Prokaryotic transcription
Encyclopedia
Prokaryotic transcription is the process in which messenger RNA
transcripts of genetic material in prokaryote
s are produced, to be translated for the production of protein
s. Prokaryotic transcription occurs in the cytoplasm
alongside translation
. Unlike in eukaryotes, prokaryotic transcription and translation can occur simultaneously. This is impossible in eukaryotes, where transcription occurs in a membrane-bound nucleus while translation occurs outside the nucleus in the cytoplasm. In prokaryotes genetic material is not enclosed in a membrane-enclosed nucleus and has access to ribosome
s in the cytoplasm.
Transcription is known to be controlled by a variety of regulators in prokaryotes. Many of these transcription factors are homodimers containing helix-turn-helix DNA-binding motifs.
, the stronger the binding is. Additional transcription regulation comes from transcription factors that can affect the stability of the holoenzyme structure at initiation.
Most transcripts originate using adenosine-5'-triphosphate (ATP
) and, to a lesser extent, guanosine-5'-triphosphate (GTP
) (purine
nucleoside triphosphates) at the +1 site. Uridine-5'-triphosphate (UTP
) and cytidine-5'-triphosphate (CTP
) (pyrimidine
nucleoside triphosphates) are disfavoured at the initiation site.
Messenger RNA
Messenger RNA is a molecule of RNA encoding a chemical "blueprint" for a protein product. mRNA is transcribed from a DNA template, and carries coding information to the sites of protein synthesis: the ribosomes. Here, the nucleic acid polymer is translated into a polymer of amino acids: a protein...
transcripts of genetic material in prokaryote
Prokaryote
The prokaryotes are a group of organisms that lack a cell nucleus , or any other membrane-bound organelles. The organisms that have a cell nucleus are called eukaryotes. Most prokaryotes are unicellular, but a few such as myxobacteria have multicellular stages in their life cycles...
s are produced, to be translated for the production of protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
s. Prokaryotic transcription occurs in the cytoplasm
Cytoplasm
The cytoplasm is a small gel-like substance residing between the cell membrane holding all the cell's internal sub-structures , except for the nucleus. All the contents of the cells of prokaryote organisms are contained within the cytoplasm...
alongside translation
Prokaryotic translation
Prokaryotic translation is the process by which messenger RNA is translated into proteins in prokaryotes.-Initiation:Initiation of translation in prokaryotes involves the assembly of the components of the translation system which are: the two ribosomal subunits , the mRNA to be translated, the...
. Unlike in eukaryotes, prokaryotic transcription and translation can occur simultaneously. This is impossible in eukaryotes, where transcription occurs in a membrane-bound nucleus while translation occurs outside the nucleus in the cytoplasm. In prokaryotes genetic material is not enclosed in a membrane-enclosed nucleus and has access to ribosome
Ribosome
A ribosome is a component of cells that assembles the twenty specific amino acid molecules to form the particular protein molecule determined by the nucleotide sequence of an RNA molecule....
s in the cytoplasm.
Transcription is known to be controlled by a variety of regulators in prokaryotes. Many of these transcription factors are homodimers containing helix-turn-helix DNA-binding motifs.
Initiation
The following steps occur, in order, for transcription initiation:- RNA polymerase (RNAP) binds to one of several specificity factors, σSigma factorA sigma factor is a bacterial transcription initiation factor that enables specific binding of RNA polymerase to gene promoters. Different sigma factors are activated in response to different environmental conditions...
, to form a holoenzyme. In this form, it can recognize and bind to specific promoter regions in the DNA. The -35 region and the -10 ("Pribnow boxPribnow boxThe Pribnow box is the sequence TATAAT of six nucleotides that is an essential part of a promoter site on DNA for transcription to occur in bacteria...
") region comprise the basic prokaryotic promoter, and |T| stands for the terminatorTerminator (genetics)In genetics, a terminator, or transcription terminator is a section of genetic sequence that marks the end of gene or operon on genomic DNA for transcription.In prokaryotes, two classes of transcription terminators are known:...
. The DNA on the template strand between the +1 site and the terminator is transcribed into RNA, which is then translatedTranslation (genetics)In molecular biology and genetics, translation is the third stage of protein biosynthesis . In translation, messenger RNA produced by transcription is decoded by the ribosome to produce a specific amino acid chain, or polypeptide, that will later fold into an active protein...
into proteinProteinProteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
.At this stage, the DNA is double-stranded ("closed"). This holoenzyme/wound-DNA structure is referred to as the closed complex.
- The DNA is unwound and becomes single-stranded ("open") in the vicinity of the initiation site (defined as +1). This holoenzyme/unwound-DNA structure is called the open complex.
- The RNA polymerase transcribes the DNA (the beta subunit initiates the synthesis), but produces about 10 abortive (short, non-productive) transcripts which are unable to leave the RNA polymerase because the exit channel is blocked by the σ-factor.
- The σ-factor eventually dissociates from the holoenzyme, and elongation proceeds.
Elongation
Promoters can differ in "strength"; that is, how actively they promote transcription of their adjacent DNA sequence. Promoter strength is in many (but not all) cases, a matter of how tightly RNA polymerase and its associated accessory proteins bind to their respective DNA sequences. The more similar the sequences are to a consensus sequenceConsensus sequence
In molecular biology and bioinformatics, consensus sequence refers to the most common nucleotide or amino acid at a particular position after multiple sequences are aligned. A consensus sequence is a way of representing the results of a multiple sequence alignment, where related sequences are...
, the stronger the binding is. Additional transcription regulation comes from transcription factors that can affect the stability of the holoenzyme structure at initiation.
Most transcripts originate using adenosine-5'-triphosphate (ATP
Adenosine triphosphate
Adenosine-5'-triphosphate is a multifunctional nucleoside triphosphate used in cells as a coenzyme. It is often called the "molecular unit of currency" of intracellular energy transfer. ATP transports chemical energy within cells for metabolism...
) and, to a lesser extent, guanosine-5'-triphosphate (GTP
Guanosine triphosphate
Guanosine-5'-triphosphate is a purine nucleoside triphosphate. It can act as a substrate for the synthesis of RNA during the transcription process...
) (purine
Purine
A purine is a heterocyclic aromatic organic compound, consisting of a pyrimidine ring fused to an imidazole ring. Purines, including substituted purines and their tautomers, are the most widely distributed kind of nitrogen-containing heterocycle in nature....
nucleoside triphosphates) at the +1 site. Uridine-5'-triphosphate (UTP
Uridine 5'-triphosphate
Uridine-5'-triphosphate is a pyrimidine nucleoside triphosphate, consisting of the organic base uracil linked to the 1' carbon of the ribose sugar, and esterified with tri-phosphoric acid at the 5' position...
) and cytidine-5'-triphosphate (CTP
Cytidine triphosphate
Cytidine triphosphate is a pyrimidine nucleoside triphosphate.CTP is a substrate in the synthesis of RNA.CTP is a high-energy molecule equal to ATP, but its role in the organism is more specific than that of ATP....
) (pyrimidine
Pyrimidine
Pyrimidine is a heterocyclic aromatic organic compound similar to benzene and pyridine, containing two nitrogen atoms at positions 1 and 3 of the six-member ring...
nucleoside triphosphates) are disfavoured at the initiation site.
Termination
Two termination mechanisms are well known:- Intrinsic termination (also called Rho-independent transcription termination) involves terminator sequences within the RNA that signal the RNA polymerase to stop. The terminator sequence is usually a palindromicPalindromeA palindrome is a word, phrase, number, or other sequence of units that can be read the same way in either direction, with general allowances for adjustments to punctuation and word dividers....
sequence that forms a stem-loop hairpin structure that leads to the dissociation of the RNAP from the DNA template.
- Rho-dependent termination uses a termination factorTermination factorTermination is part of the process of transcribing RNA. In eukaryotes, a termination factor is required to release the newly made RNA from the transcription complex...
called ρ factorRho factorA ρ factor is a prokaryotic protein involved in the termination of transcription.Rho factor is an essential transcription protein in prokaryotes. In Escherichia coli, it is a ~275 kD hexamer of identical subunits. Each subunit has an RNA-binding domain and an ATP-hydrolysis domain...
(rho factor) which is a protein to stop RNA synthesis at specific sites. This protein binds at a rho utilisation site on the nascent RNA strand and runs along the mRNA towards the RNAP. A stem loop structure upstream of the terminator region pauses the RNAP, when ρ-factor reaches the RNAP, it causes RNAP to dissociate from the DNA, terminating transcription.
External links
- Prokaryotic Transcription - animation