You Have Discovered an Operon in a Bacterium That Is

Essay

You have discovered an operon in a bacterium that is turned on only when sucrose is present and glucose is absent.You have also isolated three mutants that have changes in the upstream regulatory sequences of the operon and whose behavior is summarized in the Table 8-58.You hypothesize that there are two gene regulatory sites, A and B, in the upstream regulatory sequence that are affected by the mutations.For this question, a plus (+) indicates a normal site and a minus (-) indicates a mutant site that no longer binds its transcription regulator. $$\begin{array}{l}\underline{\text { Transcription of the operon in different media }}\\\begin{array} { l c c c } & \begin{array} { c } \text { Glucose } \\\text { only }\end{array} & \begin{array} { c } \text { Sucrose } \\\text { only }\end{array} & \begin{array} { c } \text { Glucose + } \\\text { sucrose }\end{array} \\\hline \text { normal } \left( \mathrm { A } ^ { + } \mathrm { B } ^ { + } \right) & \text {OFF } & \text { ON } & \text { OFF } \\\hline \text { mutant 1 } & \text { OFF } & \text { OFF } & \text { OFF } \\\hline \text { mutant 2 } & \text { OFF } & \text { ON } & \text { ON } \\\hline \text { mutant 3 } & \text { OFF } & \text { OFF } & \text { OFF }\end{array}\end{array}$$
Table 8-58
A.If mutant 1 has sites A^- B⁺, which of these sites is regulated by sucrose and which by glucose?
B.Give the state (+ or-) of the A and B sites in mutants 2 and 3.
C.Which site is bound by a repressor and which by an activator?