In contrast to C1C2 T2B T3 Á163(G) , C1C2 T2B T3 Á163(A) and C1C2W 2B T3 Á163(A) underwent recombination 22 nt further along the TAV RNA 3 molecule after nucleotide 2245, adjacent to stem-loop E (in Figure 7C) , regardless of the nature of the 2b protein. Therefore, while the nature of the receptor RNA and the apparently slow selection of the recombinants (Figures 2 and 6) were determined by the 2b gene, the generation of recombinants at position 2245 was determined by the absence of the first repeat. Since the first and second repeats differ by only one nucleotide (underlined blue letters in Figure 7A and B, at positions 1966 and 2129), we propose that there must be a difference in structure between the virus containing only one repeat, when this is the second repeat versus the first repeat, and that this difference in structure affects pausing by the polymerase, leading to switchover to another template. This difference could be the presence (or absence) of a weak pseudoknot structure formed between the three A residues in the loop of stem-loop G and the three U residues in the non-base-paired region between stem-loops K2 and J2 (see blue letters in Figures 7) . This pseudoknot would not be stable in T3 Á163(A) , since it would only contain two A:U base pairs. This weak pseudoknot structure may be sufficient to affect pausing by the polymerase in the T3 Á163(G) variant, if it can re-form after the polymerase has gone through it the first time and then transcribed through the various, following stem-loop structures (stem-loops J, I and H in Figure 7 ), leading to template switching in stem-loop G. However, as this pseudoknot is absent in T3 Á163(A) , the polymerase would continue through stem-loop G until it pauses at the base of stem-loop E ( Figure 7C ; ÁG = À6.8 kcal/mol), before switching to the receptor RNA.