<Li> ONPG is cleaved to produce the intensely yellow compound, orthonitrophenoland galactose, 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) which is an artificial substrate for B - galactosidase whose cleavage results in galactose and 4 - Cl, 3 - Br indigo thus producing a deep blue color . </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> <P> The experimental microorganism used by François Jacob and Jacques Monod was the common laboratory bacterium, E. coli, but many of the basic regulatory concepts that were discovered by Jacob and Monod are fundamental to cellular regulation in all organisms . The key idea is that proteins are not synthesized when they are not needed--- E. coli conserves cellular resources and energy by not making the three Lac proteins when there is no need to metabolize lactose, such as when other sugars like glucose are available . The following section discusses how E. coli controls certain genes in response to metabolic needs . </P>

Events of gene regulation by the lac operon