| dc.description | Growth of a bacterial cell is viewed as a process whereby one type of macromolecule (E) is used to transform medium components into monomers (amino acids, ATP ...). These molecules in turn, are consumed by another type of macromolecule (C) responsible for the synthesis of both types of macromolecules (M = E + C). Simple model systems are presented in which the effect of macromolecular composition on steady state growth rate $(\mu)$ is studied. Sufficient conditions for optimal growth are determined. In the models studied, it was found that the optimal macromolecular composition (expressed as $\Phi\sb2$ $\equiv$ C/M or as $\alpha\sb{\rm r}$ $\equiv$ ribosomal proteins / total proteins) varies with the quality of the medium and that $\alpha\sb{\rm r}$ (optimal) is proportional to $\mu$. This occurs even when there is a mechanism for the regulation of ribosome activity that decreases the rate of initiation of protein synthesis with decreasing levels of monomers. Comparison with experimental results shows that, while the optimal situation depicts satisfactorily what occurs at medium to high growth rate, the experimentally observed composition $(\alpha\sb{\rm r})$ at low growth rate is in excess of what would be optimally required. In this semi-optimal situation, the above mentioned regulation of ribosome activity effectively changes the fraction of active ribosomes $(\Theta)$ and maintains monomer concentration sufficiently high so as to essentially stabilize the saturation level of the Macromolecular Synthesizing System (the C fraction) despite poorer growth conditions. The properties of monomers and free ribosomes taken as signals for the regulation of the gross macromolecular composition $(\Phi\sb2)$ are also considered. Free ribosomes were found, in the models presented here, to be satisfactory signals only in poor medium. The relationship between the intensity of regulation at the level of ribosomes activity and the intensity of regulation at the level of macromolecular composition is also examined. | en_US |
