I

translation produces three proteins jn/

protein A protein B protein C

Figure 6.6. A bacterial operon is transcribed into a polycistronic mRNA.

the substrates of j-galactosidase are compounds like lactose that contain a j -galactoside linkage and are therefore called j-galactosides.

lac, an Inducible Operon jj-Galactosidase is encoded by one of the genes that make up the lactose (lac) operon, which is shown in Figure 6.8. The operon contains three protein-coding genes called lac z, lac y, and lac a. j -Galactosidase is encoded by the lac z gene. As noted before, gene names are always italicized, while the protein product is always in standard type. Lac y encodes jj-galactoside permease, a carrier (page 316) that helps lactose get into the cell. The lac a gene codes for transacetylase. This protein is thought to remove compounds that have a structure similar to lactose but that are not useful to the cell.

In the absence of j -galactoside compounds like lactose, there is no need for E. coli to produce j -galactosidase or j -galactoside permease, and the cell contains only a few molecules of these proteins. The lac operon is said to be inducible because the rate of transcription into RNA increases greatly when a j-galactoside is present. How is the transcription of the lac z, lac y, and lac a genes switched on and off? A repressor protein (the product of the lac i gene) binds to a sequence in the lac operon known as the operator. The operator lies next to the promoter so that, when the repressor is bound, RNA polymerase is unable to bind to the promoter. In the absence of a j-galactoside, the lac operon spends most of its time in the state shown in Figure 6.8a. The repressor is bound to the operator, RNA polymerase cannot bind, and no transcription occurs. Only for the small fraction of time allolactose

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