Walter J. Dobrogosz

Studies on the regulation of ribosephosphateisomerase activity in Pediococcus pentosaceus

Abstract A number of normal cellular catabolites have been found to interfere with the conversion of ribose 5-phosphate to acetyl phosphate ( via phosphoketolase (EC 4.1.2.9) pathway reactions) in crude, cell-free extracts of Pediococcus pentosaceus . Fructose diphosphate, glucose 6-phosphate and 6-phosphogluconate inhibited this conversion; whereas, fructose 6-phosphate, glucose 1-phosphate and free glucose were without effect. In the case of 6-phosphogluconate, the interference was shown to occur, at least in part, at the level of ribosephosphate isomerase activity. Ribosephosphate isomerase ( d -ribose-5-phosphate ketol-isomerase, EC 5.3.1.6) in the organism was partially purified and analyzed in respect to its substrate affinity and reaction equilibrium. The ability of 6-phosphogluconate to control the rate of isomerization of ribose 5-phosphate to ribulose 5-phosphate in vitro has lead to suggestions concerning possible implications in vivo . In this connection, a tentative working model has been proposed suggesting that 6-phosphogluconate control over ribosephosphate isomerase may, under certain conditions, regulate the intracellular availability of ribose 5-phosphate for nucleic acid synthesis. It has been further speculated that such a regulatory function could provide some basis for delineating the mechanism of catabolite repression of formation of certain inducible enzymes.

Stimulation of cytochrome synthesis in Escherichia coli by cyclic AMP.

Abstract A cyclic AMP-requiring mutant of Escherichia coli K12 which grows slowly on glucose was found to contain reduced levels of cytochrome b 1 and cytochrome oxidase o . The addition of exogenous cyclic AMP stimulated the synthesis of these cytochrome components and restored growth on glucose to the normal rate observed with the parental strain. Cytochrome synthesis in the parental strain was also stimulated by exogenous cyclic AMP. These studies have provided evidence that cyclic AMP participates in regulating cytochrome synthesis in E. coli , and, coupled with other observations, have suggested a role for this cyclic nucleotide in controlling the construction and operation of the organisms membrane system.

General physiological considerations of catabolite repression in Pediococcus pentosaceus

The general physiology of catabolite repression of l-arabinose isomerase (l-arabinose ketol-isomerase, EC 5.3.1.4) in Pediococcus pentosaceus has been investigated. Particular emphasis has been placed on the regulation of enzyme formation as influenced by pentose or hexose metabolism. The capacity of cells to form the isomerase was found to vary widely with (a) the physiological age of the cultures and (b) the nature of the growth substrates. The early stages of growth on glucose, fructose and mannose, for example, were associated with complete repression of isomerase formation. This repression was released as the substrates became limiting for the growth of the organism. Cultures in a stationary phase of growth, however, rapidly lost the capacity to synthesize the isomerase unless a fresh supply of yeast extract or casein hydrolysate was added to the medium. Although some repression was noted, growth on ribose or xylose did not appear to result in the typical “glucose effect” repression observed when the hexoses were employed as growth substrates. As growth on the pentoses became limited by substrate depletion, a greatly stimulated capacity for isomerase formation was observed. These general observations were also noted during the formation of a galactose-induced s-galactosidase (EC 3.2.1.23). These results have been discussed in light of a recently proposed, tentative model for catabolite repression in this organism.