Normal wt lacy gene y mutant lacy gene this cant be

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= normal (wt) LacY gene Y- = mutant LacY gene, this can’t be transcribed A+ = normal (wt) LacA gene A- = mutant LacA gene, this can’t be transcribed Activity: Inducible (I) = R+ = O+ = Z+ = Y+ = A+ Constitutive (C) = R- = O- Uninducible (U) = R S = P- = Z- = Y- = A- Rule of thumb: Constitutive > Inducible > Uninducible I will use my short-hand I , C , and U later below.
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How to solve Lac operon problems: Given the following example merodiploid: F’ R- P+ O+ Z+ Y+ A- Chr. R+ P+ O- Z+ Y- A+ 1. The first step is to look at each “line” separate from each other. Look at the expression of the structural genes on the F’ plasmid separately from the bacterial chromosome (Chr.). You will see the following: F’ R- P+ O+ Z+ Y+ A- I I U Chr. R+ P+ O- Z+ Y- A+ I U I 2. Now look at the expression of the repressor, promoter, and operator. Revise its effects on the structural genes: F’ R- P+ O+ Z+ Y+ A- I I U Now: C I C C U Chr. R+ P+ O- Z+ Y- A+ I U I Now: I C C U C Z and A are C because of O-. (Note: I don’t believe one can say that a P+ is either constitutive or inducible because that depends on other genes.) 3. Now take into consideration trans -acting elements. Note that the repressor is the only trans - acting element, everything else is cis -acting. Therefore, only the repressor is able to affect expression on the “other line.” Once again, revise the effects on the structural genes: F’ R- P+ O+ Z+ Y+ A- I I U C I C C U Now: I (R+) I I I U R+ “fixes” or “replaces” R-. Chr. R+ P+ O- Z+ Y- A+ I U I I C C U C Now: I C C U C
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4. “Tally” up the expression for each structural gene. You may see differential expression depending on which “line” you’re looking at. So: For Z: F’ = inducible Chr. = constitutive For Y: F’ = inducible Chr. = uninducible For A: F’ = uninducible Chr. = constitutive 5. Finally, apply the “rule of thumb” above to see which kind of expression wins out. So your final answers for each of the structural genes are: Rule of thumb: Constitutive > Inducible > Uninducible LacZ (Z) Constitutive LacY (Y) Inducible LacA (A) Constitutive
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