18 Bacterial _ Eukaryotic Gene Expression 09

18 Bacterial _ Eukaryotic Gene Expression 09 - Biol 61...

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(Ch.18) Control of Gene Expression in Bacteria We know that the genotypes of organisms consists of combinations of different alleles, which create alternative forms of the polypeptides encoded by genes, and it is the function of these polypeptides that determines the various physical and behavioral traits that make up the phenotype of an organism. We have talked about how genes are first transcribed into RNA and then translated into proteins, but we haven’t really discussed how a cell knows “the right place and the right time and the right amount” to transcribe a gene and translate it into a protein. We’ll start by discussing this in bacteria. “Gene expression ” includes the transcription and translation of a gene. For a gene to be “ expressed ” means that the gene is transcribed (to produce an mRNA copy of a DNA template sequence) and translated into a polypeptide that can do its job. Some proteins are always needed in all the cells of a prokaryotic species (or in all the cells of a multicellular eukaryote). These are “housekeeping” genes, such as the glycolytic enzymes, needed in both a heterotrophic bacteria or a multicellular animal. But not all proteins are needed at all times in a cell, so not all genes are expressed in every cell all the time. Of course, some cells don’t ever need to turn on certain genes (think of a pea plant – do you think a purple flower pigment needs to be made in the roots?), and many proteins are only needed in certain situations. How do cells control which proteins are active? More importantly, how do cells control which genes are expressed as proteins in the first place and which are not? Bacteria often have a short generation time (~20 min.) and live in highly variable environments (think about your gut bacteria and all the different types of food compounds they have to deal with over the course of a day). One way bacteria can deal with changing environments is through feedback regulation of biochemical pathways. For example, the synthesis of the amino acid tryptophan involves 5 enzymes in a pathway that works like an assembly line to take a particular chemical compound and turn it into tryptophan. If the concentration of tryptophan (the final product of the pathway) increases in the cell, it begins to act as a non-competitive inhibitor of the first enzyme of the pathway (allosteric regulation), reducing the formation of tryptophan through feedback inhibition. Why waste your energy making more tryptophan when there is clearly plenty available? While this type of feedback regulation is common, transcriptional responses are also fast and reversible, and this saves even more energy than regulating a biochemical pathway (the cell saves all the energy it would have used up transcribing and translating all those enzymes in the first place). In fact, the main mechanism that bacterial cells use to control gene expression is to
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This note was uploaded on 01/15/2011 for the course BIOL 61 taught by Professor Vierra during the Spring '08 term at Pacific.

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18 Bacterial _ Eukaryotic Gene Expression 09 - Biol 61...

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