Answer:
positive control
Explanation:
The primary focus of prokaryotic regulation is the regulation of transcription initiation, although in some cases the regulation of translation initiation takes place and other processes such as recombination can intervene. In bacteria, most mRNAs are polycistronic and a single promoter is required to regulate the transcription of a cluster of genes, it is at this point that the expression of these genes is regulated. An operon is a set of adjacent genes that have related functions, are transcribed into a single mRNA molecule and their regulation is coordinated. They therefore present a unique promoter.
The initiation of transcription is regulated by transcription factors that can act in two ways
:
»Suppressors if the regulation is negative. They block the access of the RNA pol to the promoters, joining the operators, these are sequences that are close or overlapping with the promoter.
»Activators if the regulation is positive. They bind near the promoter, increase the interaction between RNA polymerase with the promoter. They link to sites adjacent to the promoter.
A classic example, the E. coli lac operon
The lac operon controls 3 genes involved in the metabolism of lactose in E. coli, which code for β-galactosidase, a permease and a transacetylase.
The enzyme responsible for the degradation of lactose (source of C), is β-galactosidase, which breaks the β-galactoside bond, resulting in glucose and galactose. If we supply lactose to the cell, the levels of β-galactosidase increase, the alolactose (produced by the action of β-galactosidase itself on lactose) acts as an inducer for β-galactosidase. If in this induced state we eliminate lactose, since there is no substrate, it makes no sense to synthesize enzyme, thereby repressing its expression thus recovering at baseline levels of this enzyme. When we say that β-galactosidase is expressed, we mean that it increases from the basal level (<5 molecules / cell) to the induced level (> 5000 molecules / cell); we see that for this example, in the induced state there is an expression 1000 times greater than in the repressed state.
