Control of Beta Galactosidase Synthesis in Escherichia coli


E. coli growing in tryptone broth have low levels of beta galactosidase. When the sugar lactose is added to the growth medium the level of beta galactosidase increases.


How does E. coli regulate the production of beta galactosidase?


Beta galactosidase is the enzyme responsible for the first step in the breakdown of lactose. Since beta galactosidase is a protein its structure is determined by the information stored in a DNA molecule. There are several steps required to transfer the information stored in a DNA molecule into the structure of a protein.

Regulation can occur between each step.


Lactose stimulates the DNA to begin transcription. It activates the DNA-RNA-Protein Pathway

Lactose is necessary for the assembly of already present polypeptide chains.

How can one distinguish between these two hypotheses?

Suppose it is possible to block the DNA-RNA-protein pathway. In your experiment chloramphenicol will be used to prevent the m-RNA from attaching to the ribosome.
Consider the consequences of creating this block.

If the first hypothesis is correct ---- the longer the pathway is open before being blocked the greater the amount of beta galactosidase the E coli will produce. Transcription---Translation---Assembly take time.

If the second hypothesis is correct --- blocking the pathway will not affect the amount of beta galactosidase produced since the polypeptides are already present and lactose stimulates their assembly.

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Edited 6-26-03 Back to 109 Resources