For a relatively small number of cases of utilized RT, the known stimulatory signals involve an mRNA structure 3 0 of the stop codon. In MuLV, in vitro translation studies showed that a compact pseudoknot structure 3 0 of the gag terminator, UAG, is essential for meaningful levels of RT, with the identity of certain nucleotides in the 8 nt 'spacer' region between the stop codon and the pseudoknot, as well as some of the nucleotides in loop 2 of the pseudoknot, being important (39) (40) (41) (42) . The location of the pseudoknot (8 nt 3 0 of the stop codon) may permit it to act at the mRNA unwinding site half-way through the mRNA entrance channel of the ribosome (43) . A very different stimulatory element is present in the plant luteoviruses, where RT at the end of the CP gene produces a much larger CP-extension protein that is important for aphid transmission (44) . In the best-studied of these viruses, barley yellow dwarf, both 3 0 -adjacent sequences and an element $700-750 nt 3 0 of the UAG stop codon have been identified as important for RT and long-range RNA base pairing between the 3 0 -proximal and 3 0 -distal elements has been suggested as a possible mechanism (45) . Similar results were found for beet western yellows virus (46) .