Prokaryotic transcription/translation

The protein synthesis machinery reads the RNA template starting from the 5 end (the end made first) and makes proteins beginning with the amino terminus. These directionalities are set up so that in prokaryotes, protein synthesis can begin even before the RNA synthesis is complete. Simultaneous transcription-translation can t happen in eukaryotic cells because the nuclear membrane separates the ribosome from the nucleus. 

Coupled transcription-translation in prokaryotes refers to the commencement of translation of an RNA molecule before its transcription from the DNA template is complete. Could such a situation arise in eukaryotes ... 

The mRNA required for in vitro translation can itself be produced by in vitro synthesis. Commercially available kits allow DNA cloned downstream of T7-, T3 or SP6-promoters to be transcribed effectively in vitro by the relevant RNA polymerases. In coupled transcription-translation, it should be remembered that translation of eukaryotic mRNA requires a 5 cap upstream of the initiation codon, and similarly, for prokaryotic translation there should be an appropriately positioned ribosome binding site. Commercial kits are also available for combined in vitro transcription and translation. 

In prokaryotes, transcription and translation are closely coupled. Several ribosomes can simultaneously translate an mRNA, forming a polysome. 

Eukaryotes contain nuclei. Therefore, transcription is separated from translation. In prokaryotes, transcription and translation occur simultaneously. 

Proteins are synthesized in the amino-to-carboxyl direction, and mRNA is translated in the 5 —>3 direction. The start signal on prokaryotic mRNA is usually AUG preceded by a purine-rich sequence that can base-pair with 16 5 rRNA. In prokaryotes, transcription and translation are closely coupled. Several ribosomes can simultaneously translate an mRNA, forming a polysome. 

In prokaryotes such as E. coli, most of the control of protein synthesis occurs at the level of transcription. (Refer to Section 18.3 for a discussion of the principles of prokaryotic transcriptional control.) This circumstance makes sense for several reasons. First, transcription and translation are directly coupled that is, translation is initiated shortly after transcription begins (Figure 19.8). Second, the lifetime of prokaryotic mRNA is usually relatively short. With half-lives of between 1 and 3 minutes, the types of mRNA produced in a cell can be quickly altered as environmental conditions change. Most mRNA molecules in E. coli are degraded by two exonucleases, referred to as RNase II and polynucleotide phosphorylase. 

At 37 C, an E. coli ribosome can synthesize a 300-residue polypeptide chain in about 20 seconds. This rate is almost exactly the same as that calculated for prokaryotic transcription. mRNA can be translated as fast as it is transcribed because it is possible for many ribosomes to simultaneously translate a single message. Such a complex is called a polyribosome. Under some conditions, as many as 50 ribosomes may be packed onto a single mRNA. 

The details of the chain of events in translation differ somewhat in prokaryotes and eukaryotes. Like DNA and RNA synthesis, this process has been more thoroughly studied in prokaryotes. We shall use Escherichia coli as our principal example, because aU aspects of protein synthesis have been most extensively studied in this bacterium. As was the case with replication and transcription, translation can be divided into stages—chain initiation, chain elongation, and chain termination. 

Kim DM, Choi CY (1996) A semicontinuous prokaryotic coupled transcription/translation system using a dialysis membrane. Biotechnol Prog 12 645-649... 

Only one RNA polymerase catalyzes the synthesis of all three classes of RNA in prokaryotes. mRNA is transcribed from DNA template carrying genetic codes necessary for protein synthesis. mRNA, which encodes for one protein is called monocitronic, whereas mRNA encoding for more than one protein are referred to as polycitronic. Prokaryotic mRNA is polycitronic and prokaryotic transcription occurs concomitantly with translation. 

The induction of new proteins in eukaryotic cell takes a much longer time (minutes for prokaryotes but hours or days for eukaryotes) because transcription takes place in the nucleus and the resulting mRNA must be transported to endoplasmic reticulum, where translation occurs (prokaryotic transcription and translation are coupled). 

While control of gene expression can involve mRNA processing, translational control, and postribosomal covalent modification, it appears that, in eukaryotes as in prokaryotes, transcriptional control is the cardinal means for regulating gene expression. 

FIGURE 11.24 The properties of mRNA molecules in prokaryotic versus eukaryotic cells during transcription and translation. 

Technically, in vitro transcription is achieved from standard expression plasmids typically carrying SP6 or T7 promoters using marketed kits. Translation into the polypeptide may be either coupled directly to the transcription (in vitro TnT) or require isolation of the RNA. Again, a large number of suitable prokaryotic and eukaryotic cell extracts as well as complementation factors are freely available. 

Answer E. TBP and CBP participate in eukaryotic gene transcription and mRNA translation, respectively. CAP regulates the expression of prokaryotic lactose operons. ACP is involved in fatty add synthesis. 

The mRNA is bound to the smaller 30S subunit of the bacterial ribosome. The mRNA is a transcription of one of the genes of DNA, and carries the information as a series of three-base codons. The message is read (translated) in the 5 to 3 direction along the mRNA molecule. The aminoacyl-tRNA anticodon (UAC) allows binding via hydrogen bonding to the appropriate codon (AUG) on mRNA. In prokaryotes, the first amino acid encoded in the sequence is A-formylmethionine (fMet). Although the codon for initiation (A-formylmethionine) is the same as... 

Protein methylation is one of the most common protein modifications found in a wide range of prokaryotic and eukaryotic proteins that are involved either in regulation of transcription or in translation. Several amino acids can be modified, mainly by either N-methylation or C-methylation. Protein methylation has been... 

In eukaryotic cells transcription and translation occur in two distinct temporal and spacial events, whereas in prokaryotic cells they occur in one step. As humans have eukaryotic cells, we will look at this process. Transcription occurs on DNA in the nucleus and translation occurs on ribosomes in the cytoplasm. 

---