8. Complete the paragraph to describe how Meml serves to repress and activate tr
ID: 268166 • Letter: 8
Question
8. Complete the paragraph to describe how Meml serves to repress and activate transcription of target genes in haploid MAT ? cells. 2 pts Then describe how this is an example of combinatorial control. 4 pts "Mcml Is a site-specific DNA-binding transcription factor that forms repressor or activator complexes. A dimer of ?2 binds cooperatively with Mcmi upstream of a-specific genes to (repress / activate) the transcription of those gene in haploid alpha cells. At the same time, Mcml binds cooperatively with ? 1 upstream of alpha-specific genes to (repress / activate) transcription of those genes in haploid ? cells."Explanation / Answer
Alpha2 binds with Mcm1 as a heterotetramer to REPRESS transcription, while alpha1 binds to ACTIVATE transcription.
When control of gene expression requires presence or absence of a combination of regulatory proteins, in particular, then the process is called “Combinatorial control”.
DNA recognition sequences for dimeric proteins contains two types of information, viz DNA sequence of each half-site and arrangement of the two sites. Mating gene MAT? encodes two proteins MAT?1 and MAT?2. Both these can work in conjugation with transcriptional factor MCM, involved in cell-specific transcription and pheromone secretion. The protein plays an important role by formation of activator as well as repressor complexes.
Dimers of yeast homeodomain protein ?2 can read the first type of information but lack the ability to assess the second. These ?2 dimers bind with equal affinity to artificial operators in which the two half-sites are arrayed either as inverted repeats, direct repeats, or everted repeats. The protein-MCM1, a second type of protein sets the exact spacing and orientation of the homeodomains in the ?2 dimer so as to accommodate only naturally occurring operators. This shows that target specificity of a homeodomain protein is raised by an auxiliary protein and also allows it to distinguish biologically correct operators (in terms of allowing specific geometry binding) from closely related sequences in the cell.
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