Question

my teacher was useless with the method for doing these problems can someone please guide me in the right direction. dont do all of them, but if you do at least explain how you came to that conclusion. thank you

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6. For E. coli strains with the lac genotypes show below, use a plus sign (+) to indicate the synthesis of B-galactosidase and permease and a minus sign (-) to indicate no synthesis of the proteins. Lactose absent Lactose present Genotype of strain B-Galactosidase Permease B-Galactosidase Permease lact+ lacP+ lacO+ lacZ lacyt lacl lacP+ lacO+ lacZ lacy+ lacl+ lacP+ lacOº lacZ+ lacY+ lac!" lacpt lacO+ lacZt lack lacl lacp- lacO+ lacZt lacY+ lacllacP+ lacO+ lacZ lacY+/ lac! lacp+ lacO+ lacZt lacr lacl" lacP+ lac lacZ lacyt/ laclt lacp+ laco+ lacZ lacy lacl" lacP+ lacO+ lacZt lacy/ lacrt lacp- lacO+ lacZ lacy+ laclt lacp- lacOlack lacY+/ lac!" lacp+ lacO+ lacZt lacY laclt lacP+ lacO+ lacZ lacY+/ lacrt lacP+ lacO+ lacZt lacy+ lacl lacp+ lacO+ lacZt lacy/ laclt lacP+ lacO+ lacZ lacy+ lacl lacp- lacO+ lacZ lacyt/ lacit lacP+ laco+ lacZt lacY+

Answer 1

Lac-operon is a repressible operon system where a specific protein LacI binds to the operator region to repress the gene expression from the operon when lactose is not present in the medium. But when lactose is present, the lactose is converted to allolactose which then binds to the LacI protein and releases it from the operator site and the genes start expressing from the operon. So, when lactose is present operon will express all the genes, but when lactose is absent in the medium the operon will not express any gene.

Now, lac-operon consists of 3 genes, lacZ ( transcribed and translated to beta-galactosidase which breakdown lactose), lacY ( transcribed and translated to permease which helps in entering of the lactose inside the cell) and lacA (transacetylase). Lac- operon also has operator site (lacO) which is a specific DNA sequence where preferential repressor binding occurs and the operon get repressed. lacP, is the promoter of the lac-operon, RNA polymerase binding site. There is another gene upstream of the lac-operon which is called lacI that transcribed and translated to LacI protein, the repressor.

Now, generally, one operon is present per genomic DNA of E. coli. There might be an extra operon present in the extracellular DNA which can sometimes complement the function of the other operon. These types of cells are called merodiploid.

Now, you have to remember two very crucial terms, 'cis-acting site' and 'trans-acting protein'. cis-acting site is usually cannot be complemented, for example, lacO is a cis-acting site. If the lacO is mutated you cannot rescue its function by providing another set of the gene. But trans-acting proteins can be complemented and function can be rescued by providing specific genes, for example, lacI, lacZ, lacY.

1. For the first, problem every gene is wild type. So, the operon will be regulated normally. When lactose is present only then there will be expression of the genes.

2. For this one, lacI is mutated. So, the operon will not be repressed. So, irrespective of presence of lactose, operon will be expressed.

3. lacO^c mutation is called operator constitutive mutation. For this mutation, repressor protein can not bind to lacO. And irrespective of the presence of lactose, operon will be expressed.

4. lacI is mutated. So, the repressor is absent. So, irrespective of presence of lactose, operon will be expressed. But lacY is also mutated. So, there will be no permease. Only beta-galactosidase will be present.

5. The repressor is mutated. So, irrespective of the presence of lactose, operon will be expressed.

6. First, lacI gene is mutated, but its function will be complemented by the second lacI. 1st lacZ is mutated but it will be complemented by second lacZ. Mutation in the second lacY will be complemented by the first lacY. So, when lactose is present both lacZ & lac Y will express.

7. As there is a lacO^c mutation in the 1st operon, irrespective of the presence of lactose, operon will be expressed.

11. lacI^s mutation is called a super-repressed mutation. This mutated, lacI cannot bind to the allolactose and permanently repress the operon expression, for all the operon present. So, there will be no operon expression

I hope you can solve the other.

I used numbering as per the following image,

7 Lactose absent Lactose present Genotype of strain B-Galactosidase Permease B-Galactosidase Permease 1. lact+ lacpt lacO+ lacZ lacY+ 2. lacl lacp+ lacO+ lacZt lacY+ 3. lacl+ lacP+ lac Oc lacZ+ lacy+ 4. lacl lacP+ lacO+ lacZ+ lace 5. lacl lacp- lacO+ lacZt lacY+ 6. lacl+ lacP+ lacO+ lacz- lacY+/ lacl lacp+ lacO+ lacZt lac 7. lacl lacP+ lac lacZt lacyt/ laclt lacp+ lacO+ lacZ lacy 8. lacl- lacP+ lacO+ lacZt lacr- lacrt lacp- lacO+ lacZ lacY+ 9. lacit lacp- lacOlacZ lacyt/ lact- lacp+ lacO+ lacZ+ lacy 10. lacl+ lacP+ lacO+ lacZ+ lacY+/ laclt lacp+ lacO+ lacZt lacY+ 11.lack lacp+ lacO+ lacZt lacY lacit lacp+ lacO+ lacZ- lacY+ 12. lac lacp-laco+ lacZ lacyt/ laclt lacP+ lacO+ lacZt lacY+