21Gene Regulation in Bacteria and Bacteriophages

21Gene Regulation in Bacteria and Bacteriophages - Gene...

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" Gene Regulation in Bacteria and Bacteriophages Introduction Gene regulation means that the level of gene expression can vary under different conditions. Unregulated genes, which have constant levels of expression, are called constitutive genes. The majority of genes are regulated. The processes of metabolism, response to environmental stress, and cell division are all regulated processes in the cell. Regulation may occur at any stage of gene expression (Figure 14.1). Transcriptional Regulation Transcriptional regulation involves the actions of regulatory proteins that bind to the DNA and affect the rate of transcription. Repressors inhibit transcription, which is called negative control. Activators increase transcription, which is called positive control. Effector molecules do not interact directly with the DNA, but rather directly with the repressors or activators. This usually results in a conformational change in the activator or repressor. Effector molecules are named by how they affect transcription when they are present in the cell. Regulatory proteins are named by how they interact with the DNA (Figure 14.2). Inducers are effector molecules that cause an increase in transcription. Genes that are regulated in this manner by inducers are called inducible. Corepressors are effector molecules that bind to repressors, causing the repressor to bind to the DNA. They decrease the rate of transcription. Inhibitors bind to an activator protein and prevent it from binding to the DNA. Genes that are regulated in this manner are called repressible. The phenomenon of enzyme adaptation is due to the synthesis of cellular proteins. Enzyme adaptation means that an enzyme appears in the cell only after the cell has been exposed to the substrate for that enzyme.
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# Enzyme adaptation was studied extensively by Jacob and Monod. They examined lactose metabolism in E. coli . Their experiments indicated that: Exposure to lactose causes a 1,000 to 10,000-fold increase in lactose enzymes. This was due to an increase in the synthesis of the enzymes. The removal of lactose abruptly terminated the synthesis of enzymes. Mutations indicated that a separate gene encoded for each enzyme. Their results provided the experimental proof of enzyme adaptation. The lac operon encodes proteins involved in lactose metabolism. In bacteria, structural genes are often arranged together under the control of a single promoter. This is called an operon. Transcription of an operon creates a polycistronic mRNA, which contains instructions for two or more structural genes. Operons have a promoter at the beginning and a terminator at the end. In addition, there is an operator site that controls the ability of the RNA polymerase to transcribe the operon. The
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This note was uploaded on 05/19/2011 for the course BIO 325 taught by Professor Saxena during the Spring '08 term at University of Texas.

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21Gene Regulation in Bacteria and Bacteriophages - Gene...

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