You Are Interested in Examining the Psf Gene $\quad$ $\quad$

You are interested in examining the Psf gene.It is known that Psf is normally produced when cells are exposed to high levels of both calcium (Ca²⁺) and magnesium (Mg²⁺) .MetA, MetB, and MetC are important for binding to the promoter of the Psf gene and are involved in regulating its transcription.MetA binds to the "A" site in the promoter region, MetB to the "B" site, and MetC to the "C" site.You create binding-site mutations in the A, B, and C sites and observe what happens to transcription of the Psf gene.Your results are summarized in Table 8-12. $\quad$$\quad$$\quad$$\quad$$\quad$$\quad$$\quad$$\quad$$\quad$$\quad$$\quad$$\underline{\text { Transcription in the presence of } \mathrm{Ca}^{2+} \text { and/or } \mathrm{Mg}^{2+}}$
$\begin{array}{lcccc}\text { Binding sites present } & \begin{array}{l}\text { No } \mathrm{Ca}^{2+} \\\text { or } \mathrm{Mg}^{2+}\end{array} & \begin{array}{l}\text { With } \\\mathrm{Ca}^{2+}\end{array} & \begin{array}{l}\text { With } \\\mathrm{Mg}^{2+}\end{array} & \begin{array}{l}\text { With } \mathrm{Ca}^{2+} \\\text { and } \mathrm{Mg}^{2 *}\end{array} \\\hline \text { A, B, and C } & - & - & - & ++ \\\hline \text { none } & - & - & - & - \\\hline \text { A only } & - & - & + & + \\\hline \text { B only } & - & - & - & - \\\hline \text { C only } & - & + & - & + \\\hline \text { A and B only } & - & - & - & + \\\hline \text { A and C only } & - & + & + & ++ \\\hline \text { B and C only } & - & - & - & +\end{array}$

$\text {For this table: -= no transcription of Pst,}$
$\text {\( += \) low level of transcription of Psf, and}$
$\text {\( ++= \) high levels of transcription of Pst.}$
Table 8-12
Which of the following proteins are likely to act as gene repressors?

A) MetA only
B) MetB only
C) MetC only
D) both MetA and MetC