What is the regulatory mechanism that allows e. coli to ignore lactose when glucose is present?

              The regulatory mechanism that allows E.coli to ignore lactose                           when glucose is present is controlled by Lac Operon. 

Explanation:
                    Lac operon is inducible and and normally turn off. It contain genes responsible for lactose metabolism and expressed when lactose is present and glucose is absent. 

Structure of Lac Operon:
                                          Lac operon is composed of following parts as shown in the figure:

1. Catabolic activator protein (CAP):
                                                              It acts as glucose sensor and activate transcription of operon through hunger signal (cAMP) when glucose level is low. 

2. Promoter:
                   It is binding site for RNA polymerase enzyme, that start transcription.

3. Operator:
                  It is binding site for Lac repressor protein. when there is no need of lactose repressor protein bind to promoter and stop transcription. 

4. Structural genes:
                                  This region is composed of Lac Y, Lac Z and Lac A genes. These genes are transcribed into lactose when needed. 

Regulatory mechanism 
                                    When glucose is present, no cAMP is made. CAP can't bind DNA with out cAMP, so transcription occur at only low rate or stop.