Polycistronic mRNA, operons

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. 

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 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). 

When RNA polymerase transcribes the structural genes of an operon, a polycistronic mRNA (i.e., an mRNA that codes for more than one protein) is produced. 

A single polycistronic mRNA is transcribed from an operon. This single mRNA codes for all the proteins of the operon. 

The physiological signal controlling the lac and ara operons is the utilization of carbon sources for metabolic energy. In contrast, the tryptophan trp) operon is sensitive to the need for biosynthetic processes and is transcribed under conditions where intracellular concentrations of the amino acid tryptophan are below an optimal level for efficient protein synthesis. The trp operon consists of a promoter and operator region which controls the expression of a polycistronic mRNA encoding five proteins needed for tryptophan biosynthesis. 

Fig. 16.2. An operon. The structural genes of an operon are transcribed as one long polycistronic mRNA. During translation, different start (AUG) and stop (shown in blue) codons lead to a number of distinct proteins being produced from this single mRNA.

Fig. 9-17 The lactose operon of E. coll. Here I, p, o, z, y, and a denote the repressor gene, promoter, operator, p-galactosidase gene, permease gene, and transacetylase genes, respectively. Because the three genes, z, y and a, are transcribed as a single unit polycistronic mRNA), they are said to be expressed coordinately. When transcription is blocked by the binding of the repressor to the operator, none of the genes are expressed.

In Section III,B, the phenomenon of polarity was discussed in relation to its implications for a polycistronic mRNA for the histidine operon. For the deduction of the presence of a polycistronic mRNA, it was not necessary to know the specific mechanism by which polar mutations exerted their effect. However, a great deal of effort has been expended in trying to understand this mechanism, and these studies have produced a greater understanding of the translation process. 

Transcriptional repression is a key mechanism to control the activity of prokaryotic promoters. Enzymes used in a specific metabolic pathway are often organized into an operon that is transcribed into a single polycistronic mRNA. Specific repressor proteins then control the transcriptional activity of the operon by regulating RNA polymerase binding to the promoter. Repressor proteins are DNA-binding proteins that typically block RNA polymerase access to the —10 and/or —35 regions in the pro-... 

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