Question

A partial diploid strain of E. coli, grown in the presence of lactose and allolactose, does NOT produce any functional beta galactosidase. What is the possible genotype of the cells? (genotypes on the chromosome / genotypes on the plasmid)

lacI+ lacP+ lacOC lacZ+ / lacIS lacP+ lacO+ lacZ+

lacI+ lacP+ lacOC lacZ+ / lacI- lacP+ lacO+ lacZ+

lacI+ lacP+ lacOC lacZ+ / lacI+ lacP- lacO+ lacZ+

lacI+ lacP+ lacO+ lacZ+ / lacIS lacP+ lacO+ lacZ+

Answer 1

Answer:   lacI+ lacP+ lacO+ lacZ+ / lacIS lacP+ lacO+ lacZ+

In case of above genotype, A partial diploid strain of E. coli, grown in the presence of lactose and allolactose, does NOT produce any functional beta galactosidase.

Explanation: Lac operon in E.Coli has structural gene Z to produce enzyme, beta galactosidase required for lactose metabolism. In E.Coli , if glucose is available lac operon will be switched off, and it will become active only if lactose/allolactose is available. Normally when lactose is not there in the medium, an allosteric repressor synthesized by “ I “ gene of lac operon will bind to the operator region of the operon, obstructing the path of RNA polymerase required for transcription of the structural genes.

If lactose is available it binds to the repressor, and the repressor is no longer able to bind the operator region because lactose/allolactose changes the confirmation of the repressor and it would not be able to bind to the operator and the RNA polymerase will be able to transcribe the structural genes for lactose utilisation.

In case of genotype lacI+ lacP+ lacO+ lacZ+ / lacIS lacP+ lacO+ lacZ+

all the genes are showing normal activity except the lacIS , this gene in its normal form i.e. lacI is able to produce normal repressor but in this case, lacIS is producing a super repressor which even after binding with lactose/allolactose does not dissociate from the operator, the repressor is a protein which is trans-acting that means the same type of repressor will bind to the operator on another operon also. And hence the RNA polymerase will not be able to transcribe the gene for beta galactosidase even in presence of lactose on both the operons.

Note: lacI+ lacP+ lacOC lacZ+ / lacIS lacP+ lacO+ lacZ+

In the above genotype there is a constitutive mutation in operator of the operon on chromosome hence it will not be able to bind any kind of repressor and it will constitutively express beta galactosidase gene. The operon on the plasmid will not be able to transcribe beta galactosidase as the super repressor will bind to the operator and will not allow RNA polymerase to move even in presence of lactose/allolactose.

lacI+ lacP+ lacOC lacZ+ / lacI- lacP+ lacO+ lacZ+

In the above genotype, the operon on the chromosome is showing constitutive mutation in the operator region, hence it will not be able to bind the repressor and it will constitutively express beta galactosidase gene. The operon on the plasmid is not producing the repressor but repressor on the chromosome lacI gene will be able to bind its operator and the operon will show normal activity.

lacI+ lacP+ lacOC lacZ+ / lacI+ lacP- lacO+ lacZ+

In the above genotype, the operon on the chromosome is showing constitutive mutation in the operator region, hence it will not be able to bind the repressor and it will constitutively express beta galactosidase gene. The operon present on the plasmid will not be able to transcribe the beta galactosidase gene, as there is mutation in the promoter gene.