Even in the cases where a bacterial cell is growing with lots of lactose measurements of bet-gal expression reveals it is low. What is likely to be in the media besides lactose and how is the affect you see happening?
When a bacterial cell is having lot of Lactose but beta -gal expression reveals it is low this is due to presence of large amount of Glucose in medium besides Lactose.
If a medium is having large amount of Glucose then cAMP concentration in that medium is very less( inverse relationship between Glucose concentration and cAMP).so a very less number of cAMP- CAP Complex will form.
Role of cAMP- CAP Complex is to induce lac operon by binding this complex to CAP binding site .This binding of cAMP-CAP trigger RNA Polymerase to bind and increase the gene expression.
In this case there should be high level of Glucose , means low level of cAMP, less cAMP-CAP complex , no trigger of RNA polymerase to bind, low level of gene expression.Therefore having Lactose in medium , presence of large amount of Glucose, decrease the expression of gene and beta-gal expression is low.
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