Lecture18S10n - BIS101/Engebrecht Lecture18 5/11/10 The lac...

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BIS101/Engebrecht Lecture18 5/11/10 The lac operon: We will consider in depth the regulation of the lac operon where both a repressor and an activator operate to precisely control the expression of these genes. This is an excellent example for how a cell responds to changes in the environment and how a cell can integrate signals into one response. Lactose is a disaccharide made up of glucose and galactose. E. coli prefers to use glucose as its carbon source, but if glucose isn’t around, it can utilize lactose as its carbon source. Thus, genes that encode proteins necessary for lactose utilization are regulated such that when glucose is absent and lactose is present, they are turned on and when lactose is absent they are turned off. Before we consider the gene structure and function of the lac operon, we need to remind ourselves how genes are organized in bacteria. Coordinately expressed genes are grouped into operons. Operons are clusters of genes that function together and thus are regulated the same . Transcription from an operon results in the production of a single polycistronic mRNA. That is, more than one protein is translated from a single mRNA. The stop codon of one polypeptide is positioned close to the start codon of the next polypeptide and therefore, translation can continue. However, consider the consequence of a nonsense mutation in the first gene of an operon. Because translation will terminate, the ribosome will become dissociated and genes further down in the operon will not be expressed. Therefore, a nonsense mutation will have a polar affect on downstream genes. ( Consider what would happen when performing complementation tests in this situation!) There are three gene encoded by the lac operon: lacZ, lacY and lacA. LacZ encodes the enzyme beta-galactosidase; this enzyme converts lactose to glucose and galactose. In addition to this reaction, beta-galactosidase also converts lactose to allolactose, an isomer of lactose and the natural inducer of the system. LacY encodes a permease that transports lactose into the cell and lacA encodes a transacetylase. Surprisingly, the function of this last gene is not really understood and therefore will not be discussed further. In addition to these structural genes, the operon contains a promoter (the cis acting signals that direct RNA polymerase to initiate transcription) and an operator, this is a sequence that is the binding site for the repressor of the system, lacI. LacI is expressed from its own promoter. In the absence of lactose, there is very low expression from the operon, ~ 3-4 molecules of beta-galactosidase are present in the cell. However, within 3-5 min of sensing lactose in the environment, ~ 5000 molecules of beta-galactosidase are present. Once lactose is depleted from the environment, transcription is no longer induced. The mRNA has a short half life and is therefore quickly depleted. Why do you think the cell has any molecules of beta-galactosidase present in the absence of lactose? This is necessary to form the inducer allolactose, since beta-
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This note was uploaded on 07/02/2010 for the course BIS 101 taught by Professor Simonchan during the Spring '08 term at UC Davis.

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Lecture18S10n - BIS101/Engebrecht Lecture18 5/11/10 The lac...

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