Eukaryotic protein synthesis operon

The control and expression of protein in eukaryotic cells is more complex than in bacteria. Research into this field is ongoing and, as such, is beyond the scope of this text. Many systems of control are known in bacteria and can be used to illustrate the type of control mechanisms and the importance of the environmental control of protein synthesis. The first and best documented example is that of the lactose (lac) operon is Escherichia coli. 

Another example of translational control in eukaryotes is the inhibition of yeast GCN4 protein synthesis by stem-loop structures present in the 50 end of the mRNA. GCN4 control, and an analogous situation in bacteria, links amino-acid biosynthesis to ribosome pausing in the 50 end of the mRNA. This mechanism was first described for the tryptophan operon in E. coli and it is often referred to as attenuation. Transcriptional and translational control of the tryptophan biosynthetic enzymes are described in Chapter 28. 

A number of differences between eukaryotes and prokaryotes affect the processes of replication, transcription, and translation, in addition to the content of their DNA. Eukaryotic DNA is complexed with histones, and prokaryotic DNA is not. In eukaryotic cells, the process of transcription, which occurs in the nucleus, is separated by the nuclear envelope from the process of translation (protein synthesis from the mRNA template), which occurs in the cytoplasm. Because prokaryotes lack nuclei, the processes of transcription and translation occur simultaneously. Transcription of bacterial DNA requires only one promoter per operon. In contrast, human DNA requires one promoter for each gene. 

The temporal genome organization of eukaryotes could have evolved from the functional organization of the prokaryotic genome. The temporal sequence of protein synthesis in prokaryotes occurs from a stable, polycistronic template which is determined by the linear sequence of cistrons (Ohtaka and Spiegelman, 1963). For instance, the enzymes for histidine synthesis are in a sequence which corresponds to the linear gene sequence in the histidine operon of Salmonella typhimurium. There is a 20 min period between the appearance of the first and last (tenth) enzymes. The data can be explained on the basis of the successive synthesis of individual mRNA s or on the successive synthesis of a polycistronic message. This... 

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