Question

Shown below are relevant genes and sites from various E. coli strains. Note: + designates the wild-type gene or site that is fully functional, and – designates a deletion of that gene or site; lacO⁺ means all operators (O₁, O₂ and O₃) are functional; assume all other genes and sites not listed are wild-type and functional (including lacA). In some cases, a plasmid containing a wild-type functional gene or site was transformed into the E. coli strain. The strains were grown with or without glucose, and with or without lactose (for example, with glucose and without lactose is indicated by +/–). Indicate (Yes or No) whether maximum transcription at the chromosomal lac operon promoter occurs in the strain indicated under the conditions given.

Chromosome	Plasmid	Glucose/Lactose	CAP Bound?	LacI Bound?	Maximum Transcription?
crp+lacI¯ lacO+lacZ+lacY+	None	– / +	Yes No	Yes No	Yes No
crp+lacI¯ lacO+lacZ+lacY+	lacI+	+ / –	Yes No	Yes No	Yes No
crp+lacI+lacO¯ lacZ+lacY+	None	– / –	Yes No	Yes No	Yes No
crp+lacI+lacO¯ lacZ+lacY+	lacO+	+ / +	Yes No	Yes No	Yes No
crp¯ lacI+lacO+lacZ+lacY+	None	+ / –	Yes No	Yes No	Yes No
crp¯ lacI+lacO+lacZ+lacY+	crp+	+ / +	Yes No	Yes No	Yes No
crp+lacI+lacO+lacZ¯ lacY+	None	+ / –	Yes No	Yes No	Yes No
crp+lacI+lacO+lacZ¯ lacY+	lacZ+	– / +	Yes No	Yes No	Yes No
crp+lacI+lacO+lacZ¯ lacY¯	None	+ / –	Yes No	Yes No	Yes No
crp+lacI+lacO+lacZ¯ lacY¯	lacZ+	– / +	Yes No	Yes No	Yes No
crp+lacI+lacO+lacZ+lacY¯	None	– / +	Yes No	Yes No	Yes No
crp+lacI+lacO+lacZ+lacY¯	lazY+	+ / –	Yes No	Yes No	Yes No

Type Yes or No

Answer 1

Answer:

Maximum Transcription

Chromosome

Plasmid

Glucose/Lactose

Yes                 crp⁺ lacI⁺ lacO ¯ lacZ⁺ lacY⁺                  None                     – / +
No                 crp⁺ lacI⁺ lacO⁺ lacZ ¯ lacY⁺                  None                     + / +
No                 crp⁺ lacI⁺ lacO⁺ lacZ ¯ lacY ¯                 None                     + / +
Yes                crp⁺ lacI⁺ lacO⁺ lacZ ¯ lacY⁺                  lacZ⁺                     – / +
Yes                crp⁺ lacI⁺ lacO⁺ lacZ ¯ lacY ¯                 lacZ⁺                     – / +
Yes                crp⁺ lacI⁺ lacO⁺ lacZ⁺ lacY ¯                  None                     – / +
No                  crp⁺ lacI⁺ lacO ¯ lacZ⁺ lacY⁺                  lacO⁺                     + / –
Yes                 crp⁺ lacI ¯ lacO⁺ lacZ⁺ lacY⁺                  None                     – / +
No                 crp ¯ lacI⁺ lacO⁺ lacZ⁺ lacY⁺                  crp⁺                      – / –
No                 crp ¯ lacI⁺ lacO⁺ lacZ⁺ lacY⁺                  None                     + / –
No                 crp⁺ lacI ¯ lacO⁺ lacZ⁺ lacY⁺                  lacI⁺                     + / –
No                 crp⁺ lacI⁺ lacO⁺ lacZ⁺ lacY ¯                  lazY⁺                     – / –

• The lactose operon also called lac operon is required for the transport and metabolism of lactose in Escherichia coli.
• Even though glucose is the preferred carbon source for most bacteria, the lac operon allows for the effective digestion of lactose when glucose is not available (if both glucose and lactose available Ecoli prefer glucose over lactose).
• This operon has three structural genes (lacZ, lacY, and lacA), and a promoter, a terminator, regulator, and an operator.
• The fourth lac gene is lacI, encoding the lactose repressor is the regulatory gene for lac operon, which produces a protein that blocks RNA polymerase from binding to the promoter of the operon.
• Lactose repressor can only be removed, if allolactose binds to it, and inactivates it.
• The lac operon uses a two-part control mechanism to ensure that the cell expends energy on producing the enzymes only when necessary.
• In the absence of lactose, the lacI stops the synthesis of the enzymes encoded by the lac operon.
• In the presence of glucose, the catabolite activator protein is the product of the gene “crp”, required for the production of the enzymes, remains inactive, and prevent transport of lactose into the cell via lactose permease.
• This dual control mechanism causes the sequential utilization of glucose and lactose in two distinct growth phases.

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