Question

To activate the transcription of permease (and the whole lac operon) lactose needs to be present in the cell. How can lactose get in the cell when the permease gene is not activated?

A. There is a second permease in the genome that initially transports lactose.
B. The repressed lac operon is never completely repressed.
C. Though the lac operon has been studied for many years, this mystery is unsolved.
D. Lactose freely moves across the plasma membrane.

Answer 1

The lac operon allows for low levels of enzyme production even when repressed, providing the means for lactose entry and subsequent operon activation when glucose levels are low.

Step-by-step explanation:

The question posed is concerned with how lactose enters E. coli cells to activate the lac operon if the permease needed for its import is not yet produced. The lac operon is indeed tightly regulated and is typically repressed in the presence of glucose.

However, it's key to note that repression is not absolute. A small amount of baseline transcription occurs, producing low levels of permease and other necessary enzymes. This baseline expression allows for some lactose to be imported into the cell and converted to allolactose, the inducer molecule. With the subsequent drop in glucose levels and presence of allolactose, the lac operon is induced, leading to the production of more permease and other enzymes required for lactose metabolism.

Therefore, the correct answer to the question is B. The repressed lac operon is never completely repressed. This ensures a minimal level of lactose import and metabolism, priming the cell for a scenario wherein the preferred energy source glucose depletes, and lactose becomes necessary for survival.