Lac Operon 1 Normally off • In the absence of lactose, the repressor binds with the operator locus and blocks transcription of downstream structural genes Lac Operon 2 1) Operon Off: No Lactose In the absence of lactose, a repressor protein (the product of a regulatory gene located elsewhere on the bacterial chromosome) attaches to the operator region of the operon. This blocks transcription of structural genes downstream (to its right). Suppression of transcription (and translation) prevents the unnecessary synthesis of enzymes for processing lactose. Lac Operon 3 Lactose turns the operon on by acting as the inducer • Binding of lactose to the repressor protein changes its shape and causes it to fall off the operator. RNA polymerase can bind to the promoter. Structural genes are transcribed The figure of “Lac Operon on.” 2) Operon on: Lactose Present Upon entering the cell, the substrate (lactose) becomes a genetic inducer by attaching to the repressor, which is rendered inactive and falls away. The operator is no longer closed off and its DNA becomes accessible to the RNA polymerase. The RNA polymerase transcribes the structural genes, and the mRNA is translated into enzymes that can digest lactose. Lac Operon 4 The figure of “Lac Operon on” continues on this slide. Arginine Operon: Repressible 1 Normally on and will be turned off when the product of the pathway is no longer required The figure of “Arginine Operon on.” 1) Operon On: Arginine being used by cell A repressible operon remains on when its nutrient products (here, the amino acid arginine) are in great demand by the cell. The repressor has the wrong shape to bind to the DNA operator without a corepressor, so that RNA polymerase is free to actively transcribe the genes and translation actively proceeds. Arginine Operon: Repressible 2 The figure of “Arginine Operon on” continues on this slide. Arginine Operon: Repressible 3 When excess arginine is present, it binds to the repressor and changes it. Then the repressor binds to the operator and blocks arginine synthesis. Arginine is the corepressor The figure of “Arginine Operon off.” 2) Operon Off: Arginine building up The operon is repressed when (1) arginine builds up and, serving as a corepressor, activates the repressor protein. (2) The activated repressor binds to the operator and blocking RNA polymerase and further transcription of genes for arginine synthesis. Arginine Operon: Repressible 4 The figure of “Arginine Operon off” continues on this slide. RNA and Gene Expression 1 Riboswitch: short nucleotide segment of mRNA that regulates translation of the same mRNA it is part of. RNA Interference: noncoding RNA molecules – including microRNA (miRNA), antisense RNA (aRNA), and small interfering RNA (siRNA), regulate gene expression in eukaryotes. RNA and Gene Expression 2 Barry Chess/McGraw Hill