Note: this site last updated in 2006
An article from "The Molecules of HIV" (c) Dan Stowell
"rev" is another of HIV's regulator genes. It stimulates the production of HIV proteins, but suppresses the expression of HIV's regulatory genes.
How does it achieve this? The messenger RNAs of HIV can either be sent to the protein-producing part of the cell intact, or they can have bits cut out of them first (splicing). The intact mRNA tends to encode HIV proteins (such as envelope and capsid proteins), while the spliced mRNA encodes regulatory genes such as tat and nef.
So what rev does is to help intact mRNA to be exported from the cell nucleus. It binds to the mRNA at a specific point (the RRE or Rev-Responsive Element), and this complex of RNA and rev is sent out of the nucleus. A molecule of rev can "shuttle" in and out of the nucleus, potentially taking a new set of RNA out each time it leaves the nucleus.
The RRE is not present in completely-spliced HIV mRNA - it will have been chopped out. Completely-spliced mRNA is sent out of the nucleus by the ordinary cell machinery (without needing help from rev) - so you could say that rev's trick is to cause the mRNA to be exported "before it's ready", in a sense.