Question

. Explain the process of alternative splicing. What does this process achieve for the virus?

Answer 1

Alternative splicing :

Alternative splicing is a specialised process during gene expression that results in a single gene coding for multiple types of protein coding mRNAs. Alternative splicing occurs when several splice donors and/or acceptors are eligible and facultative. This leads to transcripts or mRNA having different splicing outcome and/or are unspliced.

Process of alternative splicing :

– During alternative splicing cis-acting regulatory in the pre mRNA sequence determine which exons will be retained in the final mRNA or will be spliced out. This cis-acting regulatory elements alter splicing by binding with the trans-acting protein factors and heterogeneous nuclear ribonucleoproteins. Trans-acting protein factors SR protein wroks as splicing facilitator. Whereas the heterogeneous nuclear ribonucleoproteins suppress the splicing. Retaining or splicing of the alternative exons is decided by cellular abundance, combinatorial effects, and competitive binding between SR activators and hnRNPs. The final results of splicing is dependent upon the interactions of splicing activators and inhibitors along with the conformation and accessibility of the splicing sites.

Different modes of alternative splicing are :

• Exon skipping or cassette exon : The exon can be retained or spliced out from the primary mRNA
• Intron retention : An sequence can be spliced out as an intron or simply retained from the primary transcript
• Mutually exclusive exons : One of the two exons is retained in mRNAs but nit both
• Alternative acceptor site : An alternative acceptor site or 3' splice junction is used changing the 5' boundary of the downstream exon
• Alternative donor site : An alternative donor site or 5' splice junction is used changing the 3' boundary of upstream exon

Achievements of alternative splicing in viruses :

• It helps to encode several protein in a few mRNAs. Adenoviridae and Retroviridae encodes up to 12 different peptides from one pre-mRNA.
• By the help of alternative splicing viruses like Papillomaviridae can regulate the early or late expression of a particular gene
• Viruses like Hepadnaviridae and Retroviridae have evolved specialised proteins to export their unspliced genomic RNA out of the nucleus
• Viruses like Herpesviridae use the mechanism of alternative splicing as a potential anti-host defense mechanism by inhibiting some host splicing factors that helps in the synthesis of key antiviral proteins

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