Chapter 13, Problem 20P

A muscle enzyme called ME $\text{1}$ is produced by transcription and translation of the ME $\text{1}$ gene in several muscles during mouse development, including heart muscle, in a highly regulated manner. Production of ME $\text{1}$ appears to be turned on and turned off at different times during development. To test the possible role of enhancers and silencers in ME $\text{1}$ transcription, a biologist creates a recombinant genetic system that fuses the ME $\text{1}$ promoter, along with DNA that is upstream of the promoter, to the bacterial lacZ( $\text{β-}$ galactosidase) gene. The lacZgene is chosen for the ease and simplicity of assaying production of the encoded enzyme. The diagram shows the structure of the recombinant, as well as bars that indicate the extent of six deletions the biologist makes to the ME $\text{1}$ promoter and upstream sequences. The blue bar is thesite of the promoter whereas the gray bars span potential enhancer/silencer modules. The table displays the percentage of $\text{β-}$ galactosidase activity in each deletion mutant in comparison to the recombinant gene system without any deletions.

a. Does this information indicate the presence of enhancer and $\text{/}$ or silencer sequences in the ME $\text{1}$ upstream sequence? If so, where is $\text{/}$ are the sequences located?

b. Why does deletion D effectively eliminate transcription oflacZ?

c. Given the information available from deletion analysis, can you give a molecular explanation for the observation that ME $\text{1}$ expression appears to turn on and turn off at various times during normal mouse development?