Monocistronic mRNAs

Does monocistronic mRNA of eukaryotes generally represent a primary RNA transcript SOLUTION... 

Can monocistronic mRNA represent a primary transcription product of bacterial cells ... 

Yes. (The lac repressor gene of E. coli would give rise to monocistronic mRNA.)... 

By contrast, eucaryal mRNAs are translated after extensive modifications of the primary transcripts that yield mature (generally capped and polyadenylated) monocistronic mRNAs. Recognition of translation start sites does not rely upon Shine-Dalgarno recognition instead, the small ribosomal subunit (generally) binds to the capped 5 end of mRNA and scans its nucleotide sequence until the initiator AUG codon is encountered. The polypeptide chains are initiated by a non-formylated methionine and the initiation reactions are aided by as many as 8-10 protein factors, some of which possess ATPase activity and perform functions not encountered in bacteria, such as cap recognition and mRNA unwinding (for a detailed review see ref [4]). 

Synthesis of lac mRNA is determined by the activity of a specific regulatory protein—the lac repressor. This protein is encoded in the lad gene, which is transcribed in a monocistronic mRNA molecule distinct from lac mRNA. When the repressor is active. 

Most Eukaryotic Genes Produce Monocistronic mRNAs and Contain Lengthy Introns... 

As discussed In Chapter 4, many bacterial mRNAs are poly-cistronic that Is, a single mRNA molecule (e.g., the mRNA encoded by the trp operon) Includes the coding region for several proteins that function together In a biological process. In contrast, most eukaryotic mRNAs are monocistronic that Is, each mRNA molecule encodes a single protein. This difference between polycistronic and monocistronic mRNAs correlates with a fundamental difference In their translation. 

A simple eukaryotic transcription unit produces a single monocistronic mRNA, which Is translated into a single protein. 

A complex eukaryotic transcription unit is transcribed into a primary transcript that can be processed into two or more different monocistronic mRNAs depending on the choice of splice sites or polyadenylation sites (see Figure 10-2b). 

The VSY genome codes for only five proteins (33), and soon after infection the five YSY proteins represent the only translation products of the infected cell, since this virus also inhibits cellular protein synthesis (34) The five YSY monocistronic mRNAs serving as templates for these proteins have been relatively well characterized (35) and thus this system lends itself to studies on the mechanism of superinfection. Ehrenfeld and Lund (36) examined the consequences of superinfecting YSY infected HeLA cells at two hours after infection with poliovirus. By I.5 to 2 hours after poliovirus superinfection, virtually all YSY polysomes disaggregated. The pattern of protein synthesis as shown on polyacrylamide gels changed from YSY specific to poliovirus specific within 2.5 hours after infection. All five of the YSY proteins were inhibited to the same extent. 

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