Question

The light-producing genes of \(V\). fischeri are organized in an operon that is under positive control by an activator protein called LuxR. Would you expect the genes of this operon to be transcribed when LuxR is bound or not bound to a DNA regulatory sequence? Explain.

Short answer

The genes in the V. fischeri operon would be transcribed when the activator protein LuxR is bound to a DNA regulatory sequence, as it is under positive control. LuxR promotes gene expression by facilitating the binding of RNA polymerase to the promoter region. Without LuxR bound to the regulatory sequence, transcription is less likely to occur.

Step-by-step solution

In gene regulation, positive control refers to the activation of gene expression by a regulatory protein. In this case, the activator protein LuxR enhances the transcription of the genes in the operon. Conversely, negative control involves the suppression of gene expression by a regulatory protein, such as a repressor. #Step 2: Role of Activator Proteins in Gene Transcription#

Activator proteins, like LuxR, promote the transcription of a gene by binding to a specific DNA regulatory sequence near the gene. This binding event increases the likelihood that RNA polymerase will bind to the promoter region of the target gene and initiate transcription. #Step 3: Genes Transcription in the Presence of LuxR#

Since LuxR is an activator protein and gene expression in the operon is under positive control, we can expect that the genes in the operon will be transcribed when LuxR is bound to the DNA regulatory sequence. When LuxR is not bound to the regulatory sequence, transcription is less likely to occur because the activator protein is not present to facilitate RNA polymerase's binding to the promoter region. #Conclusion#

In conclusion, the genes in the V. fischeri operon would be transcribed when the activator protein LuxR is bound to a DNA regulatory sequence, as it is under positive control. Without LuxR bound to the regulatory sequence, transcription is less likely to take place. This is consistent with the mechanism of positive control in which an activator protein promotes gene expression by facilitating the binding of RNA polymerase to the promoter region.

Key concepts

Positive Control of Gene Expression

The ability of cells to adjust their genetic activity is fundamental to life, and one such adjustment mechanism is through positive control of gene expression. This process involves upregulation, where specific genes are turned on or their expression is enhanced, leading to an increased production of their respective proteins. Imagine a light switch that only turns on when you insert a special key; similarly, in positive gene control, an activator protein acts like that key, initiating the transcription of certain genes.

For example, in the bioluminescent bacterium Vibrio fischeri, the activator protein LuxR must bind to a DNA regulatory sequence to switch on the genes responsible for light production. This process shows how cells can respond to changes in their environment – such as the availability of LuxR – to regulate gene expression and achieve a desired outcome, like illuminating the ocean's depths where V. fischeri often resides.

Activator Protein

Activator proteins play a crucial role in positive gene regulation by serving as molecular 'on' switches. These proteins, such as LuxR mentioned in the exercise, bind to specific sites on the DNA called enhancers or promoters, which are near the genes that need to be transcribed. The binding of an activator protein modifies the structure of the DNA and recruits the transcriptional machinery to the site. This action paves the way for RNA polymerase to attach to the promoter region of the gene and start transcribing the DNA into messenger RNA (mRNA), which will later be translated into the corresponding protein.

An activator protein is not always active; it might require a signal molecule or a specific condition to initiate its binding to DNA. Once active, it can exert a significant influence over whether a gene is expressed at low levels, in abundance, or not at all.

Transcription Process

The transcription process is akin to a scribe copying a sacred text, but here, the script is the DNA code, and the copy is a strand of RNA. Transcription is the first step in decoding the genetic instructions stored in DNA to produce proteins. It begins when RNA polymerase binds to a section of DNA known as the promoter, located at the start of a gene. This binding is often facilitated by activator proteins in the case of positive gene regulation.

RNA polymerase then travels along the DNA strand, unwinding it and synthesizing a complementary strand of mRNA in a 5' to 3' direction. The sequence of the mRNA mirror's that of the gene, except for the substitution of the base uracil (U) for thymine (T). Once the entire gene is transcribed, the RNA strand detaches, and the DNA helix re-forms. The mRNA strand carries the genetic information to the ribosomes, where it serves as a template for protein synthesis. It's this meticulous process that ensures the precise control over when and how much of each protein is made, thereby orchestrating the complex symphony of life.