The metabolic control of development in the cellular slime mould Dictyostelium discoideum.
2015-11-19T09:08:36Z (GMT) by
Axenically-grown myxamoebae of Dictyostelium discoideum strain. Ax-2, containing various amounts of glycogen, develop in a manner chronologically and morphologically similar to myxamoebae of Dictyostelium discoideum strain NC-4 grown on Aerobacter aerogenes or Escherichia coli. Isotopic and other studies indicated the presence of two incompletely separate pools of cellular glycogen during this development: (1) Myxamoebal glycogen; present in the cell cytoplasm in granular form unbounded by membranes and degraded immediately upon the onset of development, probably by combined amylase and maltase action, to products which are mainly oxidised to carbon dioxide. (2) Developmental glycogen; synthesised in an unknown cellular location during aggregation, regardless of myxamoebal glycogen content, using hexose derived at least in part via gluconeogenesis. Net degradation of this glycogen occurred only during sorocarp construction, possibly by the action of glycogen phosphorylase which reaches peak specific activity at this time. This data is inconsistent with the model of Wright et al (1968) and was discussed in this context. Whilst end-product saccharides (trehalose, a mucopolysaccharide, cell wall polysaccharide) may in part be synthesised from developmental glycogen, analyses revealed that myxamoebal glycogen can also act as precursor. Investigations of this system showed that the pathways of end-product saccharide synthesis are not coordinately controlled. Moreover, trehalose accumulation is regulated not by the cellular content of trehalose-6-phosphate synthase nor trehalase but rather by the cellular concentrations of UDP-glucose and glucose-6-phosphate, the substrates for trehalose synthesis. Although some myxamoebal glycogen can be used for end-product saccharide synthesis, most was oxidised to carbon dioxide probably with the production of metabolically useful energy, but this failed to inhibit developmental protein or RNA degradation and oxidation. It is therefore suggested that protein and RNA degradation is essential for successful development for reasons other than energy production. The remarkable versatility of metabolism during the developmental phase of Dictyostelium discoideum is discussed.