113lecture1306 - BIOLOGY 113 - MICROBIOLOGY Lecture 13:...

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BIOLOGY 113 - MICROBIOLOGY Lecture 13: Microbial Genetics - Regulation by repression, induction and attenuation Much of the energy provided by catabolic metabolism is needed for synthesis of proteins - This can be appreciated by consideration of some the steps of protein synthesis = Each time that a nucleotide is incorporated into an mRNA molecule the equivalent of an ATP is hydrolyzed (in fact, some of them are ATPs!); actually, 2 ATP, since both phosphates are removed from the nucleoside triphosphates when they are incorporated into RNA = When amino acids are attached to tRNAs, ATP is hydrolyzed = Several of the steps in translation involve hydrolysis of ATP or GTP - It is clear that it is in the "interest" of a cell to synthesize a particular protein only when it is needed. Thus = an enzyme used in a catabolic pathway for oxidation of a particular nutrient should only be synthesized when the nutrient is available = an enzyme used in an anabolic pathway for synthesis of a particular molecule should only be synthesized when that molecule is needed by the cell - For greatest efficiency, it is at the level of transcriptional initiation that gene expression is regulated; thus, if an enzyme is not needed, the mRNA encoding it is not synthesized. - Regulation of gene expression allows cells to respond to changes in their environment How expression of genes is regulated depends on how the enzyme product of the gene is used by the cell - Some enzymes are present at all times in a cell = These are called constitutive enzymes = The genes for constitutive enzymes are transcribed at a relatively constant rate because they are always needed by cells = RNA polymerase is a good example of a constitutive enzyme - Some enzymes are needed only when a particular compound, such as a nutrient, is available to a
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113lecture1306 - BIOLOGY 113 - MICROBIOLOGY Lecture 13:...

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