This model assumes that intramolecular recombination is a more frequent event than intermolecular 1-2060 T3 1902-2386 À À C1C2 T2B T3 1-2060 T3 2065-2386 À À '+' indicates infection, while 'À' indicates no infection. The nature of the plasmid constructs is described in Figure 1 . recombination. Therefore, generation of TAV RNA 3 progeny with deletion of one of the 163-nt repeats is more likely to occur than recombination between TAV RNA 3 and another viral genomic RNA. That is, if one repeat is missing, then only intermolecular recombination can occur, which may explain why the recombinants were seen to accumulate more rapidly in the plants directly inoculated with virus containing only one repeat ( Figure 2 ). On the other hand, loss of a repeat was not a prerequisite for intermolecular recombination, since intramolecular recombination would involve loss of the first repeat [163(A)], while the recombinants formed from the full-length T3 all retained the first repeat. Intramolecular recombination would be expected to occur during (+) TAV RNA 3 synthesis, if the polymerase jumped from the beginning of the first repeat in stem-loop L2 ( Figure 7A ) to the beginning of the second repeat in stemloop L1 ( Figure 7B ). These sequences also contain the putative RNA 5 subgenomic promoter sequence (33, 38) that would facilitate this recombination and those described previously that involved this 'hot spot' for intermolecular recombination between TAV and CMV (33, 38) . The presence of another stem-loop structure (K1/K2 in Figure 7 ) immediately adjacent to the putative subgenomic promoter may slow the polymerase down sufficiently to stutter and in some cases switch templates. In the case of the first TAV-CMV RNA 3 recombinant described (30) , this occurred just before another stem-loop structure in the first repeat (J2 in Figure 7A ; see arrow), but in that instance, the polymerase jumped to the putative RNA 5 subgenomic promoter of (À) CMV RNA 2, leading to an initial recombinant that had a duplication of the two, putative subgenomic RNA promoters, which then led to a second recombination event, with the intervening TAV RNA 3 sequences deleted, giving the final, stable recombinant RNA 3 described (30) . The full-length TAV RNA 3, as well as the T3 Á163(G) variant gave rise to similar recombination products, with the exact switchover position on the donor RNA of the recombinant determined by the 2b gene present (T2B versus W2B), while the switchover position on the receptor RNA is the beginning of the RNA 5 sequence in either CMV RNAs 1 or 2, again determined by the specific 2b gene present. In the case of C1C2 T2B T3 and C1C2 T2B T3 Á163(G) , the recombination occurred immediately after nucleotide 2060. For C1C2 T2B T3, switchover would have had to occur at the base of stem-loop L1 ( Figure 7B ), while for C1C2 T2B T3 Á163(G) , switchover would have had to occur within stem-loop G ( Figure 7B and C), since the sequences comprising stem-loop L1 (plus stem-loops K1 to H1) would not exist in C1C2 T2B T3 Á163(G) . Since stem-loops L1 and G are quite different in most of their sequence and structure, as well as stability (ÁG = -1.3 kcal/mol for stem-loop G and À6.9 kcal/mol for stem-loop L1), it seems likely that the common structures preceding these stem-loops (J2-I2-H2 versus J1-I1-H1; ÁG = À3.8, À4.8 and À2.9 kcal/mol, for each stem-loop structure) may have slowed the polymerase sufficiently to allow it to pause at nucleotide 2060, in either stem-loop L1 or G ( Figure 7B ).