IDENTIFICATION OF A SEQUENCE ELEMENT IMMEDIATELY UPSTREAM OF THE POLYPURINE TRACT THAT IS ESSENTIAL FOR REPLICATION OF SIMIAN IMMUNODEFICIENCY VIRUS

A short stretch of T-rich sequences immediately upstream of the polypu rine tract (PPT) is highly conserved in the proviral genomes of human and simian immunodeficiency viruses (HIV and SIV). To investigate whet her this 'U-box' influences SIVmac239 replication, we analyzed the pro perties of mutants with changes in this region of the viral genome. Al l mutants were either retarded in their growth (up to one month delay) or did not replicate detectably in CEMx174 cells. When U-box mutants did replicate detectably; compensatory changes were consistently obser ved in the viral genome. The most common compensatory change was the a cquisition of thymidines immediately upstream of the PPT, but marked e xpansion in the length of the PPT was also observed. U-box mutants pro duced transiently by transfection were severely impaired in their abil ity to produce reverse transcripts in infectivity assays. Analysis of transiently produced mutant virus revealed no defect in RNA packaging or virus assembly These results identify a new structural element impo rtant for an early step in the viral life cycle that includes reverse transcription.