Name: operons in bacterial gene control
Uploaded: 2020-11-23T11:47:37.000Z
Duration: 2 min 9 s
Channel: JoVE

In prokaryotic cells, an operon is a group of genetic sequences that contain regulatory elements and several protein-coding genes, the structural genes, that are transcribed together. One of the best studied cases is the lac operon in bacteria that contains three genes, lacZ, lacY, and lacA, that encode the enzymes necessary for the metabolism of lactose, along with the promoter, operator, and terminator sequences that regulate the expression of the lac genes.
Normally, when glucose concentration is abundant, the lac repressor protein binds tightly to the operator and prevents transcription of the lac genes by blocking RNA polymerase from binding to the promoter. The lac repressor is constitutively expressed, meaning that the gene encoding it is turned on by default. When glucose concentration is very low, the cell will use lactose as an energy source. Once present inside the cell, some of the lactose is converted to a modified version called allolactose, which is known as the inducer of the lac operon because it binds to and inhibits to the repressor, triggering the expression of the lac genes.
Additionally, with low glucose levels, the amount of the signaling molecule cyclic AMP increases, and binds to the catabolite activator protein, or CAP. Together, they bind to a regulatory sequence, proximally upstream of the promoter and help recruit RNA polymerase to significantly increase transcription. During transcription, a single mRNA strand is produced and released when the polymerase reaches the terminator sequence. From this mRNA, the three proteins that are necessary to process lactose are translated.

operons

Operons

Prokaryotes can control gene expression through operons&mdash;DNA sequences consisting of regulatory elements and clustered, functionally related protein-coding genes. Operons use a single promoter sequence to initiate transcription of a gene cluster (i.e., a group of structural genes) into a single mRNA molecule. The terminator sequence ends transcription. An operator sequence, located between the promoter and structural genes, prohibits the operon&rsquo;s transcriptional activity if bound by a repressor protein. Altogether, the promoter, operator, structural genes, and terminator form the core of an operon.

Operons are usually either inducible or repressible. Inducible operons, such as the bacterial lac operon, are normally &ldquo;off&rdquo; but will turn &ldquo;on&rdquo; in the presence of a small molecule called an inducer (e.g., allolactose). When glucose is absent, but lactose is present, allolactose binds and inactivates the lac operon repressor&mdash;allowing the operon to generate enzymes responsible for lactose metabolism.

Repressible operons, such as the bacterial trp operon, are usually &ldquo;on&rdquo; but will turn &ldquo;off&rdquo; in the presence of a small molecule called a corepressor (e.g., tryptophan). When tryptophan&mdash;an essential amino acid&mdash;is abundant, tryptophan binds and activates the trp repressor&mdash;preventing the operon from making enzymes required for its synthesis.

Operons may also be constitutively (i.e., continuously) active. For example, bacterial ribosomal RNA (rRNA) operons are always &ldquo;on&rdquo; because rRNAs are constantly required for translation.

Other regulatory elements contribute to an operon&rsquo;s coordinated gene expression as well. Regulatory genes encode transcriptional activator or repressor proteins. The lacI and trpR genes, for example, encode for their respective operon&rsquo;s repressors. Additional regulatory sequences, such as the lac operon&rsquo;s catabolite activator protein (CAP) binding site, provide binding sites for other activators or repressors. For instance, when glucose is low, a signaling molecule (i.e., cyclic AMP) activates CAP&mdash;permitting it to bind the CAP site, recruit RNA polymerase, and initiate lac operon transcription.

Prokaryotes can control gene expression through operons&mdash;DNA sequences consisting of regulatory elements and clustered, functionally related ...

Education

JoVE Core

Molecular Biology

Unit 1

Gene Expression

KEY TERMS AND CONCEPTS

operons in bacterial gene control

Operons help bacteria control groups of genes at the same time. An operon is a set of genes that are regulated together and can be turned on or off as a unit. This shared control helps bacteria respond quickly to changes in their environment.

In many operons, a promoter gives RNA polymerase a place to bind and start transcription. The operon also includes a regulatory region that helps control whether the genes are expressed. When conditions change, the cell can use this setup to manage gene activity efficiently.

Operons are a key example of gene regulation in prokaryotes. The transcript highlights the lac operon as a model for this process. Studying operons shows how bacteria coordinate gene expression without separating the genes into different control systems.

Operons in Bacterial Gene Control

cell specific gene expression

How Cells Switch Genes On and Off

Cells switch genes on and off to make different proteins for different jobs. In multicellular organisms, many cell types are built from the same DNA, but they do not use the same genes in the same way. This difference in gene expression helps explain why cells can look and act so differently. Liver cells are a clear example. They help detoxify blood, make bile to help digest fats, and produce proteins needed for metabolism. To do these jobs, liver cells must express a specific set of genes.

regulation of expression occurs at multiple steps

regulation-of-expression-occurs-at-multiple-steps

How Cells Control Gene Expression

Cells control gene expression at several points between DNA and protein. The most common control point is transcription. At this stage, proteins bind to short regulatory DNA sequences and can either increase or reduce transcription of the linked gene. After transcription, a precursor mRNA is made. This pre-mRNA contains exons, which code for protein, and introns, which do not. It must be processed before translation, and splicing removes the introns and joins the exons. Cells can also regulate...

cis-regulatory sequences

How Cis-Regulatory DNA Controls Transcription

Cis-regulatory sequences are short, non-coding DNA segments that control gene transcription. They sit on the same chromosome as the genes they regulate and act as binding sites for transcriptional regulators. In eukaryotes, these proteins help control gene expression patterns in different cell types. These DNA sequences can be close to a gene or thousands of bases away. When they are far from the gene in the DNA sequence, they are often brought close together in space because chromosomes are...

cooperative binding of transcription regulators

cooperative-binding-of-transcription-regulators

How Transcription Factors Bind DNA

Transcriptional regulators bind to cis-regulatory DNA sequences to control gene transcription. These DNA sequences are very short, usually fewer than ten nucleotide pairs long. Because they are so short, the same sequence can appear by chance in many places across the genome. Some regulators can also bind to groups of similar sequences. That raises the chance of random binding even more. To lower this risk, transcription regulators often form dimers, which are pairs of proteins that bind a DNA...

prokaryotic transcriptional activators and repressors

prokaryotic-transcriptional-activators-and-repressors

Operon Control by Activators and Repressors

Operon control in prokaryotes depends on activators and repressors. Prokaryotic genes are often arranged in groups, not one by one like in eukaryotes. Genes that help carry out the same biochemical process are placed together with their regulatory elements. This group is called an operon, and it is transcribed into a single polycistronic mRNA strand. Activators and repressors are DNA-binding proteins that control transcription in an operon. Activators bind near the promoter, which is the site...

Operons help bacteria control groups of genes at the same time. An operon is a set of genes that are regulated together and can be turned on or off as a unit. This shared control helps bacteria respond quickly to changes in their environment. In many operons, a promoter gives RNA polymerase a place to bind and start transcription. The operon also includes a regulatory region that helps control whether the genes are expressed. When conditions change, the cell can use this setup to manage gene...

the eukaryotic promoter region

Eukaryotic Promoter Motifs and Gene Control

The eukaryotic promoter region is a stretch of DNA located upstream of a gene. It includes an RNA polymerase binding site, a transcription start site, and several cis-regulatory sequences. These DNA signals help control when transcription begins. The promoter can be divided into proximal and distal regions. The proximal promoter lies close to the gene and includes cis-regulatory sequences and the core promoter. The core promoter is the RNA polymerase binding site, and it is usually found...

co-activators and co-repressors

Chromatin Control in Gene Transcription

Chromatin control plays a major role in gene transcription in eukaryotes. Gene expression is regulated by several proteins that work together at a gene regulatory site. Some of these proteins bind directly to specific DNA sequences, while co-regulators attach to those regulators but do not bind DNA on their own. Co-regulators can act as co-activators or co-repressors, depending on the complex they join. One co-regulator may even switch roles in different settings. For example, the...

eukaryotic transcription activators

How Transcription Activators Turn Genes On

Transcription activators are proteins that help turn genes on by promoting transcription from DNA to RNA. They often have two parts. One part binds to DNA, and the other part activates transcription. In some proteins, such as the glucocorticoid receptor and MyoD, one domain can do both jobs. The DNA-binding part recognizes specific regulatory sequences on the DNA. These binding domains are grouped into families based on their structure. Common examples include the leucine zipper, zinc finger,...

eukaryotic transcription inhibitors

How Eukaryotic Gene Repressors Work

Eukaryotic gene repressors help turn specific genes off when the cell needs to control growth, development, or other biochemical processes. These DNA-binding proteins regulate gene expression in eukaryotes. Their presence at the right time and place can be triggered by hormones and signals from other cells. Most transcription inhibitors have two main parts. One part binds DNA, and the other is a repressor domain, which helps shut down transcription. Repressor domains can connect with basal...

combinatorial gene control

Gene Expression by Transcription Factor Combinations

Gene expression can depend on combinations of transcription factors working together. In combinatorial gene control, several transcription factors act in a coordinated way to regulate a single gene. If one or more of these factors are missing, the level of gene expression or repression can change a lot. This kind of control helps explain how a relatively small number of transcription factors can regulate a very large number of genes. More than 30,000 genes are controlled by about 2,000 to...

induced pluripotent stem cells

Reprogramming Cells into Stem Cells

Induced pluripotent stem cells are created by reprogramming mature cells back into a stem-cell state. This process shows that specialized cells can be changed into cells with broader developmental potential. The topic is important in biology because it links cell identity, gene control, and stem cell research. These cells begin as ordinary body cells and are pushed to de-differentiate, or lose their specialized features. After reprogramming, they behave like pluripotent stem cells, which means...

master transcription regulators

MEF2C and Gene Control Cascades

MEF2C is a master transcription regulator that controls gene expression in coordinated cascades. These regulators are proteins that manage the activity of multiple genes that often work together to produce a complex outcome. When a master transcription regulator is activated, it can start a chain of transcriptional activation that supports that outcome. Master transcription regulators can act in two main ways. They may bind directly to the regulatory sequences of several genes involved in a...

epigenetic regulation

DNA Methylation and Histone Changes

Epigenetic regulation controls gene activity without changing the DNA sequence. It helps determine when genes are turned on or off in a cell. Two major epigenetic mechanisms are DNA methylation and histone modification. DNA methylation adds chemical groups to DNA, while histone changes affect how tightly DNA is packed around histone proteins. These changes can make genes easier or harder to use. In high school biology, epigenetic regulation is important for understanding how cells with the...

genomic imprinting and inheritance

Genomic Imprinting in Human Disease

Genomic imprinting affects how some inherited genes are turned on or off in human cells. Diploid organisms inherit chromosomes from both parents, and each gene usually has two alleles. In most cases, both alleles are expressed. If one allele is missing or mutated, the other can often compensate, but some genes do not work that way. In genomic imprinting, a gene or its regulatory sequence carries chemical tags, such as methyl groups, on the copy inherited from only one parent. These tags are...

key terms and concepts

Learn how operons let bacteria regulate groups of genes together, with a focus on promoters, regulatory regions, and the lac operon model.

Operons in Bacterial Gene Control | JoVE K12

molecular biology

JoVE Core Molecular Biology showcases the molecular processes of the cell through clear and concise animated video lessons. Additionally, in scientist-in-action videos, these concepts' application is demonstrated through real experiments in modern laboratories.

myJoVE Corporation

Operons in Bacterial Gene Control

Description

Transcript

Tags