For an E. coli strain with the lac operon genotype I⁺&nb...

Question

For an E. coli strain with the lac operon genotype I⁺ P⁺ O⁺ Z⁺ Y⁺, identify the level of transcription of the operon genes in each growth medium listed. Specify transcription as 'none,' 'basal,' or 'activated' for each medium, and provide an explanation to justify your answer.

Growth medium contains lactose and glucose.

Accepted Answer

Hi, everyone. Here's our next question. When an E coli strain is grown in a medium containing glucose and lactose. Which carbon and energy source do they prefer? Choice? A lactose? B glucose, C, both lactose and glucose or D, none of the options are correct. Well, let's go ahead and cross out choice. D we know glucose and lactose are both sources of carbon and energy for E coli. So that's not going to be our answer. So let's think this through um there's advantages to using glucose over lactose glucose is a monosaccharide can be just metabolized right away versus the disaccharide lactose where the cell needs to break it down into single sugar molecules first. So it's an extra step, extra work. And the cell does have a mechanism for causing it to choose glucose. And that would be that C A MP cap binding system as part of the lack operon. So the lac operon governs the synth or governs the transcription of the enzymes that allow the cell to metabolize lactose. And in this case, when glucose is present C A MP levels in the cell are low and you won't have your maximal level of transcription of the lac enzymes unless C A MP vines to the cap protein that complex then binds to a specific binding site, which optimizes the binding of R N A polymerase to the promoter of the lac operon. And therefore optimizes lack operon transcription. So that's kind of a long way to work through that. If glucose is present, C A MP levels are low and all of that process, the C A MP cap binding does not occur. So I've crossed that out. So the C A MP cap system causes the cell to prefer glucose as its carbon and energy source over lactose when they are both present. So our answer will be choice B glucose and we can eliminate choice. A lactose and C both lactose and glucose. Because this cap regulatory system forces the cell to keep the lac operon transcription at a low level when glucose is present. So our answer here as to which carbon and energy source the E coli prefers is choice B glucose. See you in the next video.

Key Concepts

Lac Operon Structure

Regulation of the Lac Operon

Transcription Levels

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