Overview

All the gene-encoded molecules of a cell – proteins and RNAs – are needed in amounts that differ from gene product to gene product and that change over the cell's lifetime. For example, if a cell has too little ribosomal RNA (rRNA), then protein synthesis is retarded; similarly, if molecule X becomes the sole carbon source for a cell and it fails to respond by synthesizing enzymes to catabolize X, then it will be starved for carbon and energy. Because RNAs encode proteins, one way to control the quantity of all gene products is to control the abundance of each kind of RNA.

Opposing forces determine the abundance of RNA: synthesizing RNA increases its cellular concentration, while enzymatic degradation of RNA, cell growth, and cell division reduce its concentration. This chapter is about the molecular machinery that controls rates of RNA synthesis and degradation. Other ways of controlling the amount of gene products are presented elsewhere.

Some of the key concepts of RNA regulation are (1) RNA stability, (2) positive and negative regulatory proteins (activators and repressors) that bind to (3) regulatory DNA sequences (promoters, upstream activating sequences, and enhancers), and (4) a regulatory system, the operon. This chapter focuses on bacteria; other simple forms are considered very briefly.

Abundance of Stable RNAs – rRNA and tRNA

In Escherichia coli undergoing exponential growth, about 97% of RNA is rRNA and tRNA. There are two reasons for this large excess: rRNAs and tRNAs are much more stable than mRNA (less susceptible to degradation), and the rates of synthesis of rRNA and tRNA genes are very high.