EFFICIENCY OF REINITIATION OF TRANSLATION ON HUMAN-IMMUNODEFICIENCY-VIRUS TYPE-1 MESSENGER-RNAS IS DETERMINED BY THE LENGTH OF THE UPSTREAMOPEN READING FRAME AND BY INTERCISTRONIC DISTANCE

In this study, we examined the mechanism of translation of the human i mmunodeficiency virus type 1 tat mRNA in eucaryotic cells. This mRNA c ontains the tat open reading frame (ORF), followed by rev and nef ORFs , but only the first ORF, encoding tat, is efficiently translated. Int roduction of premature stop codons in the tat ORF resulted in efficien t translation of the downstream rev ORF. We show that the degree of in hibition of translation of rev is proportional to the Length of the up stream tat ORF. An upstream ORF spanning 84 nucleotides was predicted to inhibit 50% of the ribosomes from initiating translation at downstr eam AUGs. Interestingly, the distance between the upstream ORF and the start codon of the second ORF also played a role in efficiency of dow nstream translation initiation. It remains to be investigated if these conclusions relate to translation of mRNAs other than human immunodef iciency virus type 1 mRNAs. The strong inhibition of rev translation e xerted by the presence of the tat ORF may reflect the different roles of Tat and Rev in the viral life cycle. Tat acts early to induce high production of all viral mRNAs. Rev induces a switch from the early to the late phase of the viral life cycle, resulting in production of vir al structural proteins and virions. Premature Rev production may resul t in entrance into the late phase in the presence of suboptimal levels of viral mRNAs coding for structural proteins, resulting in inefficie nt virus production.