Chapter VIII: Transcription Starts and Stops in the lac Operon

Abstract

The levels of different proteins in single cell bacteria vary over a wide range. One example of a control mechanism which enables the cell to regulate the levels of different proteins under changing environmental conditions is provided by the Jacob-Monod theory (Jacob and Monod, 1961). The amounts of a given protein can be varied as much as 1000-fold by the use of specific inducers and repressors.

However, one finds that the maximal levels of different proteins also vary greatly (Pardee and Beckwith, 1962). A good example is the lac repressor, compared with β-galactosidase. Under conditions of full induction, β-galactosidase comprises as much as 3% of the soluble cell protein. In contrast the repressor, apparently synthesized constitutively, comprises only about 0.003% of the soluble cell protein (Müller-Hill, Crapo, and Gilbert, 1968; Gilbert and Müller-Hill, 1966). Since β-galactosidase (monomer) is 3.5 times the weight of the repressor (monomer), this corresponds to a difference, in terms of numbers of molecules, of several thousand-fold (Zabin, 1963). What types of control mechanisms exist which enable the cell to vary the maximal levels of different proteins over such a wide range?

Operons undoubtedly have discrete starting and stopping points for messenger-RNA synthesis. The starting point for transcription has been termed the promoter. By varying the nucleotide sequence of the promoter, the cell can fix the rate of transcription for any given operon. Alternatively, the level of various proteins could be governed by varying the efficiency of the ribosomal binding site on the mRNA. Also, different messengers...