Question

In gene regulation in bacteria lactose is required to turn on the cell and permease brings lactose into the cell,how does it work?

Answer 1

Answer: In the regulation of lac operon, lactose is necessary to turn ON the gene regulation i.e. transcription of the genes followed by translation. Lactose gets inside the cell and isomerised into allolactose by Beta-galactosidase, which binds to the repressor. Binding of allolacose to the repressor takes off repressor from the operator site, which enables RNA polymerase to bind to the promoter site and begin the transcription.

However, the question arises that when lactose is required to synthesise permease that brings lactose inside, how can the regulation begin?

A basal level of activity of beta-galactosidase and permease is always present inside the cell even when it is uninduced. A minimum of 10-20 molecules of beta-galactosidase and some molecule of permease is always present inside the cell under uninduced state (Erlich HA et al., 1978; X. Sunney Xie et al., 2008).

The basasl activity of these enzymes are because of mis-match in the affinity of repressor and inducer. According to functional analysis, the higher basal levels of the enzymes in some strains are due to lower affinity of the repressor or the less amount of repressor molecules (Peter Overath., 1968).

Therefore, the basal level of permease present inside the cell under uninduced state brings in the Lactose which is isomerised to allolactose by the existing beta-galactosidase. Allolactose acts as inducer helps in starting the transcription process. Now the expression of beta-galactosidase and permease increases which further inducce the cell untill the cell is fully induced.

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Reference:

1. Erlich HA, Cohen SN, McDevitt HO. A sensitive radioimmunoassay for detecting products translated from cloned DNA fragments. Cell. 1978. 13(4):681-9. doi:10.1016/0092-8674(78)90218-0 p.686.

2. X. Sunney Xie, Paul J. Choi, Gene-Wei Li, Nam Ki Lee, and Giuseppe Lia, 'Single-Molecule Approach to Molecular Biology in Living Bacterial Cells', Annu. Rev. Biophys. 2008. 37:p.419.

3. Peter Overath, Control of basal level activity of ?-galactosidase in Escherichia coli. Molecular and General Genetics. 1968. 101(2), pp 155–165.