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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.