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Figure 2<br />

In vivo reactions carried out <strong>by</strong> HIV integrase.<br />

Page 86<br />

two nucleotides from each 3' end of the viral DNA (Figure 2b). The two nucleotides are removed as a<br />

dinucleotide rather than in two individual steps. The specificity for this reaction is conferred <strong>by</strong> the third<br />

and fourth nucleotides from each 3' end, a -CA sequence that is absolutely conserved. Once two<br />

nucleotides have been removed, leaving recessed 3' hydroxyl groups, the next step is the joining of the 3'<br />

ends to target DNA (Figure 2c, d). This process, known as double-ended integration, occurs on opposite<br />

strands such that the joining sites on each of the target DNA strands are separated <strong>by</strong> five base pairs. The<br />

final step in integration is the repair of the single-stranded gaps generated <strong>by</strong> the staggered insertion of<br />

the viral 3' ends on opposite strands; this regenerates an intact double-stranded DNA molecule (Figure<br />

2e and f). Gap repair is probably carried out <strong>by</strong> host cell DNA repair systems.<br />

One necessary consequence of retroviral integration is the duplication of five base pairs of host cell<br />

DNA on either side of the integrated provirus.<br />

http://legacy.netlibrary.com/nlreader/nlReader.dll?bookid=12640&filename=Page_86.html [4/5/2004 4:54:34 PM]

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