Control of gene expression

Bacteria have operons, which are protein-encoding gene clusters. Prokaryotic genes lack introns. RNA transcribed from prokaryotic operons is polycistronic – multiple proteins are encoded in a single transcript.

In prokaryotes, operons are defined as groups of genes transcribed under the control of a single operator gene. The operon comprises a functionally integrated genetic unit for the control of gene expression, and comprises one or more genes together with an adjacent promoter (activator or repressor) and an operator that controls expression through interaction with a regulator protein.

Control of prokaryotic gene expression is brought about by control of the rate of transcriptional initiation by two DNA promoter sequence elements. In prokaryotic cells, all RNA classes are synthesized by a single polymerase. Regulatory accessory proteins alter the activity of RNA polymerase at a given promoter by affecting the ability of RNA polymerase to recognize start-sites. These regulatory proteins can act both positively (activators) and negatively (repressors).

In bacteria, regulons comprise several operons, and are global regulatory systems that participate in pleiotropic regulatory domains.


 Table gene regulation in E.coli :

More detail:

Control of prokaryotic gene expression is brought about by control of the rate of transcriptional initiation by two DNA promoter sequence elements – the -35 and -10 positions. These elements are approximately 35 bases and 10 bases upstream of the site of transcriptional initiation. These promoter sequences promote recognition of transcriptional start sites by RNA polymerase. The consensus sequence for the -35 position is TTGACA, and for the -10 position, TATAAT. (The -10 position is also known as the Pribnow-box.) These promoter sequences are recognized and contacted by RNA polymerase.

Regulatory accessory proteins alter the activity of RNA polymerase at a given promoter by affecting the ability of RNA polymerase to recognize start-sites. These regulatory proteins can act both positively (activators) and negatively (repressors). Proteins with sequences termed operators regulate the accessibility of promoter regions to prokaryotic DNA. The operator region is adjacent to the promoter elements in most operons, and in most cases the sequences of the operator bind a repressor protein. However, E. coli possesses several operons that contain overlapping sequence elements, one that binds a repressor and one that binds an activator.

Two major modes of transcriptional regulation in bacteria (E. coli) utilize repressor proteins to control the expression of operons.
1. Catabolite-regulated operons employ repressor proteins to down-regulate operons that produce gene products necessary for the utilization of energy. A classic example of a catabolite-regulated operon is the lac operon, responsible for obtaining energy from b-galactosides such as lactose.
2. Attenuated operons regulate operons that produce gene products necessary for the synthesis of small biomolecules such as amino acids. Expression of the an attenuated operon class of operons is repressed by sequences within the transcribed RNA. A classic example of an attenuated operon is the trp operon, responsible for the biosynthesis of tryptophan.

Table  gene regulation in E.coli :

E. Coli Research Identifies Two New Keys To Regulation Of Bacterial Gene Expression :
Gourse investigated the interaction between RNA polymerase and promoters from the E. coli chromosome. RNA polymerase reads the information in DNA and transcribes it into chains of RNA, which are later translated into proteins. Promoter regions are specific sequences within the DNA chain that tell RNA polymerase when and where to begin transcription, and how much product to make from specific genes.

Gourse's group found that there is a specific region within DNA promoters that makes contact with a highly conserved but previously underappreciated segment of the sigma subunit of RNA polymerase. While the contact with sigma is very strong at promoters for most genes, it is particularly weak at promoters that make ribosomal RNA, which means that other factors like nutritional and environmental signals ultimately regulate the expression of those genes.

"In this case, regulation is achieved not because the promoter makes a special contact, but because it can't establish contact at all," says Gourse. "This is an example of how sometimes less is more, and a probably very ancient example of one of the methods that arose through evolution to regulate gene expression."

Ribosomal RNA makes up the bulk of ribosomes, the molecular machines that make proteins and are present in huge numbers in all cells. Since so much of the cell's energy is used to make ribosomes, control of ribosomal RNA transcription is particularly crucial to a cell's well-being.

1 Glossary:

Blogger qtr said...

A consensus sequence may be a short sequence of nucleotides that is found several times in the genome and is thought to play the same role in its different locations. For example, many transcription factors recognise particular consensus sequences in the promoters of the genes they regulate.

Thus a consensus sequence defines a putative DNA recognition site: it is obtained by aligning all known examples of a certain recognition site and defined as the idealized sequence that represents the predominant base at each position. All the actual examples shouldn't differ from the consensus by more than a few substitutions.

In general, a consensus sequence is that idealized sequence in which each position represents the base/amino acid most often found when many sequences are compared. A genetic consensus sequence is a sequence of nucleotides that is common to different genes or genomes. There may be some variations but such sequences show considerable similarity. So, a consensus sequence is the prototype sequence that most others approach.

9:29 AM  

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