Effect of 3'U3 alterations on virus titre. (A) Titres of vectors containing either the SV40 or bGH polyadenylation sites, and either a 12 or 19 bp att sequence were compared using 0.125 ug vector per well (n = 3). bGHatt12, pHIV-1SDm2SEbGHatt12; bGHatt19, pHIV-1SDm2SEbGHatt19; SVatt12, pHIV-1SDm2SESVatt12; SVatt19, pHIV-1SDm2SESVatt19. * = P < 0.05 ANOVA on Ranks. All results are presented as Mean ± SD. (B) Schematic of 3' LTR structures. In both cases R is truncated at the 3' end, att is 12 or 19 bp in length. SV40, SV40 early polyadenylation signal; bGH, bovine growth hormone gene polyadenylation signal (see Results for details).