Q.19 In E. coli, pur repressor regulates the expression of more genes as compared to lac repressor because
1. pur repressor binds to DNA without corepressor
2. pur repressor is expressed at a much higher level
3. there are more pur operator sequences in E. coli genome
4. pur repressor is expressed at a much lower level
Pur repressor in E. coli regulates more genes than lac repressor due to more pur operator sequences across the genome. This makes option 3 the correct answer for this molecular biology MCQ on bacterial gene regulation.
Correct Answer: More Pur Operators
PurR controls ~9-19 purine biosynthesis operons (purA, purB, purC, etc.) plus purR itself and others like guaBA, thanks to multiple 16-bp operator sequences genome-wide. Lac repressor targets mainly the lac operon (lacZYA) with fewer sites. This expanded regulon reflects purine metabolism’s complexity versus lactose catabolism.
Corepressor Binding (Option 1)
PurR requires hypoxanthine/guanine corepressors for DNA binding, unlike apo-PurR; it doesn’t bind operators without them. This doesn’t explain broader regulation—lacI also needs no corepressor but regulates fewer genes.
Higher Expression Level (Option 2)
PurR::LacZ fusions show modest autoregulation (1.6-2.2 fold), not “much higher” expression than lacI. Steady-state levels are comparable; operator abundance, not repressor concentration, determines target scope.
Lower Expression Level (Option 4)
PurR is not expressed at lower levels—autoregulation maintains steady concentrations similar to lac repressor. Low expression would limit, not expand, regulation.
Comparison Table
| Option | Explanation | Why Incorrect/Correct |
|---|---|---|
| 1. Binds without corepressor | PurR needs hypoxanthine corepressor | False mechanism |
| 2. Higher expression | Comparable to lacI via autoregulation | Not primary reason |
| 3. More pur operators | ~19 sites vs. lac’s few | Correct |
| 4. Lower expression | Steady levels, not lower | Reduces regulation |
