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

CATABOLIC REPRESSION...                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

TRP Operon* - 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,