![]() By converting the sequence of, for example, HIV gag to the human sequence bias, its expression becomes Rev-independent without altering the RRE. The HIV genome is particularly Adenosin (A)-rich and Cytidin (C)-low, resulting in the utilization of codons that are untypical for human cells. Very recently, we could show that spliceosomal factors of the B complex function as retention factors for the unspliced HIV RNA. The current view is that a combination of unused splice sites (SS) in the intron-containing RNAs and an HIV-specific sequence bias result in nuclear retention and ultimately degradation of these transcripts. ![]() The sequence features that lead to Rev-dependency have been debated in the last decades. Thus, Rev dispatches the incompletely spliced viral RNAs to the cellular protein export pathway, resulting in export of intron-containing viral RNAs. Rev binds to the Rev-responsive element (RRE) present in all intron-containing viral RNAs and subsequently engages with the cellular protein export factor CRM-1. HIV has evolved an export adaptor protein named Rev to overcome this restriction. These mRNAs are normally retained in the nucleus. Gene expression of HIV is complicated by the use of intron-containing mRNAs for translation. An extension of this concept, named mRNP code, takes into account how the primary RNA sequence dictates all steps of mRNA biogenesis including splicing, processing, translation, and localization based on differential protein recruitment. Using iCLIP data and bioinformatics approaches, the term splicing code was established to describe and predict splicing events. Splicing enhancers and silencers are recognized by RNA-binding proteins (RBPs), namely, SR proteins and hnRNPs, respectively. In essence, two major features determine splice site usage: splicing enhancers/silencers and dynamic RNA secondary structures. Many of these have served as blueprints to understand the regulation of cellular alternative splicing. A variety of mechanisms have been described as to how the virus maintains its splicing ratios using HIV reporter constructs. In order to generate various translation templates, HIV uses alternative splicing to obtain many mRNAs from a single pre-mRNA. Most retroviruses are faced with the problem of expressing all genes from a single, polycistronic primary transcript (pre-mRNA), which also serves as the genomic RNA to be packaged in newly formed viral particles. ![]() This is a strong indication of an HIV-specific mRNP code in the 5′ gag region wherein the primary RNA sequence bias creates motifs for RNA-binding proteins and controls the fate of the HIV-RNA in terms of viral gene expression and infectivity. Thus, the phenotypes solely depend on the nucleotide composition of the two GFP versions. Of note, the adaptation yielded completely different primary sequences although encoding the same amino acids. ![]() Interestingly, an adaptation of the inserted GFP sequence toward an HIV-like nucleotide bias reversed these phenotypes completely. Increasing the GC content by insertion of either GFP or silent mutations activates a cryptic splice donor site in gag, entirely deregulates the viral splicing pattern, and lowers infectivity. Here, we show that these processes are extremely sensitive to sequence alterations in the 5’coding region of the HIV genomic RNA. Both alternative splicing and Rev-dependency are regulated by the primary HIV RNA sequence. To facilitate the otherwise hampered nuclear export of non-fully processed mRNAs, HIV encodes the Rev protein, which recognizes its intronic response element and fuels the HIV RNAs into the CRM-1-dependent nuclear protein export pathway. Alternative splicing and the expression of intron-containing mRNAs is one hallmark of HIV gene expression. ![]()
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |