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Transcript
The Arabinose Operon (http://faculty.clintoncc.suny.edu/faculty/michael.gregory/files/bio%20101/ Bio%20101%20Lectures/Gene%20Regulation/gene.htm) The arabinose operon uses both positive and negative control. The operon contains three different genes (ara A, ara B, and ara D) that code for three enzymes needed to convert arabinose to a usable form. A fourth gene codes for a protein (ara C) that acts to regulate the structural genes. The regulator protein (ara C) is needed for transcription of the three structural genes (ara A, ara B, and ara D). It binds to its own structural gene preventing its own transcription, thus autoregulating its own level. When the level of ara C is low, transcription occurs and more ara C is synthesized. Ara C also binds to other sites within the operon, inhibiting transcription of the three structural genes. The genes therefore are normally not active. When arabinose is present, it binds to ara C causing it to change shape. The new shape promotes the attachment of RNA polymerase to the DNA, thus allowing transcription to occur. Questions. 1) The lac operon is known as a “negative control” operon. Review the function of the operon and discuss why it would be seen in this way. 2) Lac is known as an “inducible operon, trp as a “repressible” operon. What is the difference between these. 3) Into which of these categories (inducible or repressible) does the arabinose operon fit? Explain your reasoning. 4) Use your idea of negative control from question 1 to explain how “positive control” would work in an operon. 5) Give one example of negative control and one example of positive control in the ara operon. 6) Evolution rarely has on “one size fits all” approach to things. Both the lac and ara operons are normally in the off position unless the right substrate comes along. Yet both operons are organized differently. Speculate on the utility of this to the organism.
