In the Absence of Glucose, E Table 8-57
Do the Results in Table 8-57 Indicate That

In the absence of glucose, E.coli can proliferate by using the pentose sugar arabinose.As shown in Figure 8-57, the arabinose operon regulates the ability of E.coli to use arabinose.The araA, araB, and araD genes encode enzymes for the metabolism of arabinose.The araC gene encodes a transcription regulator that binds adjacent to the promoter of the arabinose operon.To understand the regulatory properties of the AraC protein, you engineer a mutant bacterium in which the araC gene has been deleted and look at the effect of the presence or absence of the AraC protein on the AraA enzyme.
Figure 8-57
A.If the AraC protein works as a gene repressor, would you expect araA RNA levels to be high or low in the presence of arabinose in the araC^- mutant cells? What about in the araC^- mutant cells in the absence of arabinose? Explain your answer.
B.Your findings from the experiment are summarized in Table 8-57. $\begin{array}{l}\quad\quad\quad\quad\quad\quad\quad\quad\quad\quad\quad\quad\quad\quad\quad\quad\quad\quad\quad\quad\quad\quad\underline{\text { araA RNA Levels }}\\\begin{array} { l l l } \text { Genotype }& \text { in the absence of arabinose } & \text { in the presence of arabinose } \\\hline \operatorname { araC } \mathrm { C } ^ { + } \text {(normal cells) } & \text { low } & \text { high } \\\hline \operatorname { araC } ^ { - } \text {(mutant cells) } & \text { low } & \text { low }\end{array}\end{array}$
Table 8-57
Do the results in Table 8-57 indicate that the AraC protein regulates arabinose metabolism by acting as a gene repressor or a gene activator? Explain your answer.