Review the lac operon

• Single promoter (P_lac) controls three genes: lacZ, lacY, lacA
• A repressor (coded by the separate gene lacI) can bind to the operator and block transcription.
• If lactose is present, allolactose binds to the repressor, which stops it from blocking transcription.
• A separate regulatory mechanism controls the binding of RNA polymerase to the promoter. If glucose is present, there is little cAMP, so the activator complex (CAP-cAMP) can not bind to the promoter region.

Partial Diploids

We reviewed the use of partial diploids figure out the genetics of the lac operon

• For example, what is the phenotype of a cell that has:
• lacI+ lacO+ lacZ- lacY+ / F' lacI- lacO+ lacZ+ lacY-

• Will there be B-galactosidase activity? With lactose? Without lactose?
• Will there be permease activity? With lactose? Without lactose?
• Are the genes inducible (is there a difference with and without lactose)?

Here's another: What is the phenotype for:

• lacI+ lacO^c lacZ- lacY+ / F' lacI+ lacO+ lacZ+ lacY-

You should know the system well enough to figure out problems like this, and other similar ones from the book.

Northern Blots

We talked briefly about using gels called Northern Blots to detect mRNA.
We looked at one example showing differences in the amount of lac mRNA when grown with glucose, lactose, or glucose+lactose.

The trp operon

• See Fig. 16.15 in the book
• trp is an example of a repressible operon
• Contains genes for the synthesis of tryptophan
• If the end product (tryptophan) is abundant, the gene is turned off.
• Two separate regulation mechanisms, repression and attenuation

Repression:

• Repressor is activated by tryptophan (the effector molecule). :
• Changes shape so it can bind to the operator.
• The repressor protein is produced by another gene (trpR) far away (not shown in fig)
• 70x reduction in synthesis

Attenuation

• The second regulatory mechanism can fine-tune production of trp
• Attenuation depends on an interaction between transcription and translation in a "leader sequence" at the beginning of the operon.
• 10x variation in transcription
• Attenuation is common in other operons that synthesize amino acids.

Fig. 16.16 and 16.17 diagram the attenuation model for trp

Gene Regulation in bacteriophage lambda

• Gene regulation is important for determining developmental patterns.
• Bacteriophage Lamba is a simple place to start
• Different sets of genes determine whether the phage will be lytic or lysogenic.
• The principles are similar to bacterial gene regulation.
• It is an example of a genetic "switch"
• Also a examlple of sequential regulatory series

Overview

• Balance between 2 activators (cro and cII) determines which lifecycle prevails
• Early genes activate middle genes which activate late genes
• Environmental conditions in the cell can trigger a switch, by affecting the abundance of cII

See Fig. 16.20 and 16.21. I don't expect you to memorize every gene, but you should understand the general sequence of events that determine which life-cycle the phage will follow.

Chapter 16 problems

3, 5, 6, 7, 8, 13, 14