Regulation

Abstract

Ribosomes are needed for protein synthesis and the control of ribosome biogenesis has to respond to the requirements for protein production at any time in the life of a cell. Protein synthesis in prokaryotes involves the interaction of short-lived mRNAs with an excess of metabolically stable ribosomes, but such metabolic characteristics are clearly not valid for eukaryotes. In fact, the formation of ribosomes in growing eukaryotic cells occurs throughout most of the cell cycle, except for a short period during mitosis (see Lloyd et al. 1982). Ribosomes are in constant turnover even in non-dividing cells (Loeb et al. 1965, Hadjiolov 1966), which—although unexpected—occurs in practically all resting cells of higher eukaryotes (see Hadjiolov and Nikolaev 1976). Many cases are known wherein ribosome biogenesis displays a faster and deeper response to exogenous or endogenous Stimuli than mRNA. For example, growth stimulation of resting fibroblasts results in a more than twofold increase in protein synthesis and formation of new ribosomes, while synthesis of mRNA remains unchanged (Rudland et al. 1975). Similarly, differentiation of myoblasts into myotubes results in a 10-fold decrease of ribosome formation, while synthesis of mRNA remains quantitatively unaltered (Krauter et al. 1979). These and other results indicate that the control of ribosome biogenesis involves mechanisms that permit rapid changes in its intensity finely tuned to the rate of protein synthesis.