Well, since Abbie doesn’t want to talk about RNAi, I suppose I will. As with my more technical posts, I’ll try to define all the not-so-common terms first.
mRNA=active RNA transcripts with introns removed
miRNA=small RNA sequences (~20 bases) which may be complementary to an mRNA transcript
dsRNA=double stranded RNA; necessary to start the RNAi pathway
Dicer=a protein which cleaves dsRNA
siRNA=small interfering RNA; small segments of dsRNA which have been cleaved by Dicer
RISC=RNA-induced silencing complex, a large protein and RNA complex which binds the antisense segment to an mRNA transcript
HIV transactivator (Tat)=one of two subtypes of proteins which increase the level of HIV expression
TAR=transactivation response region, a portion of RNA which reacts to Tat by forming a stem loop of dsDNA
Now, Tat has an RNA binding domain as does Dicer, and since Dicer is required to initiate RNAi, when Dicer binds to TAR (this is dsRNA) which is also the target of Tat, Tat then interacts with the helicase region of Dicer and suppresses the RNase activity. This suppression of Dicer at the location where it would interact (TAR) prevents the formation of RISC and the degradation of the HIV transcript.
HIV-1 Tat interaction with Dicer: requirement for RNA. Bennasser et al. Retrovirology 2006 3:95
HIV-1 TAR RNA Subverts RNA Interference in Transfected Cells through Sequestration of TAR RNA-binding Protein, TRBP. Bennasser et al. J. Biol. Chem., Vol. 281, Issue 38