling up of intermediates between a-ketoglutarate and citrate or its precursory C2 fragments within the fungus. Somehow, the sequence of biosynthetic reactions leading to the genesis of a resistant sporangium was thus initiated (Cantino, 1951, 1952). In a recent progress report (Cantino, 1953), it was suggested that "If our interpretations regard-ing the function of bicarbonate are correct, it should be possible, in cell-free preparations, (1) to demonstrate the presence of at least some of the enzyme systems associated with the tricar-boxylic acid cycle, (2) to verify the supposed effect of biotin, bicarbonate, and the various intermediates and metabolic inhibitors on the oxidation of a-ketoglutarate, and (3) to establish the relation of such inhibition to the biosynthesis of fat, chitin, and melanin. " (The latter syntheses are associated with the genesis of a resistant sporangium.) It is our purpose, now, to report the progress that has been made in this direction.