Polycistronic mRNA transcripts

FIGURE 28-5 Representative prokaryotic operon. Genes A, B, and C are transcribed on one polycistronic mRNA. Typical regulatory sequences include binding sites for proteins that either activate or repress transcription from the promoter. 

The function of the leader sequence is to fine tune expression of the trp operon based on the availability of tryptophan inside the cell. It does this as follows. The leader sequence contains four regions (Fig. 2, numbered 1-4) that can form a variety of base-paired stem-loop ( hairpin ) secondary structures. Now consider the two extreme situations the presence or absence of tryptophan. Attenuation depends on the fact that, in bacteria, ribosomes attach to mRNA as it is being synthesized and so translation starts even before transcription of the whole mRNA is complete. When tryptophan is abundant (Fig. 2a), ribosomes bind to the trp polycistronic mRNA that is being transcribed and begin to translate the leader sequence. Now, the two trp codons for the leader peptide lie within sequence 1, and the translational Stop codon (see Topic HI) lies between sequence 1 and 2. During translation, the ribosomes follow very closely behind the RNA polymerase and synthesize the leader peptide, with translation stopping eventually between sequences 1 and 2. At this point, the position of the ribosome prevents sequence 2 from interacting with sequence 3. Instead sequence 3 base-pairs with sequence 4 to form a 3 4 stem loop which acts as a transcription terminator. Therefore, when tryptophan is present, further transcription of the trp operon is prevented. If, however, tryptophan is in short supply (Fig. 2b), the ribosome will pause at the two trp codons contained within sequence 1. This leaves sequence 2 free to base pair with sequence 3 to form a 2 3 structure (also called the anti-terminator),... 

Question Does transcription in eukaryotes yield polycistronic mRNA ... 

How many transcription termination sequences would be present in polycistronic mRNA SOLUTION... 

Polycistronic mRNA, which forms as a primary transcript in bacteria, is a continuous length of RNA transcribed from a single promoter. It will therefore contain only one normal termination sequence (i.e.. ignoring a possible attenuator sequence before the first initiation codon). 

Metabolic pathways normally consist of a large number of enzymes in some cases, the individual enzymes are used in a particular pathway and nowhere else. In these cases, it is efficient to regulate expression of all or none of the enzymes. In bacterial systems, all enzymes of the pathway are encoded in a single polycistronic mRNA molecule, and synthesis of the mRNA produces all the enzymes. In eukaryotes, common signals for transcription of different genes may be used or the primary transcript can be differentially processed to yield a set of mRNA... 

When a single mRNA transcript contains the coding information for more than one protein, and those proteins are translated as independent polypeptides, it is called a polycistronic mRNA. Such polycistronic mR-... 

Attenuation Controls Transcriptional Elongation of trp Polycistronic mRNA... 

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