<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> <P> During World War II, Monod was testing the effects of combinations of sugars as nutrient sources for E. coli and B. subtilis . Monod was following up on similar studies that had been conducted by other scientists with bacteria and yeast . He found that bacteria grown with two different sugars often displayed two phases of growth . For example, if glucose and lactose were both provided, glucose was metabolized first (growth phase I, see Figure 2) and then lactose (growth phase II). Lactose was not metabolized during the first part of the diauxic growth curve because β - galactosidase was not made when both glucose and lactose were present in the medium . Monod named this phenomenon diauxie . </P>

When is the bacterial lac operon switched on