Question

Only the proteins that the bacteria Escherichia coli (E. coli) needs are synthesized at any given time. For example, when lactose is available in the environment, E. coli will express the lac genes that allow it to break down lactose to obtain energy. When lactose is not present, these genes are not expressed. mc012-1.jpg Which prevents the lac genes in the DNA of E. coli from being expressed most of the time? the lac repressor the lac operator the lac promoter the lac locator

Answer 1

The repressor binds to the lac operator when glucose is available and E. Coli does not need to digest lactose. So, in a form, it is both the repressor and the operator. 

Answer 2

The right answer is the lac repressor.

The lactose operon is the set of genes transcribed into polycistronic RNA whose translation products are involved in the catabolic pathway of lactose.

The lactose operon of Escherichia coli (6,237 bp) contains the following elements:

• 3 structural genes: Lac Z, Lac Y and Lac A coding for the enzymes of the metabolic pathway.

• Regulatory elements, promoter P, operator O.

• Upstream of the operon, the lac I regulatory gene codes for a regulatory protein.

Lac I (repressor lac): with a promoter different from the structural genes; code for the repressor, which inhibits gene expression of the operon by specifically binding to the DNA at the operator level. The expression of this repressor is constitutive: it is expressed whatever the growth conditions of the bacterium. Its affinity for the operator is, in turn, modified by the presence of lactose.