Is lac repressor cis or trans?
One of the major trans-regulators of the lac operon is encoded by lacI. Four identical molecules of lacI proteins assemble together to form a homotetramer called a repressor (Figure 12.1. 2). This repressor binds to two operator sequences adjacent to the promoter of the lac operon.
What binds to the lac repressor?
Allolactose (rearranged lactose) binds to the lac repressor and makes it let go of the operator. RNA polymerase can now transcribe the operon.
Why are mutations of the lac operator often called cis dominant?
Mutations in the operator only affect the genes linked to it on the same DNA molecule, so they are said to be cis-dominant. This is typical of transcription control elements like promoters, etc.
How does the lac repressor bind to DNA?
Lac repressor binds to DNA non-specifically (initial scene derived from 1osl, 20 NMR models), enabling it to slide rapidly along the DNA double helix until it encounters the lac operator sequence (“facilitated diffusion”). The DNA-binding domain employs a helix-turn-helix motif (Alpha Helices, Turns).
What are the cis and trans-acting regulatory elements of the lac operon?
cis- and trans- Regulatory Elements
In the lac operon, these sequences are called P (promoter), O (operator), and CBS (CAP-binding site).
Which type of regulation takes place in lac operon?
negative inducible
The type of regulation that the lac operon undergoes is referred to as negative inducible, meaning that the gene is turned off by the regulatory factor (lac repressor) unless some molecule (lactose) is added.
Where does lacI bind to?
LacI, which is a tetramer, binds to O1 through two of its subunits. The other two subunits then bind either O2 or O3, forming a loop between the operators. Either configuration prevents the binding of RNA polymerase to the promoter.
Which binds to the operator of the lac operon?
The repressor
The lac operon is repressed by LacI, encoded by lacI. The lacI gene is upstream of lacZYA and faces in the opposite direction. The repressor, LacI, binds to the operator sequence upstream of lacZYA and prevents transcription of those genes unless the inducer molecule is present.
What is a cis dominant mutation?
mutations (eg of an operator) that alter the functioning of genes on that same piece of DNA.
How would the lactose operon work if there is a mutation in its repressor gene?
An “up” mutation would make the lac operon no longer dependent on the positive regulation by the cAMP‑CAP complex (when the operon is induced). A “down” mutation would not allow expression even in the de‑repressed state (presence of inducer) and hence would show a non‑inducible phenotype.
Is the lac repressor lacI?
The lac repressor (LacI) is a celebrated DNA-binding protein that regulates the function of the lac operon (1), a set of genes responsible for the lactose metabolism in Escherichia coli.
Where does a repressor bind an operon?
A repressor protein binds to a site called on the operator. In this case (and many other cases), the operator is a region of DNA that overlaps with or lies just downstream of the RNA polymerase binding site (promoter). That is, it is in between the promoter and the genes of the operon.
What is the difference between cis and trans-acting sequences?
Cis-acting elements are DNA sequences in the vicinity of the structural portion of a gene, which are required for gene expression. Trans-acting factors are usually protein factors that bind to the cis-acting sequences to control gene expression.
Is lac operon inducible or repressible?
inducible system
The lac operon is an example of an inducible system. With repressible systems, the binding of the effector molecule to the repressor greatly increases the affinity of repressor for the operator and the repressor binds and stops transcription.
Which type of regulation takes place in lac operon Toppr?
The lac operon has one regulatory gene which is present upstream the operator gene. The genes that code for any RNA / protein product which do not serve as regulatory protein are known as structural genes. The lac z, lac y and lac a genes in lac operon are the structural genes. Option B is the correct answer.
What is LacI repressor?
The Lac repressor protein, LacI, prevents the transcription of genes involved in lactose utilization (lac genes) in E. coli. Like many other repressors, LacI utilizes multiple operators to increase the efficiency of repression.
Where is LacI in lac operon?
The lacI gene is present just upstream of the lac operon, and in fact there are three operator sites where the LacI tetramer can bind and affect transcription (5). The LacI tetramer contains two identical dimers, connected at their C-terminal region.
Does the repressor bind to the operator?
The repressor binds to the operator gene and prevents it from initiating the synthesis of the protein called for by the operon. The presence or absence of certain repressor molecules determines whether the operon is off or on.
What does CIS dominant mean?
The term cis-dominant means that the mutation is in an element that has to be physically joined to the gene to exert its effect (ie, in cis), not separated from it on another chromosome.
What is Interallelic complementation?
Referring to the change in the properties of a multimeric protein as a consequence of the interaction of subunits coded by two different mutant alleles (in contrast to the protein consisting of subunits derived from a single mutant allele).
Which of the following is are involved in repression of the lac operon?
The lac operon is repressed by LacI, encoded by lacI. The lacI gene is upstream of lacZYA and faces in the opposite direction. The repressor, LacI, binds to the operator sequence upstream of lacZYA and prevents transcription of those genes unless the inducer molecule is present.
What would be the effect of a mutation in the lacI gene that prevented the repressor from binding to lactose?
What would be the effect of a mutation in the lacI gene that prevented the repressor from binding to lactose? The lac Z, Y, and A genes would be repressed by lactose.
What is lac repressor made of?
Structurally, the lac repressor protein is a homotetramer. More precisely, the tetramer contains two DNA-binding subunits composed of two monomers each (a dimer of dimers). Each monomer consists of four distinct regions: An N-terminal DNA-binding domain (in which two LacI proteins bind a single operator site)
Where is the binding site of the repressor protein?
Repressors can also have two binding sites: one for the silencer region and one for the promoter. This causes chromosome looping, allowing the promoter region and the silencer region to come in proximity of each other.
What regulates the lac operon?
The activity of the promoter that controls the expression of the lac operon is regulated by two different proteins. One of the proteins prevents the RNA polymerase from transcribing (negative control), the other enhances the binding of RNA polymerase to the promoter (positive control).