Transcriptional Activator Proteins in Prokaryotes Can Experience Shape Changes in the Presence

Transcriptional activator proteins in prokaryotes can experience shape changes in the presence or absence of small molecules. As a consequence, some may:

A) activate genes only when the small molecule is present in the cell.
B) activate genes only when the small molecule is absent from the cell.
C) repress genes only when the small molecule is present in the cell.
D) repress genes only when the small molecule is absent from the cell.