THE PACKAGING PHENOTYPE OF THE SE21Q1B PROVIRUS IS RELATED TO HIGH PROVIRAL EXPRESSION AND NOT TRANS-ACTING FACTORS

The avian packaging cell line SE21Q1b produces particles which encapsi date cellular RNAs. Such RNAs can be reverse transcribed by endogenous polymerase and integrated into the genomes of newly infected cells (M . Linial, Cell 49:93-102, 1987). Genomic RNA is not packaged because t he packaging (Psi) region of the provirus is deleted. The provirus als o lacks the negative-strand primer binding site, which prevents effici ent reverse transcription of randomly packaged genomic RNA. Previous w ork from our laboratory suggested that the trass-acting defect which a llows packaging of cellular mRNA mapped to the provirus but did not ma p to the nucleocapsid region of the gag gene (D. J. Anderson, P. Lee, K. L. Levine, J. Sang, S. A. Shah, O. O.Yang, P. R. Shank, and M. L. L inial, J. Virol. 66:204-216, 1992). We have found, using proviral reco mbinants between SE21Q1b and wild-type Rous sarcoma virus, that packag ing of cellular RNAs does not map to the gag gene. Rather, the propens ity of SE21Q1b particles to package cellular mRNA is a function of the high level of particle production in these cells and not of any speci fic viral structural proteins.