The accompanying graph shows a growth curve of E. coli growing in a defined medium containing equal molar amounts of glucose and lactose
Based on your knowledge of the regulation of the lac operon, the best explanation of these results is that
A) early on, due to the presence of lactose, the bacteria use the lac operon. Glucose is used later.
B) the bacteria use both sugars simultaneously.
C) the presence of glucose causes the formation of cAMP catabolic repressor protein (CRP) complexes, which shut down the lac operon until the glucose is used up.
D) glucose is used first. Once glucose becomes limiting, intracellular cAMP increases, forming cAMP–CRP complexes that activate the lac operon to use lactose.
E) early on the glucose binds to the repressor protein lacI and shuts down the lac operon.
Answer: D
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