Diauxic growth of microorganisms

[garytse86]

Hi. Just a problem with diauxic growth. If there are glucose and lactose in a medium, the glucose will exhibit catabolic repression, inhibiting the syntheis of lactase. However when glucose is in scarce supply a microogranism will start making lactase.

The problem is, when lactase hydrolyses lactose, giving galactose and glucose, wouldn't the glucose from lactose cause catabolic repression?

 

[iansmith]

Hi. Just a problem with diauxic growth. If there are glucose and lactose in a medium, the glucose will exhibit catabolic repression, inhibiting the syntheis of lactase. However when glucose is in scarce supply a microogranism will start making lactase.

The problem is, when lactase hydrolyses lactose, giving galactose and glucose, wouldn't the glucose from lactose cause catabolic repression?

The regulation of the lac operon, or any other catabolic genes, by glucose is a bit more complex than described by books. It is technically not glucose doing the repression.

First when glucose is present, the ATP pool is high in concentration and cAMP is low. As glucose is used and the concentration is low, the amount of ATP produce by glucose metabolism decreases and cAMP increases. When cAMP is high, it is bound by Crp (cAMP recepetor protein) and it forms cAMP-CRP complex which induce genes that require it as an inducer. As lactose is used, the ATP pool is increasing and the cAMP is decreasing. Therefore the balanced shifts from cAMP to ATP. concentration of lac mRNA will decrease partily due to the decrease of cAMP and the increase of lacI that are not bond by lactose.

So to answer your question, yes metabolisim of lactose will inhibit the lac operon eventually but most of the resources will be used by that time.