Retroviral Psi packaging element

Retroviral Psi packaging element is a cis-acting RNA element identified in the genomes of the retroviruses Human immunodeficiency virus (HIV) and Simian immunodeficiency virus (SIV). It is involved in regulating encapsidation of the retroviral RNA, an essential step in replication. This RNA element is known to have a secondary structure composed of four hairpins called SL1 to SL4 (SL is for Stem Loop). It has been shown that SL1 may provide a secondary binding site for the viral Rev protein. Each of the stem loops SL1, SL2 , SL3 and SL4 have been independently expressed and structurally characterised.