RNA metabolism and the control of protein synthesis in fish
This thesis examines the regulation of protein synthesis by pre - translational events; with particular reference to the means by which RNA is able to reduce the energetic cost of protein synthesis. Protein synthesis was measured by the application of a "flooding dose" of 3H-phenylalnine. Protein synthesis rates are then calculated from the "free intracellular" and "protein bound" specific radioactivity (dpm nmole-1 phenylalanine). A similar approach was used to investigate RNA synthesis: i.e. a flooding dose of 3H-uridine. As with protein synthesis RNA synthesis rates was assessed by the relating precursor and product (uridine nucleotide and RNA) radioactivity. Oxygen consumption was measured by monitoring the decline in partial pressure in calibrated respirometery chambers. In fish cells protein synthesis was regulated in terms of the amount (ie the "capacity" for protein synthesis) and the translational efficiency of the RNA. Translationally efficient RNA equated to RNA with an increased turnover. In order to minimise RNA production costs, rapidly synthesised RNA places more reliance on the salvage of exogenous nucleosides, as opposed to the relatively expensive alternative of intracellular synthesis. During yolk sac larval development of the African wels (Clarius gariepinus) protein synthesis rates decline whilst oxygen consumption and the amount of RNA (relative to protein) remains constant. Thus the increasing protein synthesis costs resulted from a reduction in RNA translational efficiency. This was mirrored by a declining RNA synthesis rate. Larval fish growth is primarily due to the repartitioning of yolk sac proteins since early life history stages are thought unable to sustain rapid rates of protein turnover. This pre - translational strategy optimises growth and regulates protein synthesis; whilst, at the same time maintaining the capacity for protein synthesis in anticipation of exogenous feeding.