The processing of U5 duplex oligo substrates by IN was as described in the legend to Fig. 3. A, HIV-1 U5 LTR duplex oligo. B, ASV U5 LTR duplex oligo. HIV-1 IN (3CSF185H), lane 1; ASV IN, lane 5; no enzyme (−), lane 6. HIV-1/ASV chimeras (see Table 1 for specific amino acid substitutions): S72, lane 2; S125; lane 3; S153, lane 4. Notations used are as in Fig. 3.