VISIT THIS LINK FOR ANOTHER GREAT VIDEO ON THE LAC OPERON
|CLICK ON IMAGE TO ENLARGE VIEW. Source: http://www.nature.com/scitable/content/the-lac-operon-of-e-coli-5825|
WHY IS THE cAMP AND CAP SO IMPORTANT FOR THE CELL WHEN IT COMES TO ENERGY USE?
The lac operon uses a two-part control mechanism to ensure that the cell expends energy producing β-galactosidase, β-galactoside permease and thiogalactoside transacetylase (also known as galactoside O-acetyltransferase) only when necessary. It achieves this with the lac repressor, which halts production in the absence of lactose, and the Catabolite activator protein (CAP), which assists in production in the absence of glucose. This dual control mechanism causes the sequential utilization of glucose and lactose in two distinct growth phases, known as diauxie. Similar diauxic growth patterns have been observed in bacterial growth on mixtures of other sugars as well, such as mixtures of glucose and arabinose, etc. The genetic control mechanisms underlying such diauxic growth patterns are known as xyl operon and ara operon, etc.
FROM MY UNDERSTANDING, THE E.COLI CELL TRIES TO EFFICIENTLY UTILIZE IT'S ENERGY USE BY ONLY PRODUCING WHAT IS NEEDED. WHEN SYNTHESIZING ENZYMES FOR GLUCOSE AND LACTOSE BREAKDOWN, THE CELL PREFER THE USE OF GLUCOSE AS AN ENERGY SOURCE SINCE THE GROWTH RATE IS FASTER. BUT WHEN GLUCOSE IS LOW, THE BUILD UP OF cAMP OCCURS. THIS ACTIVATES THE CAP TO BIND TO THE PROMOTER ON THE DNA TO INCREASE THE PRODUCTION OF THE THREE COMPONENTS TO METABOLIZE LACTOSE. SO WHEN LEVELS OF GLUCOSE INCREASE THE CAP UNBINDS TO THE PROMOTER AND THE INACTIVE LAC REPRESSOR LOSES IT'S ALLOLATOSE (THE INDUCER), WHICH CHANGES REPRESSOR BACK TO A SHAPE SUITABLE FOR BINDING TO THE OPERATOR TO STOP THE PRODUCTION OF LACTOSE METABOLISM.