After the positive stranded RNA virus GM402 infects its host, its genome is unco
ID: 10007 • Letter: A
Question
After the positive stranded RNA virus GM402 infects its host, its genome is uncoated and translated to yield 5 polypeptides that are required for viral replication and morphogenesis. Describe how expression of these proteins might be likely to differ if the virus infects a prokaryotic versus a eukaryotic organism. Specifically focus on how the mechanism of translation requires different ways of producing the mature viral polypeptides and incorporate into your description at least two mechanisms that are known in eukaryotes to generate multiple mature polypeptides.Explanation / Answer
Initially its important to know that all +RNA viruses Baltimore class IV after infection undergo translation rather than transcription immediately. Thus, these viruses do not require access to the nucleus of the host. Non-eukaryotic mRNA is essentially mature upon transcription and requires no processing, except in rare cases. Eukaryotic pre-mRNA, however, requires extensive processing. mRNA in eukaryotes is monocistronic but prokaryotic mRNA is polycistronic. Furthermore, bacterial mRNA is never capped and rarely polyadenylated. While mRNA of eukaryotic cells is. There are two things that occur for eukaryotic viruses. Initially you may have splicing of the mRNA to remove introns and leave exons. A primary transcript may be cut and spliced in more than one way to produce two or more mRNA species, splice variants. In many bicistronic mRNAs the ORFs (open reading frames ) overlap in others there is an ORF within an ORF . One mechanism to read the second ORF involves leaky scanning; a 40S ribosomal subunit may overlook the ORF 1 start codon and initiate translation at the start of ORF 2. The ORFs for the two proteins are in different reading frames, so the proteins that they encode are unrelated. Of course it is essential that the sequence ‘makes sense’ in both reading frames! Another mechanism for reading a second ORF in an mRNA involves ribosomal frameshifting; a ribosome shifts into a different reading frame towards the end of ORF 1. It therefore does not recognize the ORF 1 stop codon, but continues along the mRNA, reading ORF 2 to produce an elongated version of the ORF 1 protein. Frameshifting occurs when the ribosome moving along the RNA encounters a frameshift signal (a speci?c sequence) followed by a secondary structure, usually a pseudoknot.