The Human Immunodeficiency Virus Type 1gagGene Encodes an Internal Ribosome Entry Site

Abstract

Several retroviruses have recently been shown to promote translation of their gag gene products by internal ribosome entry. In this report, we show that mRNAs containing the human immunodeficiency virus type 1 (HIV-1) gag open reading frame (ORF) exhibit internal ribosome entry site (IRES) activity that can promote translational initiation of Pr55 gag . Remarkably, this IRES activity is driven by sequences within the gag ORF itself and is not dependent on the native gag 5-untranslated region (UTR). This cap-independent mechanism for Pr55 gag translation may help explain the high levels of translation of this protein in the face of major RNA structural barriers to scanning ribosomes found in the gag 5 UTR. The gag IRES activity described here also drives translation of a novel 40-kDa Gag isoform through translational initiation at an internal AUG codon found near the amino terminus of the Pr55 gag capsid domain. Our findings suggest that this low-abundance Gag isoform may be important for wild-type replication of HIV-1 in cultured cells. The activities of the HIV-1 gag IRES may be an important feature of the HIV-1 life cycle and could serve as a novel target for antiretroviral therapeutic strategies.Like other complex retroviruses, human immunodeficiency virus type 1 (HIV-1) employs a variety of mechanisms to express numerous proteins from a single-genomic-length RNA transcript. One such mechanism is the regulated splicing of the primary transcript into more than 30 distinct mRNA species (65,(74)(75)(76). Other mechanisms for the production of different polypeptides from mature viral mRNA species rely on translational events that are atypical of normal host protein translation.The initiation of translation of most eukaryotic mRNAs is thought to occur by a process involving ribosome scanning. In this model, the 40S ribosomal subunit binds to the 5Ј end of a capped mRNA (80, 81), commences scanning toward the 3Ј end, and initiates translation upon encountering an AUG codon in suitable context (often referred to as a "Kozak" context) (47,49,50). The 60S ribosomal subunit is then recruited, and translation of a single polypeptide begins.Translation of some HIV-1 mRNAs can depart from this basic model in several ways. Translation of the pol open reading frame (ORF) requires a Ϫ1 frameshift at a "slippery" sequence within the gag ORF. This frameshift allows roughly 5 to 10% of translating ribosomes to bypass the normal gag stop codon, enabling translation of a Gag-Pol precursor protein (reviewed in reference 32). Other HIV-1 gene products, including Env and Nef, have been shown to be translated by "leaky" scanning of ribosomes past the vpu and rev AUGs, respectively (74,75,77).Another alternative mechanism for translational initiation is driven by RNA structural elements referred to as internal ribosome entry sites (or segments) (IRESs). IRESs are thought to promote initiation of translation by directly binding ribosomes, with participation of other host cell factors, in a manner independent of the mRNA cap or of scann...

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