Polycistronic unit

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

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. 

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.

Gonzalez, F., Barragan Monasterio, M., Tiscornia, G., Montserrat Pulido, N., Vassena, R., Batlle Morera, L., Rodriguez Piza, I., Belmonte, J. C. I. 2009. Generation of mouse-induced pluripotent stem cells by transient expression of a single nonviral polycistronic vector. Proceedings of the National Academy of Sciences of the United States of America, 106, 8918-22. 

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. 

As we have seen in the case of virus polycistronic RNA templates, despite their integral structure, they produce different proteins in sharply different amounts. Each cistron of such a template functions at its own intensity and different cistrons produce different numbers of molecules per unit time depending on the amount of them required for virus reproduction. In this situation it is easier to assume that each cistron is not functionally connected with each other, but works independently at its own individual speed. 

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