in E. coli, the ability to utilize lactose as a carbon source requires the prese
ID: 271336 • Letter: I
Question
in E. coli, the ability to utilize lactose as a carbon source requires the presence of the enzymes B-galactosidase and B-galactoside permease. These enzymes are encoded by two closely linked genes, lacZ and lacY, respectively. Another gene, proC, controls, in part, the ability of E. coli cells to synthesize the amino acid proline. The alleles str^r and str^s, respectively, control resistance and sensitivity to streptomycin. Hfr H is known to transfer the two lac genes, proC, and str, in that order, during conjugation. A cross was made between Hfr H of genotype lacZ- lacY+ proC+ strs and an F- strain of genotype lacZ+ lacY- proC- strr. After about 2 hours, the mixture was diluted and plated out on medium containing streptomycin but no proline. When the resulting proC+ strr recombinant colonies were checked for their ability to grow on medium containing lactose as the sole carbon source, very few of them were capable of fermenting lactose. When the reciprocal cross (Hfr H lacZ+ lacY- proC+ strs X F- lacZ- lacY+ proC- strr) was done, many of the proC+ strr recombinants were able to grow on medium containing lactose as the sole carbon source. What is the order of the lacZ and lacY genes relative to proC? ( the answer is lacY—lacZ—proC, however what to the step by step explanation of getting there ) Please provide explanation. I do not need to know the answer, I need to know how to arrive to the answer which is lacY lac Z proC strr.
Explanation / Answer
If you talk about the order you have lacY—lacZ—proC , to understand this first you need to know that lac Y and Lac Z are very closely related gene so always go together duringthe transfer hrom Hfr H strain to F- strrain.
step 1- lacZ+ & lac Y- goes first . butin F- , Y- enter first then LacZ+ , that's why in hybrid colonies the dominate of Y which is Y+ is express first then the lacZ+
step 2 - likewise then pro C and strs goes together from Hfr Hto F- but in hybrid the dominant one is express so we have pro C- & strr which provide ressistance from streptomycine.
order lacY—lacZ—proC
first you need permiase to allow entry of lactose and then Z which makes beta galactosidase to breaking the lactose and proC- because there is no proline .
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