LAC Operon* is an INDUCIBLE operon   (i.e.,exhibits negative control).
                                          it is always off and is turned on by an inducer molecule (allolactose)

               as long as glucose is present LAC operon is OFF, even if allolactose is present
             glucose prevents the action of the LAC operon through another regulator-like
             protein, the Catabolite Activator Protein or CAP and DNA binding site -
CAP gene
     CAP is an allosteric protein, regulated by cAMP    
           when glucose is low - all the ATP is hydrolyzed favoring high cAMP amounts
                 cAMP-CAP conformation can bind to CAP DNA region
      - favors rapid transcription
           when glucose is high - lots of ATP & little cAMP
                 CAP-alone conformation doesn't bind to CAP DNA region
- favors slow transcription

* - is a REPRESSIBLE operon... is always on & turned off by end product - tryptophan
                                                                  (turns off operon by attenuating transcription)
  back                 DNA recognition is via Repressor Proteins & DNA-Binding Protein Motifs
interaction of inducer & repressor  &  structure of repressor protein 
                                                         helix-turn-helix  & 
binding to DNA