Bacteriology 304: The Beta-Galactosidase Page
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Beta-Galactosidase Page 		Page Last
Modified:
8/10/07

Despite the "archived" nature of this site, this and other pages which contain background material may get updated from time to time. Included herein is a new (2007) summary of regulation associated with the lac operon. Also on this page is the same old (2003) set of very rough class data that can be used to graph beta-galactosidase activity vs. time (on linear paper) and absorbance vs. time (on semilog paper), comparing two strains and their growth with glucose vs. lactose.

I.  A general summary of the lac operon.

  • The Catabolic Activator Protein (CAP) attaches to the CAP Binding Site when enabled by cAMP and allows RNA Polymerase to attach to the Promoter.
  • The Repressor Protein (RP) attaches to the Operator when not disabled by Allolactose and blocks transcription of the lac operon by RNA Polymerase.

A question from a Bacteriology 304 quiz given August, 2007: What should theoretically happen when a typical strain of Escherichia coli is inoculated into a medium containing high amounts of both glucose and lactose? The two-part answer:

  • Elevated glucose causes a decrease in cAMP. With less or no cAMP, the Catabolic Activator Protein (CAP) is disabled. Therefore there is no attachment of the RNA polymerase and thus no transcription of the lac operon. This way, the glucose is used preferentially to the lactose as a C and energy source.

  • When the glucose runs out, the increase in cAMP enables the CAP to attach, allowing the RNA polymerase to attach and proceed with transcription of the lac operon which – with the elevated lactose present – is not blocked by the repressor protein.

A review of activation and repression of the Lac Operon:

 

GLUCOSE
cAMP

CAP disabled.

GLUCOSE
cAMP

CAP enabled by cAMP to allow attachment of RNA polymerase.

LACTOSE
Allolactose

RP disabled by allolactose.

RNA polymerase:

  • cannot attach.

  • therefore, no transcription of the lac operon (even though there is no blockage by RP).

RNA polymerase:

  • can attach.

  • can transcribe lac operon.

LACTOSE
Allolactose

RP enabled to block lac operon transcription.

RNA polymerase:

  • cannot attach.

  • therefore, no transcription of the lac operon (which is blocked by RP anyway).

RNA polymerase:

  • can attach.

  • cannot transcribe (due to blockage by RP).

II.  Some data for graphing practice.

strain & medium time (min) A420 A600 time of
assay (min)		 U/ml U/(ml X A600)
mutant strain in MM+glucose 0 0.033 0.03 8.33 1.44E-05 4.80E-04
15 0.34 0.05 7.033 1.76E-05 3.52E-04
30 0.34 0.04 6.67 1.86E-05 4.64E-04
45 0.04 0.05 7.0 2.08E-05 4.16E-04
60 0.066 0.06 7.0 3.43E-05 5.71E-04
75 0.109 0.12 7.033 5.64E-05 4.70E-04
90 0.24 0.28 5.33 1.64E-04 5.85E-04
105 0.246 0.24 5.5 1.63E-04 6.78E-04
120 – – – – –
mutant strain in MM+lactose 0 0 0.009 10.0 0 0
15 0.035 0.246 10.0 1.27E-05 5.17E-05
30 0.055 0.303 10.0 2.00E-05 6.60E-05
45 0.198 0.376 8.5 8.47E-05 2.25E-04
60 0.207 0.468 4.53 1.66E-04 3.55E-04
75 0.247 0.562 4.33 2.07E-04 3.69E-04
90 0.349 0.608 2.83 4.48E-04 7.38E-04
105 0.85 0.756 1.69 1.83E-03 2.42E-03
120 – – – – –
wild-type strain in MM+glucose 0 0 0.03 5.0 3.64E-06 1.21E-04
15 0.015 0.078 5.0 1.09E-05 1.40E-04
30 0.008 0.08 5.0 5.82E-06 7.27E-05
45 0.02 0.17 5.0 1.45E-05 8.56E-05
60 0.021 0.18 5.0 1.53E-05 8.48E-05
75 0.024 0.26 5.0 1.75E-05 6.71E-05
90 0.019 0.4 5.0 1.38E-05 3.45E-05
105 0.016 0.49 5.0 1.16E-05 2.37E-05
120 – – – – –
wild-type strain in MM+lactose 0 0 0 10.0 0 0
15 0 0.07 10.0 0 0
30 0.038 0.03 10.0 1.38E-05 4.61E-04
45 0.024 0.04 10.0 8.73E-06 2.18E-04
60 0.078 0.06 10.0 2.84E-05 4.73E-04
75 0.21 0.07 10.0 7.64E-05 1.10E-03
90 0.247 0.14 10.0 8.98E-05 6.42E-04
105 – – – – –
120 – – – – –

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Archived Bacteriology 304 Website
for Fall Semester, 2003

Page last modified on 8/10/07 at 9:15 PM, CDT.
John Lindquist, Department of Bacteriology
University of Wisconsin – Madison