<Ul> <Li> ONPG is cleaved to produce the intensely yellow compound, orthonitrophenol, and is commonly used as a substrate for assay of β - galactosidase in vitro . </Li> <Li> Colonies that produce β - galactosidase are turned blue by X-gal (5 - bromo - 4 - chloro - 3 - indolyl - β - D - galactoside). </Li> </Ul> <Li> ONPG is cleaved to produce the intensely yellow compound, orthonitrophenol, and is commonly used as a substrate for assay of β - galactosidase in vitro . </Li> <Li> Colonies that produce β - galactosidase are turned blue by X-gal (5 - bromo - 4 - chloro - 3 - indolyl - β - D - galactoside). </Li> <Li> Allolactose is an isomer of lactose and is the inducer of the lac operon . Lactose is galactose - (β1 -> 4) - glucose, whereas allolactose is galactose - (β1 -> 6) - glucose . Lactose is converted to allolactose by β - galactosidase in an alternative reaction to the hydrolytic one . A physiological experiment which demonstrates the role of LacZ in production of the "true" inducer in E. coli cells is the observation that a null mutant of lacZ can still produce LacY permease when grown with IPTG but not when grown with lactose . The explanation is that processing of lactose to allolactose (catalyzed by β - galactosidase) is needed to produce the inducer inside the cell . </Li>

Describe in detail how the lac operon is regulated in the presence of lactose