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Inductive effects of isopropyl-beta-D-thiogalactoside (IPTG) and lactose: two inducers of beta-galactosidase production

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Introduction

The bacterium E.coli has the ability to use lactose as its carbon source in the absence of glucose. In order to metabolise lactose E.coli requires the presence of beta-galactosidase, an enzyme that catalyses the hydrolysis of lactose to produce glucose and galactose. Previous experiments have shown that lactose alone can stimulate beta-galactosidase production, ... The purpose of this subsequent experiment was to find out ... It was hypothesised ... To test this, ...

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Results

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(Figure not shown)

Figure 1. Specific Activity of beta-galactosidase in E.coli culture incubation containing IPTG with periodic lactose additions. An E.coli culture, grown at 37degrees C to log phase in glycerol medium, ... extinction coefficient of 4.5mM-1cm-1 after correcting for turbidity at 550nm (correction factor: A420 - 1.3 * A550).

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Interpretation

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The graph shows a general increase in activity over time with frequent minima and maxima points corresponding to the addition of lactose. Beta-galactosidase activity exhibits a fairly linear increase to ~1250nmol/min/mg after 15 minutes. After each addition of lactose at 15, 30 and 45 minutes a decline was observed followed by a steep increase in activity. A maximum value of ...

Group 5 performed an experiment that observed beta-galactosidase activity in IPTG over a 60 minute period of time in which the activity linearly increased. This group attained a maximum activity of ~10 after 50 minutes. This is much higher than the value of ~2200 achieved ... This significant difference in results suggests that beta-galactosidase ....

Some lactose only experiments were performed with high (4mM) and moderate (0.4mM) concentrations of lactose. The results show a lag period then an increase of beta-galactosidase activity followed by a smaller increase to ~2200 and ~1600 respectively. In comparison to this experiment the end results were similar. In all these experiments lactose stimulated the production of newly synthesised beta-galactosidase, which ... In the lactose only experiments a decrease in lactose would occur over time and would lead to a plateau observed in the graph of beta-galactosidase activity as a result of no stimulant available to induce enzyme production. This was observed in a previous experiment ... The difference between the lactose only experiment and this one is the presence of IPTG, which will induce beta-galactosidase production ...

From the results when IPTG and lactose are present a decline is observed immediately after each lactose addition. An explanation of this is ... Therefore, a decrease in beta-galactosidase activity is observed in these areas because .... A steep increase is then observed as IPTG ... Free beta-galactosidase will then metabolise any left over lactose present in the cell which is why ... When another addition of lactose is added, the cycle .... However, as time progresses ...

In practice, using lactose as an inducer results in lower activity levels of beta-galactosidase in comparison with IPTG. The results support lactose metabolism by newly synthesised beta-galactosidase and therefore quantitatively IPTG is a more effective inducer of beta-galactosidase synthesis than lactose.

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