Control of Beta Galactosidase Synthesis in Escherichia coli

Observation:

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.

Question:

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

Background:

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.

• Transcription involves copying the code in a DNA molecule into a messenger RNA molecule (m-RNA). In eukaryotes, this process occurs in the nucleus of the cell.
• Translation of this message into the sequence of amino acids in beta galactosidase occurs in the cytoplasm at the ribosome.
• Once the polypeptide (amino acid) chains are formed they are assembled into a functional enzyme

Regulation can occur between each step.

Hypotheses:

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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