Cis -acting factors are physically linked Trans -acting factors are separate entities Partial diploids demonstrate that they can act in cis and trans because the diffusible lac repressor protein can bind to the operator sequence found on the lac operon of the chromosome and the (F’ lac) because the lac O (operator region) can only act in cis The lac O can only act in cis because a functional operator must be located near the promoter of the linked lac genes on the same DNA molecule Catabolite repression of the lac operon Glucose, the catabolite of the breakdown of lactose, prevents high-level induction of the lac operon by inhibiting the enzyme adenylcyclase (which converts ATP to cAMP) resulting in low cellular levels of cAMP When glucose levels are low, cAMP levels are increased and bind to the catabolite activator protein CAP CAP is a positively acting regulator protein that binds to the control region of a gene when it is bound by the effector molecule cAMP Binding of CAP-cAMP to the CAP binding site in the promoter helps to recruit RNA polymerase to the promoter resulting in the induction of the lac operon CAP-cAMP mediates positive regulation of the lac operon The trp operon and attenuation Trp operon is a repressible system under negative control In the absence of tryptophan, the trp repressor protein is unable to bind to the trp operator and therefore RNA polymerase binds to the promoter and expression of the trp operon occurs (the derepressed ON state) When tryptophan is present it acts as a co-repressor and binds to the trp repressor allowing the repressor to bind to the operator and inhibit RNA polymerase binding thereby shitting of transcription (the repressed OFF state)
The trpL gene encodes a leader peptide with 2 tryptophan residues The trpL mRNA can form hairpin loop structures, one of which acts as a rho- independent terminator sequence When tryptophan levels are low, translation of the trpL mRNA pauses at the tryptophan codons allowing the formation of a hairpin loop structure that inhibits the formation of the terminator hairpin loop resulting in complete transcription of the trp operon When tryptophan levels are high, the entire trpL mRNA is translated allowing the formation of the terminator hairpin loop resulting in the termination of transcription of the trp operon
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- Fall '13