[Inhibition of gene expression by antisense DNA]

We demonstrated that unmodified and modified (phosphorothioate) oligonucleotides prevent cDNA synthesis by AMV, MMLV, or HIV reverse transcriptases. Antisense oligonucleotide/RNA hybrids specifically arrest primer extension. The blockage involves the degradation of the RNA fragment, bound to the antisense oligonucleotide, by reverse transcriptase associated RNase H activity. However, the phosphorothioate oligomer inhibited polymerization, by binding to the AMV- and MMLV-RTs, rather than to the template RNA, whereas there was no competitive binding of the phosphorothioate oligomer on the HIV RT during reverse transcription. Furthermore, the RNase H activity of HIV-RT was only slightly affected by the phosphorothioate oligonucleotide. The anti-HIV activities of phosphorothioate- or 5'-linked lipid-oligonucleotides are also described and some of the problems that still need to be solved are pointed out.