Question

2) A sugar named sugarose induces synthesis of two enzymes from the sug operon of E. coli. Mutations that affect the appearance of these enzymes were mapped to four adjacent loci (A, B, C, and D) in the order A B C D on the E. coli chromosome. Deletion mutants were characterized in haploids and partial diploids, and the results are shown below. (Here, I = enzyme induced normally, i.e., synthesized only in the presence of the inducer; C = enzyme synthesized constitutively; 0 = enzyme cannot be detected.) Deduce which of the following represents enzyme 1, enzyme 2, a promoter, and a repressor.

                                                           

                                                     Enzyme 1                       Enzyme 2

A^- B⁺ C⁺ D⁺                                                                      I                                   0

A⁺ B^- C⁺ D⁺                                              0                                  I

A⁺ B⁺ C^- D⁺                                             0                                  0

A⁺ B⁺ C⁺ D^-                                              C                                  C

A⁺ B^- C⁺ D⁺ / A^- B⁺ C⁺ D⁺                                   I                                   I

A⁺ B^- C^- D⁺ / A^- B⁺ C⁺ D⁺                           I                                   0

A^- B⁺ C^- D⁺ / A⁺ B^- C⁺ D⁺                                         0                                  I

A^- B⁺ C⁺ D⁺/ A⁺ B^- C⁺ D^-                            I                                   I

Answer 1

Ans) A=code for enzyme 2

B code for enzyme 1

C Code for promoter

D code for repressor

Explaination

Mutation in repressor ie Din A+ B+ C+ D- enzyme 1 =C, enzyme2=C ie, constitutive expression of operon means mutation in repressor either prevents repressor binding to operator or inhibition of synthesis of repressor so no turning off of operon leads to constitutive expression of both enzynes hence normal D code for repressor      

in A+ B+ C- D+ enzyme1=   0 enzyme2= 0

OnlyC is mutated while expression of both enzyme is absent means C code for promoter in which mutation prevents binding of RNA polymerase to promoter and no expression of operon.

A- B+ C+ D+ Enzyme1=I ,enzyme2=0 it indicate A gene code for enzyme2 because mutation in A gene causes no expression of only enzyme2.

A+ B- C+ D+ Enzyme1=0 , enzyme2=1

It indicate mutation in B gene only prevent expression of enzyme 1 so B codes for enzyme1