Polycistronic transcription unit

Trypanosomatid Genes Are Organized as Polycistronic Transcription Units... 

Polycistronic transcription units are typical of prokaryotic organisms and of some eukaryotic viruses. Trypanosomes (and nematodes which also trans-splice, see below) represent the only examples of eukaryotes which transcribe their genes in this way. At present, there is no evidence that the various genes encoded in polycistronic transcription units in trypanosomes are related in their function as are operons in bacteria. Likewise, we do not know what the average size of a transcription unit is in trypanosomatids and whether there is any attenuation of transcription in regions that are promoter-distal relative to those that are promoter-proximal. This latter consideration has obvious relevance for the control of gene expression in these organisms. 

As discussed above, trans-splicing serves to mature 5 ends of mRNAs embedded with polycistronic transcription units in trypanosomes and may serve the same function in nematodes. However, trans-splicing may be important in other aspects of mRNA metabolism. 

A transcription unit represents the combination of regulatory and coding DNA sequences that together make up an expressible unit, whose expression leads to synthesis of a gene product that often is a protein but also may be an RNA molecule. In prokaryotes, proteins in a specific metabolic pathway are often encoded by genes that are clustered and transcribed into one polycistronic mRNA. A polycistronic mRNA encodes for multiple proteins. In such mRNAs, ribosomes are recruited to internal translational initiation sites through an interaction between the 16S ribosomal RNA and the so-called Shine-Delgamo sequence located immediately upstream of the translational start codon that is used to initiate protein synthesis. 

The analysis in vitro demonstrated the synthesis of a polycistronic messenger RNA (see page 65). Later such a transcription unit was also shown in vivo (page 66). As a consequence, one has to expect the existence of a common promoter which was indeed shown (Davis and Hyman, I97O Gomez and... 

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.

Fig. 105. Model of polycistronic unit of transcription. (After Scherrer and Mar-caud, 1968)...

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