Point mutations in the avian sarcoma/leukosis virus 3 ' untranslated region result in a packaging defect

The 3' untranslated region (3' UTR) between the 3' end of env and the long terminal repeat is well conserved among avian retroviruses and is essential for efficient replication. Deletion of the dr1 element within the 3' UTR h as been reported to have various effects, including reduced: levels of unsp liced RNA in the cytoplasm, decreased stability of unspliced RNA, decreased particle production, and decreased genomic RNA packaging. To probe the rol e of specific sequences within dr1 in virus replication, site-directed muta genesis was utilized to perturb parts of the predicted secondary structure of dr1. Seven of thirteen mutations had no significant effect; the others r esulted in an approximately 10- tea 20-fold reduction in replication. These mutants were further characterized and found to impair cytoplasmic accumul ation of unspliced RNA only slightly. Furthermore, no decreases were observ ed in the stability of the unspliced RNA or in the production of virus part icles. Genomic RNA packaging, however, was reduced by about 10-fold. Simila r amounts of particles were produced by cells containing the mutant and wil d-type DNA, and all particles contained similar levels of reverse transcrip tase activity. The results suggest that the region of the dr1 disrupted by the mutations plays a role in genomic RNA packaging, although that packagin g may not be the only role for dr1.