The lac region has been shown to contain three operators whose locations are sho

Question

The lac region has been shown to contain three operators whose locations are shown below:

Recently, experimenters genetically-engineered wild-type and mutated lac operons into the chromosome of an E. coli host strain using bacteriophage lambda. The wild-type lac operon contained the three operators, the lac promoter and the lacZ gene. Mutant operons had mutations in one or more of the three operators. The lac operon was deleted from the host bacterium, but it did retain a wild-type lacI gene (encoding the repressor). The ?-galactosidase levels produced in the presence and absence of the artificial inducer, IPTG, were measured in all strains and the ratio of activity recorded in the column on the right:

                                    Ewt 312                                    1,300

                                    Ewt 310                                    440

                                    Ewt 012                                    700

                                    Ewt 010                                    18

                                    Ewt 302                                    1.9

                                    Ewt 300                                    1.0

                                    Ewt 002                                    1.0

                                    Ewt 000                                    1.0

For example, the repression observed with all three functional operators (Ewt 312) was 1,300-fold. Write a report explaining these results. Comment specifically on the difference in activity ratios seen in constructs Ewt 310 (functional O1 and O3), Ewt 012 (functional O1 and O2): comment on the minimal activity of Ewt 302. In particular, how do you explain the significant difference in activity ratios between Ewt 123 (functional O1, O2 and O3) and Ewt 103 and Ewt 120, especially when the models argue that the binding of the Lac tetrameric repressor to only two operators is sufficient to bring about almost full repression?

Explanation / Answer

* The repression is high in Ewt 312 as both the pseudo operator regions O2 and O3 are functional in this. The results suggest Lac repressor must be able to bind two lac operator sites at the same time. The lac repressor may form loops by binding simultaneously to O1 and O2, or O1 and O3.

* Ewt 310 has an inactive O2 region, which led to about a 3 fold decrease of repression. This shows that repression shows redundancy. Repressor binding to O1 and any one of the other operators, that is O2 or O3, bring about full repression. Presence of an extra operators exerts only enhancing effects of repression.

* Ewt 302 hs minimum activity may be owing to desctruction of promotor O1. O2 or O3 alone or in combination, could not exert any detectable repression with tetrameric Lac repressor.

The results infer that O1 is for sure the main site of repression.

There are no details given about Ewt 123 or Ewt 103, 120 in the question.

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