Polycistronic mRNAs

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. 

A single polygenic (polycistronic) mRNA is transcribed from these three genes starting at the 5 -end of the strand being synthesized... 

Spieth, J., Brooke, G., Kuersten, S., Lea, K. and Blumenthal, T. (1993) Operons in C. elegans polycistronic mRNA precursors are processed by trans-splicing of SL2 to downstream coding regions. Cell 73, 521-532. 

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),... 

Eukaryotic ribosomes are larger (80S) and more complex than prokaryotic ribosomes (70S). Initiation is basically similar in prokaryotes and eukaryotes except that in eukaryotes at least nine initiation factors are involved (cf. three factors in prokaryotes), the initiating amino acid is methionine (cf. N-formylmethionine in prokaryotes), eukaryotic mRNAs do not contain Shine-Dalgarno sequences (so the AUG initiation codon is detected by the ribosome scanning instead), and eukaryotic mRNA is monocistronic (cf. some polycistronic mRNAs in prokaryotes). Initiation in eukaryotes involves the formation of a 48S preinitiation complex between the 40S ribosomal subunit, mRNA, initiation factors and Met-tRNA 61. The ribosome then scans the mRNA to locate the AUG initiation codon. The 60S ribosomal subunit now binds to form the 80S initation complex. 

Eukaryotic genes may be clustered (for example, genes for a metabolic pathway may occur on the same region of a chromosome) but are independently controlled. Operons or polycistronic mRNAs do not exist in eukaryotes. This contrasts with prokaryotic genes, where a single control gene often acts on a whole cluster (for example, lac I controls the synthesis of p-galactosidase, permease, and acetylase). 

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). 

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