Answer:
In the absence of glucose, the molecule that binds to the promoter region of the lac operon to allow RNA polymerase to bind effectively is called cAMP-CRP (cyclic AMP-cAMP receptor protein).
The lac operon is a group of genes in bacteria that are involved in the metabolism of lactose. The expression of the genes in the lac operon is regulated by the presence or absence of lactose and glucose in the cell. When glucose is absent and lactose is present, the lac operon is activated and the genes are transcribed into mRNA.
cAMP is a small molecule that is produced when glucose levels are low in the cell. When glucose is scarce, the levels of cAMP rise, and cAMP binds to the cAMP receptor protein (CRP). The cAMP-CRP complex then binds to a specific site on the DNA near the promoter region of the lac operon, which allows RNA polymerase to bind more effectively to the promoter region and initiate transcription.
Therefore, in the absence of glucose, cAMP-CRP binds to the promoter region of the lac operon and enhances the binding of RNA polymerase, which leads to increased expression of the genes in the lac operon and the metabolism of lactose.