Alternative splicing

Alternative splicing occurs when several splice donors and/or acceptors are eligible and facultative. This leads to transcripts having different splicing outcome and/or are unspliced. Alternative splicing is regulated by cellular and viral proteins which modulates locally the activity of splicing factors U1 and U2.
-It offers the opportunity to encode several proteins in few messengers, like for Adenoviridae and Retroviridae encoding up to 12 different peptides from one pre-mRNA.
-It is a way to regulate early and late expression for viruses like Papillomaviridae and maybe Orthomyxoviridae.
-Cellular unspliced mRNA cannot be exported out of the nucleus. Hepadnaviridae and Retroviridae have evolved proteins to export their unspliced genomic RNA.
-It is used by Herpesviridae as a potential anti-host defense mechanism. By inhibiting some host splicing factors, these viruses prevent the synthesis of key antiviral proteins like PML or STAT1.

Virus	viral factor	cellular factor	Ref
ssDNA viruses
Anelloviridae
Circoviridae
Parvoviridae
dsDNA viruses
Adenoviridae	33k : expression of late proteins
Baculoviridae
Herpesviridae	Unspliced RNA nuclear export EBV- BM2 HCMV- UL69 HSV1- ICP27		EBV HCMV HSV-1
Papillomaviridae	E2 activates cellular SR	hnRNP A1 inhibit L1 mRNA expression
Polyomaviridae		SF2 (SV40)
Reverse-transcribing viruses
Hepadnaviridae	Capsid?: pg-RNA nuclear export
Retroviridae, Alpharetrovirus	GAG genomic RNA nuclear export
Retroviridae, Lentivirus	genomic RNA nuclear export HIV-1 Rev HTLV-1 Rex		HIV-1 HTLV-1
ss(-)RNA viruses
Bornaviridae
Orthomyxoviridae	NS1	SF2