Adaptation in micro-organisms. II. Transient behaviour following shift-up and shift-down

Abstract

This is essentially an extension of the work presented in an earlier communication (Singh, 1976) describing a model of bacterial population in steady states of exponential growth. The paper consists of two parts: in Part 1 a quantitative analysis of observed variations in ribosomal content of bacterial cells with growth rate on the basis of a multi-level regulatory mechanism originally proposed in the above paper has been carried out. It is concluded that variations in (i) functional efficiency of ribosomes as inferred from the fraction of ribosomes (polyribosomes) actively engaged in protein synthesis and (ii) differential synthesis rate of ribosomal proteins constitute the two essential modes of regulation of macromolecular synthesis in exponentially growing cells. Relative changes in polyribosomal content of bacterial cells with growth rate as predicted from these analyses are shown to be quantitatively compatible with experimentally observed values. Mutually complimentary roles of (i) and (ii) are indicated by a reciprocal relationship between the corresponding curves. The formalism developed in Part 1 has provided the basis for an analysis of transient changes in relative amounts and synthesis rates of various macromolecular components - mRNA, rRNA, proteins etc. following shift-up and shift-down presented in Part 2. Several pertinent questions such as the mechanism of regulation of rRNA synthesis by free ribosomal proteins and its implications in the observed differences in elongation rates of mRNA and rRNA chains, role of ppGpp, changes in apparent growth rates following perturbations in nutritional level etc. are discussed.