Adaptation in micro-organisms: variation in macromolecular composition with growth rate

A model of bacterial population maintained in a steady state of sustained exponential growth is developed. The analysis is based essentially on a stochastic model of protein synthesis on labile mRNA's described in earlier communications. Two basic postulates: (1) co-ordinate synthesis of ribosomal proteins and (2) free ribosomal proteins in the pool functioning as a positive control element in the synthesis of rRNA provide the necessary framework for biogenesis of ribosomes. Macromolecular compositions of bacterial cells, with respect to RNA and protein contents at a gross level, are defined in terms of various kinetic parameters related to protein synthesis and maturation of ribosomes. Three alternative modes of regulation operating at the level of (i) functional efficiency of ribosomes, (ii) transcription of rRNA and (iii) differential synthesis rate of ribosomal proteins have been analysed. It is concluded that while the observed variations in macromolecular compositions of bacterial cells with growth rate constant at medium to high growth rates are compatible with (iii), the regulation at the level of functional efficiency of ribosomes (i) may be important at low growth rates