The Enzymes of the Galactose Operon in Escherichia coli

The frequencies of translation of the structural genes for uridine diphosphogalactose 4-epimerase and for galactokinase have been determined in wild type Escherichia coli and found to be equivalent under a variety of conditions. As these structural genes are the terminal cistrons in the galactose operon, it is concluded that polarity is not an inherent characteristic of this operon. The addition of amino acids to the growth medium doubles the growth rate, and hence the sum of all translation frequencies in the cell, but causes little, if any, change in the translation frequency of the galactose operon. This surprising result indicates that the galactose operon belongs to a subset of units of transcription the translation frequency of which is relatively insensitive to changes in the intracellular concentration of amino acids and ribosomes, and suggests that this translation frequency is strictly controlled by the transcription frequency. The model of Morse, Baker, and Yanofsky (Proc. Nat. Acad. Sci. U. S. A., 60, 1428 (1968)), in which each messenger RNA is translated once by a limited cluster of ribosomes following immediately behind the grow point for transcription, allows such a strict control. Test of this model against the existing data on the translation of the galactose operon indicates a remarkably good fit.